2019

Activity Report 2019
TABLE OF CONTENTS
pp
7
8
11
13
15

Yoshihisa Harada
Preface :Status of Beamline BL07LSU at the SPring-8
Status of spin-and angle-resolved photoelectron spectroscopy with laser light at LASOR
Workshop & Meetings
Seminar
Activities

1
2
3
4
5

Synchrotron Radiation Experiments (SPring-8)
16

1)

SOFT X-RAY EMISSION SPECTROSCOPY STUDY OF A CATHODE MATERIAL FOR
LI-ION BATTERIES IN COMBINATION WITH FULL-MULTIPLET CALCULATION
Daisuke Asakura, Yusuke Nanba, Eiji Hosono, Hideharu Niwa, Hisao Kiuchi, Jun Miyawaki and Yoshihisa Harada

18

2)

DEVELOPMENT OF ALL-SOLID-STATE LI-ION-BATTERY CELL FOR OPERANDO SOFT
X-RAY EMISSION SPECTROCOSPY
Daisuke Asakura, Eiji Hosono, Kosuke Yamazoe, Jun Miyawaki and Yoshihisa Harada

20

3)

VERIFICATION OF TWO-CARRIER STATE IN THE CUPRATE SUPERCONDUCTOR
Pr1.4-xLa0.6CexCuO4
Kenji Ishii, Shun Asano, Masaki Fujita, Kohei Yamagami, Jun Miyawaki, Yoshihisa Harada

22

SPIN-POLARIZED ELECTRONIC STATE OF SINGLE CRYSTAL Co2MnSi HEUSLER
4) ALLOY PROBED BY RESONANT INELASTIC SOFT X-RAY SCATTERING (SX-RIXS)IN MAGNETIC FIELD
Rie Y. Umetsu, Hidenori Fujiwara, Jun Miyawaki, Kohei Nishimoto, Toshiyuki Kashiuchi, Akira Sekiyama,
Akinori Irizawa, Yoshihisa Harada and Shigemasa Suga

24

5)

MULTIORBITAL BOND FORMATION FOR STABLE OXYGEN-REDOX REACTION
IN BATTERY ELECTRODES (1)Masashi Okubo

26

6)

MULTIORBITAL BOND FORMATION FOR STABLE OXYGEN-REDOX REACTION
IN BATTERY ELECTRODES (2)Masashi Okubo

28

UNDERSTANDING OF SOLVATION AND DESOLVATION STATE OF ACTIVE
7) MATERIALS FOR REDOX-FLOW BATTERIES DERIVED FROM HYDROGEN-BONDING
ANALYSIS BY SOFT X-RAY SPECTROSCOPY
Akihiro Ohira, Eiji Hosono, Daisuke Asakura, Takashi Funaki, Kosuke Yamazoe, Jun Miyawaki,
Yoshihisa Harada

30

8)

PHOTOEXCITED CARRIER DYNAMICS IN LEAD SULFIDE QUANTUM DOT
PHOTOVOLTAIC CELL
Kenichi Ozawa, Susumu Yamamoto, Iwao Matsuda

32

9)Photo-induced valence transition dynamics of SmS studied by time-resolved x-ray absorption
spectroscopy

H. Watanabe, T. Nakamura, Y. Shibata, K. Yamagami, Y. Hirata, K. Ikeda, Y. Zhang, H. Wadati, K. Imura,
H. S. Suzuki, N. K. Sato, S. Kimura

34

10)

Relaxation dynamics of Photo-induced valence transition of SmS studied by time-resolved x-ray
absorption spectroscopy
H. Watanabe, T. Nakamura, Y. Shibata, K. Yamagami, Y. Hirata, K. Ikeda, Y. Zhang, H. Wadati, K. Imura,
H. S. Suzuki, N. K. Sato, S. Kimura

36

11)

LOW-TEMPERATURE METHANE OXIDATIVE COUPLING ON MODIFIED PD MODELCATALYSTS STUDIED BY OPERANDO AMBIENT-PRESSURE XPS
Takanori Koitaya, Susumu Yamamoto, Iwao Matsuda, Jun Yoshinobu and Toshihiko Yokoyama

38

12)

X-ray Spectral Imaging Analysis of Interface States at p +-WSe2/n -MoS2 Hetero Junctions in
TFET Device Structure
Naoka Nagamura, Keigo Nakamura, Hayami Kawamoto, Naoki Higashitarumi, Keishi Akada, and Kosuke Nagashio

40

13)

SOFT X-RAY ADSORPTION AND EMISSION SPECTROSCOPY OF THE INTERMEDIATEWATER IN BIOCOMPATIBLE POLLYMERS
Shin-nosuke Nishimura, Keishi Akada, Daiki Murakami, Kosuke Yamazoe, Tomoya Ueda, Jun Miyawaki,
Yoshihisa Harada, Masaru Tanaka

42

14)

DEVELOPMENT OF TIME-RESOLVED X-RAY PHOTOELECTRON DIFFRACTION FOR
THE OBSERVATION OF THE STRUCTURAL DYNAMICS OF EPITAXIAL SILICENE
Kouichi Hayashi, Artoni Kevin R. Ang, Koji Kimura, Yuichiro Fukatsu, Susumu Yamamoto, Iwao Matsuda,
Takahiro Yonezawa, Antoine Fleurence, Yukiko Yamada-Takamura

44

15)

TIME-RESOLVED X-RAY PHOTOELECTRON DIFFRACTION STUDY OF EPITAXIAL
SILICENE
Kouichi Hayashi, Artoni Kevin R. Ang, Koji Kimura, Yuichiro Fukatsu, Susumu Yamamoto, Iwao Matsuda,
Takahiro Yonezawa, Antoine Fleurence, Yukiko Yamada-Takamura

46

16)

48

17)

ELECTRIC STRUCTURE OF WATER IN THE VICINITY OF MODEL ORGANIC SURFACESBY X-RAY EMISSION SPECTROSCOPY
Tomohiro Hayashi
MEASUREMENT OF THE THICKNESS DEPENDENCE OF THE MAGNET-OPTICALCONSTANTS OF METAL FE USING FAST POLARIZATION SWITCHING
Yasuyuki Hirata, Keisuke Ikeda, Kohei Yamagami, Yujun Zhang, Hiroki Wadati, Souliman El Moussaoui,
Iwao Matsuda

50

18)

Creation of multi-dimensional operando photoelectron spectroscopy (I)Selection of industry-government-academia collaborative model sample
Hirokazu Fukidome, Takamasa Kamogawa, Naoka Nagamura, Masaharu Oshima, Masato Kotsugi, andIssei Watanabe

52

19)

OPERAND MEASUREMENT OF ALL SOLID-STATE LI-ION BATTERYS BYPHOTOELECTRON MICROSPECTROSCOPY
Eiji Hosono, Wenxiong Zhang, Daisuke Asakura, Naoka Nagamurae, Masaharu Oshima, Yoshihisa Harada

54

20)

MAPPING MEASURMENT OF SINGLE CRYSALLINE LITHIUM METAL OXIDE BY
ALL SOLID-STATE LI-ION BATTERYS
Eiji Hosono, Wenxiong Zhang, Daisuke Asakura, Naoka Nagamurae, Masaharu Oshima, Yoshihisa Harada

5621) Resonant inelastic soft x-ray diffraction of nanoparticles
J. Miyawaki, T. Yachi, S. Maki, K. Kanie, Y. Harada

5822) Development of X-ray Polarization Precession Spectroscopy
J. Miyawaki, Y. Hirata, K. Yamagami, I. Matsuda, Y. Harada

6023) Resonant Inelastic Soft X-ray Scattering of Y3Fe5O12
J. Miyawaki, Y. Harada

62

24)

Development of Coherent Soft X-ray Diffraction Imaging Technique for Observation of
Ultra-fast Magnetic Dynamics
Yuichi Yamasaki, Kohei Yamamoto, Yuichi Yokoyama, Yasuyuki Hirata, Hiroki Wadati, and Taka-hisa Arima

64

25)

REVERSIBLE AND IRREVERSIBLE ANIONIC REDOX FOR HIGH-CAPACITYBATTERY MATERIALS
Naoaki Yabuuchi and Daisuke Asakura

66

26)

ELECTRONIC STATE OF INTERMEDIATE WATER IN BIOCOMPATIBLE POLYMERSINVESTIGATED BY XAS AND XES
Koichi Ozaki, Jumpei Yahiro, Masaru Nakada, Masahiro Kunisu, Kosuke Yamazoe, Jun Miyawaki and
Yoshihisa Harada

68

27)

Present status of the development of time-resolved ambient pressure X-ray photoelectron
spectroscopy system at SPring-8 BL07LSU
Susumu Yamamoto and Iwao Matsuda

70

28)

Development of time-resolved X-ray absorption and photoelectron spectroscopy system using
time-of-flight electron analyzer
Susumu Yamamoto, Rei Hobara, Kenichi Ozawa and Iwao Matsuda

7229) SOFT X-RAY ABSORPTION SPECTROSCOPY OF Ca2RuO4 UNDER ELECTRIC FIELD
Daiki Ootsuki, Yukie Takasuka, Ayuko Hishikawa, Tatsuhiro Ishida, Teppei Yoshida, Kosuke Yamazoe,
Tomoyuki Tsukada, Jun Miyawaki, Yoshihisa Harada, Chanchal Sow, Shingo Yonezawa, Yoshiteru Maeno andFumihiko Nakamura

74

30)

OPERANDO XPS STUDY OF CO2 HYDROGENATION ON A SINGLE-ATOM ALLOY
Pd-Cu MODEL CATALYST
J. Yoshinobu, S. Tanaka, K. Mukai, W. Osada, T. Matsuda, S. Nakano, Y. Choi, S. Yamamotoand I. Matsuda

76

31)

AP-XPS STUDY OF ELECTRONIC STATES OF METAL NANO PARTICLES ANDMETAL HYDRIDE COMPLEX
J. Yoshinobu, W. Osada, Y. Choi, S. Tanaka, K. Mukai,T. Koitaya, S. Yamamoto and I. Matsuda

78

32)

DYNAMICS OF PHOTOCATALYTIC MATERIALS FOR WATER SPLITTING
REACTION STUDIED BY TRANSIENT X-RAY ABSORPTION SPECTROSCOPY
Yohei Uemura, Ahmed Ismail, Hebatalla Elnaggar, Federica Frati, Hiroki Wadati, Yasuyuki Hirata, Yujun Zhang,Kohei Yamagami, Susumu Yamamoto, Iwao Matsuda, Frank de Groot

80

33)

INVESTIGATION OF THE ELECTRONIC STRUCTURE OF COBALT OXIDE SPINEL ON
ELECTRODES FOR WATER OXIDATION CATALYSTS USING CO 2P3D RIXSMinmin Chen, Kosuke Yamazoe, Jun Miyawaki, Yoshihisa Harada, Serena DeBeer, Olaf Rüdiger

82

INVESTIGATION OF THE ELECTRONIC STRUCTURE OF ELECTRODEPOSITED
34) COBALT OXIDE FILMS ON ELECTRODES FOR WATER OXIDATION CATALYSTS USING
CO 2P3D RIXS
Minmin Chen, Kosuke Yamazoe, Jun Miyawaki, Yoshihisa Harada, Serena DeBeer, Olaf Rüdiger

Experiments at E-Labo
841) SPIN-POLARIZED SURFACE STATES ON MAX PHASE COMPOUND V2ALC
Takahiro Ito, Masashi Ikemoto, Damir Pinek, Koichiro Yaji, Shik Shin and Thierry Ouisse

86

2)

ABSENCE OF HYBRIDIZATION BETWEEN SPIN-POLARIZED BANDS INQUASI-ONE-DIMENSIONAL GIANT RASHBA SYSTEM BI/INAS(110)-(2×1)
Takuto Nakamura, Yoshiyuki Ohtsubo, Ayumi Harasawa, Koichiro Yaji, Shik Shin, Fumio Komori,Shin-ichi Kimura

88

3)

TOPOLOGICAL SURFACE STATE MODIFIED BY ADSORPTION OF ORGANIC DONOR
MOLECULE
Tatsuya Kitazawa, Koichiro Yaji, Kosuke Shimozawa, Hiroshi Kondo, Takayoshi Yamanaka, Hiroshi Yaguchi,
Yukiaki Ishida, Kenta Kuroda, Ayumi Harasawa, Takashi Iwahashi, Yukio Ouchi, Fumio Komori, Shik Shin and
Kaname Kanai

904) INVESTIGATION OF TOPOLOGICAL PROPERTIES IN SUPERCONDUCTING TASE3
Chun Lin, Ryo Noguchi, Kenta Kuroda, Koichiro Yaji, Ayumi Harasawa, Shik Shin, Atsushi Nomura,
Masahito Sakoda, Masakatsu Tsubota, Satoshi Tanda and Takeshi Kondo

92

5)

ANGLE- AND SPIN-RESOLVED PHOTOEMISSION SPECTROSCOPY RESEARCH ON
TYPE-II WEYL SEMIMETAL WTe2
Yuxuan Wan, Lihai Wang, Keisuke Koshiishi, Masahiro Suzuki, Peng Zhang，Jaewook Kim, Kenta Kuroda,
Koichiro Yaji, Ryo Noguchi, Ayumi Harasawa, Shik Shin, Atsushi Fujimori, Sang-Wook Cheong, Takeshi Kondo

94

6)

DEVELOPMENT OF TIME-, SPIN- AND ANGLE-RESOLVED PHOTOEMISSION
SPECTROSCOPY MACHINE AT ISSP
K. Kawaguchi, K. Kuroda, Z. Zhao, A. Harasawa, K. Yaji, R. Noguchi, S. Tani, M. Fujisawa, S. Shin,
F. Komori, Y. Kobayashi, T. Kondo

967) THE WEAK TOPOLOGICAL INSULATOR STATE IN ZRTE5Peng Zhang, Ryo Noguchi, Kenta Kuroda, Chun Lin, Kaishu Kawaguchi, Koichiro Yaji, Ayumi Harasawa,
Simin Nie, Hongming Weng, V. Kandyba, A. Giampietri, A. Barinov, Qiang Li, G.D. Gu, Shik Shin andTakeshi Kondo

988) Observation of spin polarized surface states in polar Weyl semimetal Td -MoTe2
M. Sakano, Y. Tanaka, M. S. Bahramy, H. Takahashi, K. Kuroda, A. Harasawa, K. Yaji, S. Shin, S. Ishiwata,
K. Ishizaka

1009) SARPES STUDY OF A PB ATOMIC LAYER ON GE(111)
Koichiro Yaji and Fumio Komori

102

10)

HIGH-RESOLUTION SPIN-RESOLVED ELECTRONIC STRUCTURE OF TOPOLOGICALHEAVY-FERMION MATERIALS
Andrés F. Santander-Syro, Franck Fortuna, Maximilian Thees, Cris Adriano11) Study of spin structure in domain-regulated alloy of atomic layer

103

Ting-Yu Chen, Li-Cheun Yang, Santosh Chiniwar, H. -T. Jeng, Woei Wu Pai, S. -J. Tang

12) Spin resolved ARPES on interfacial electronic states of Bi/NbSe2

106

R. Stania, H. Jung, H.W. Yeom

108

13)

HIGH-RESOLUTION SPIN- AND ANGLE-RESOLVED PHOTOEMISSION SPECTROSCOPY
ON DOPED MAGNETIC TOPOLOGICAL INSULATORS
Keisuke Fukutani, Chandan De, Roland Stania, Koichiro Yaji, Kenta Kuroda, Jaeyoung Kim and Han-Woong
Yeom

Half-metallicity of the ferrimagnet Mn2VAl revealed by resonant inelastic soft x-ray scattering in amagnetic field

110

R.Y. Umetsu, H. Fujiwara, K. Nagai, Y. Nakatani, M. Kawada, A. Sekiyama, F. Kuroda, H. Fujii, T. Oguchi, Y.
Harada, J. Miyawaki, and S. Suga
PHYSICAL REVIEW B 99, 134414 (2019)

Influence of interface dipole layers on the performance of graphene field effect transistors

120

Naoka Nagamura, Hirokazu Fukidome, Kosuke Nagashio, Koji Horiba, Takayuki Ide, Kazutoshi Funakubo,
Keiichiro Tashima, Akira Toriumi, Maki Suemitsu, Karsten Horn, Masaharu Oshima
Carbon 152 (2019) 680-687

Spectrum adapted expectation-maximization algorithm for high-throughput peak shift analysis

128

Tarojiro Matsumura, Naoka Nagamura, Shotaro Akaho, Kenji Nagata, Yasunobu AndoSCIENCE AND TECHNOLOGY OF ADVANCED MATERIALS (2019), VOL. 20, NO. 1, 733–745

142

6

Staff

143

7

Publication List

Preface
The Synchrotron Radiation Laboratory (SRL) of the Institute for Solid State Physics(ISSP) has been cooperating with the Synchrotron Radiation Research Organization of the
University of Tokyo since 2006 to operate the BL07LSU soft X-ray beamline andexperimental end-stations in SPring-8. The beamline has a 27-m-long polarizationcontrolled undulator and a monochromator covering the photon energy range from 250 eV
to 2 keV with an average photon flux of 1012 photons/sec. As one of the central facilities forpromoting advanced materials science and development of unique spectroscopic methods inthe soft X-ray region, the SRL fully opened the beamline to public users in 2009. Since
then, the SRL staff members have played an essential role in promoting both domestic andinternational joint researches. Four end-stations, i.e. time-resolved photoelectron
spectroscopy, ambient pressure X-ray photoelectron spectroscopy, three dimensional
nanoESCA, and high-resolution soft X-ray emission spectroscopy stations, have excellent
time resolution (~50 ps), high pressure (~20 mbar), spatial resolution (~70 nm), and energy
resolution (E/E ≈ 10,000), respectively. They have been installed in the beamline toaccept joint-research proposals, while a free-port station equipped with a focusing mirror isavailable to outside users.
In 2018, the Japanese government has decided to construct a next generation synchrotron
radiation facility dedicated to the soft and tender x-ray regions in Sendai. SRL has alsodecided to make a significant commitment and will gradually transfer the techniques
cultivated in SPring-8 to the new synchrotron facility.
In addition to the cutting-edge activities using synchrotron radiation, SRL joined theLaser and Synchrotron Research (LASOR) Center as a member in 2012. Since then, SRL
has promoted the scientific use of high-harmonic generation by lasers in the vacuum ultraviolet and soft X-ray regions at the Kashiwa Campus. In 2014, SRL constructed a new
high-resolution laser spin- and angle-resolved photoelectron spectroscopy (SARPES)
apparatus that is designed to provide high-energy (1.7 meV) and -angular resolutions withhigh-efficiency spin detectors for various types of solids, such as spin-orbit coupledmaterials and ferromagnetic materials. Since 2015, the SARPES system has been open forusers and accepting joint-research proposals.
Our goal is to provide users with a platform that uses both synchrotron radiation andhigh-harmonic generation by lasers by strong collaboration with other LASOR groupmembers.
June 30, 2020
Yoshihisa Harada
Director of SRL-ISSP

1. Status of Beamline BL07LSU at SPring-8
The University-of-Tokyo high-brilliance synchrotron soft X-ray outstation beamline
BL07LSU at SPring-8 has been maintained by the permanent staff members with adjuncts
for user operations. The scientific aim of the beamline is to promote advanced
spectroscopy for solid state and soft (including bio-) materials. There are four regular
endstations for time-resolved soft X-ray spectroscopy (TR-SX spectroscopy), ambientpressure X-ray photoelectron spectroscopy, 3D-scanning photoelectron microscope (3Dnano-ESCA) and high-resolution soft X-ray emission spectroscopy (HORNET) that areopen for users. There is also a free port station for users who bring their own experimental
apparatus.
The beamline BL07LSU is equipped with a segmented cross-type undulator. By using
phase shifter among the undulator segments, a polarization control of soft X-ray wasstarted since 2016. Circularly and linearly polarized soft X-rays at full energy range (250
– 2000 eV) have been available by tuning the permanent magnet type phase shifter.
At the endstations, various scientific researches were carried out by both the laboratory
staffs and general users (G-type and S-type applications). In addition, from this fiscal year,
we started to accept priority issues (P type problem) for the purposes of utilizing theadvanced synchrotron radiation light source technology, especially high-speed
polarization switching, at BL07LSU and of promoting developments of the beamline andendstation technologies for the next generation synchrotron light source.
Recent activities at each station is briefly introduced below.
(1) Time-Resolved soft X-ray spectroscopy station (TR-SX spectroscopy)
The station is to perform time-resolved photoemission spectroscopy experiments bysynchronizing the high-brilliant soft X-ray and the ultra-short laser pulses. A twodimensional angle-resolved time-of-flight (ARTOF) analyzer has been equipped for theefficient time-resolved measurements and the measurement temperature can be controlledfrom 15 K to 1150K. The station adopts two different optical laser systems synchronizedwith synchrotron soft X-ray; low repetition rate and high pulse energy (1 kHz, mJ) andhigh repetition rate and low pulse energy (208 kHz, J).
In 2019, Hayashi group succeeded in observing the temporal variation of the Si 2p
photoelectron diffraction pattern of a silicene layer after the laser pulse irradiation. The
time-resolved pattern agreed with the simulation results and revealed the lattice dynamics.
The research paper is submitted as the world's first time-resolved photoelectron
diffraction data. The system of time-resolved photoelectron spectroscopy will betransferred to Kashiwa Campus in 2020 for experiments with the vacuum ultraviolet/soft

X-ray lasers. The time-resolved photoemission measurement will be continued users with
the AP-XPS system, see the following section, that has been updated for this purpose
recently.
(2) Ambient-pressure X-ray photoelectron spectroscopy (AP-XPS)
AP-XPS station allows Operando observation of catalysts under reaction conditions.
The AP-XPS station was constructed in 2014 by an external funding (JST ACT-C project),
and was opened to external users since 2018. The AP-XPS system is equipped with adifferentially pumped electron analyzer (SPECS, PHOIBOS 150 NAP) and an ambientpressure gas cell. XPS measurements can be performed both under ultrahigh vacuum andin near-ambient gas pressure up to 20 mbar. Catalytic activity is monitored by mass
spectrometer, and simultaneous evaluation of adsorbate and catalyst electronic states isperformed. In addition to the experiments using synchrotron soft X-ray, off-line
experiments using a twin-anode X-ray source can be performed.
In 2019, a variety of research projects have been conducted using the AP-XPS station:(i) Methanol synthesis on Cu-Zn catalysts, (ii) Sabatier reaction on Ni catalysts, (iii) H2adsorption/absorption in Pd nanoparticles and Pd alloys (PdAg and PdCu), (iv) CH4
partial oxidation on Pd catalysts, (v) CO2 adsorption on graphene support, (vi) bandalignment at a semiconductor photoelectrode and electrolyte interface, and (vii)development of time-resolved AP-XPS. Koitaya et al. published a research paper on CO2hydrogenation on a Zn modified Cu surface in ACS Catalysis (IF 11.384). Tang et alreported articles on H2 adsorption and storage on a PdCu alloy surface in Appl. Surf. Sci.
(3) 3D-scanning photoelectron microscope (3D nano-ESCA)
3D-nano-ESCA can be used for sub-100 nm range microscopic 2D mapping and depth
profile of the chemical structure of functional materials and devices.
In 2019, elucidation of facet-wise Li de-insertion characteristics of LTO in all-solidstate LIB was carried out as a joint research by Asakura and Hosono group at AIST. Bychanging the solid electrolyte from LLZ and dry polymer to sulfide solid electrolyte, thedegas rate was reduced, and a clearer image was obtained. Also conducted wasphotoelectron spectromicroscopy of SnS as a joint research by Nagamura at NIMS andProf. Nagashio group at UTokyo. The reason behind the monolayer growth of SnS by thesimple PVD remains unclear. A single SnS film was not grown from high-purity SnS
powder. By nano-ESCA observation, it is confirmed that the lateral growth of monolayer
SnS is facilitated due to the enhanced surface diffusion of SnS precursor molecules by
Sn2S3 contaminants in low-purity powder. Lastly, super-resolution imaging and depth
profiling analysis of hetero-junction transistor fine structures using III-V compoundsemiconductors were performed as a joint research by Fukidome group at Tohoku Univ.
In the viewpoint of measurement informatics, sparce modeling approach is applied toovercome the limits of spatial resolution and make it possible to observe objects in thesize of several nms.
(4) Ultra high-resolution soft X-ray emission spectroscopy (HORNET)
The station is dedicated for soft X-ray emission (or resonant inelastic X-ray scattering:
RIXS) spectroscopy measurements with high-resolution (E/ΔE > 10,000) and undervarious environmental conditions (gas, liquid, and solid).
In 2019 the station accepted 19 joint researches. Studies on solid state physics includei) O K-edge RIXS of holes doped in Pr1.4-xLa0.6CexCuO4, ii) Observation of magnoninduced state of spintronics material Y3Fe5O12 by operando RIXS by application of highfrequency electromagnetic wave, iii) XAS and RIXS studies of the electric-field-induced
metal insulator transition of Ca3(Ru1-xTix)2O7, iv) Fe L-edge RIXS of diluted magnetic
semiconductors (Ga, Fe)Sb and (In, Fe)As. For the battery and catalyst materials, thevalence state of Li-ion battery electrode materials LiMn1-xFexPO4 and Li(Li2/3, Mn4/3)O4
during charge and discharge as well as water decomposition catalyst CoxFeyO4 wereobserved by operando RIXS. For the studies on the behavior of interfacial water, water
hydrating thermo-responsive polymer PNIPAM, polyvinyl alcohol aqueous solution,water confined in a subnanopore of liquid crystalline membrane, interfacial water in thepolymer-brush form of PBuA and PMEA were observed. All the measurements weresuccessful in the determination of specific hydrogen bond structure of water responsible
for materials functions such as phase transition, folding, ice nucleation process, water
filtration, and biocompatibility.

Status of spin- and angle-resolved photoelectron spectroscopy
with laser light at Laser and Synchrotron Research Laboratory
Spin-and angle-resolved photoelectron spectroscopy (SARPES) is a powerful technique toinvestigate the spin-dependent electronic states in solids. This has been used for studying exchange
splitting of ferromagnetic materials over 50 years. Recently, spin textures have been intensively
studied in strongly spin-orbit coupled materials, such as Rashba spin-split systems and topologicalinsulators. We have been developing a SARPES apparatus using vacuum-ultraviolet (VUV) lasers at
Laser and Synchrotron Research Laboratory in the Institute for Solid State Physics, named LOBSTER(Laser-Optics-Based Spin-vecTor Experimental Research) machine. The LOBSTER machine iscurrently utilized to obtain precise information on spin-dependent electronic structures near the Fermi
level in solids. We started a project to construct the LOBSTER machine from FY 2014 and joint
researches at this station have started from FY2015.
Figure 1 represents an overview of the LOBSTER machine [1]. The apparatus consists of ananalysis chamber, a carousel sample-bank chamber connected to a load-lock chamber, and a molecularbeam epitaxy (MBE) chamber,which are kept in an ultra-high vacuum (UHV) environment and areconnected with each other via UHV gate valves. The hemispherical electron analyzer is a

Fig. 1. Overview of the LOBSTER machine developed at the Laser and Synchrotron ResearchLaboratory at the Institute for Solid State Physics [1].

custom-made ScientaOmicron DA30-L, modified to attach the very-low-energy-electron-diffraction
type spin detectors. The electrons are currently excited by 6.994-eV photons, yielded by 6th harmonicof a Nd:YVO4 quasi-continuous wave laser with repetition rate of 120 MHz. A helium discharge lamp
(VG Scienta, VUV5000) is also available as a photon source. At the MBE chamber, samples can beheated by a direct current heating or electron bombardment. The surface evaluating and preparing
instruments, such as evaporators, low energy electron diffraction, sputter-gun and quartz microbalance,
can be installed. At the carousel chamber, 16 samples can be stocked in the UHV environment.
In FY2019, eleven research proposals from outside of the institute were accepted and conducted.
Spin-polarized states were investigated in both bulk and surface of various topological materials
including magnetic and superconducting ones, atomic layers, and ferromagnetic compounds. For
extending the use of laser-SARPES to time-resolved study, we have been developing a new laser
beamline of 10.7 eV femtosecond laser pulses based on the Yb-doped fiver with the high repetition
rate (1 MHz) [2].

References[1] K. Yaji, A. Harasawa, K. Kuroda, S. Toyohisa, M. Nakayama, Y. Ishida, A. Fukushima, S. Watanabe, C.-T.
Chen, F. Komori and S. Shin, Rev. Sci. Instrum. 87, 053111 (2016).
[2] Z. Zhao and Y. Kobayashi., Opt. Exp. 25, 13517 (2017).

3. Workshops & Meetings
ISSP workshop: Innovations to the Next-Generation Synchrotron Radiation
Date: 2020/2/14(Fri)
Place: Lecture Room (A632), 6th Floor, ISSP, The University of Tokyo

Program:
10:30- Address
Hatsumi Mori (ISSP, The University of Tokyo)
10:35- Conceptual design of public beamlines at the next generation synchrotron facility
Takahasi Masamitu (National Institutes for Quantum and Radiological Science and Technology)
11:05- Overview of the partner beamlines design in the next generation synchrotron radiation facility
Tetsuya Nakamura (Tohoku University, PhoSIC)
11:35- Poster short presentation
12:00- lunch
13:00- Poster session
14:15- AI-Robot-driven inorganic materials research
Ryota Shimizu (Tokyo Institute of Technology, JST-PRESTO)
14:45- Expanding Availability of Spectromicroscopy by Informatics
Naoka Nagamura (National Institute for Materials Science, JST-PRESTO)
15:15- Soft X-ray Focusing using Ellipsoidal mirrors
Hiroto Motoyama (Center for Ultrafast Intense Laser Science, School of Science, The University of Tokyo)
15:45- Coffee Break
16:00- Operando spectroscopic studies of heterogeneous catalysts by soft X-ray ambient-pressure XPS
Takanori Koitaya (Institute for Molecular Science, JST-PRESTO)
16:30- Challenges towards Ultrahigh Resolution Resonant Inelastic Soft X-ray Scattering
Jun Miyawaki (ISSP, The University of Tokyo)
17:00- High-Brilliance Soft-X-Ray Beamline for Magnetic and Spintronic Materials Science
Tetsuro Ueno (National Institutes for Quantum and Radiological Science and Technology)
17:30- Closing Address
Takahisa Arima (SRRO, The University of Tokyo)

A plan for the next-generation synchrotron radiation facility was launched in Japanand the user-experiments are expected from 2023. Ten beamlines are planned by the
Photon Science Innovation Center (PhoSIC) and the National Institutes for Quantum andRadiological Science and Technology (QST). There have been vigorous discussions on
experimental methods for the individual beamlines that are expected to innovate ourscience and technology, including novel X-ray optics, advanced measurements,
informatics analyses and automatic controls by AI robots.
To promote our project toward the new synchrotron radiation facility, we organized
this workshop at the ISSP lecture room with broadcast live at SPring-8. It has successfully
brought together 91 participants with fruitful discussion on these new technologies andforesights of the coming science. The program started with invite talks by Dr. Tetsuya
Nakamura (PhoSIC) and Dr. Masamitsu Takahashi (QST), introducing their beamlines. In
the following, there were invited talks by leading young researchers who madepresentations on technical innovations for synchrotron radiation. We had a poster session
by graduate students and postdocs who shared their new results and actively discussedwith each other. Participants were confident to find that our next-generation synchrotron
radiation facility would be developed by our next generations.

4. Seminar

Date: August 5, 2019
Title: Prospect of Solid State Physics Research using Coherent Soft X-ray in Next-generation
Synchrotron Radiation
Speaker: Dr. Yuichi Yamasaki (National Institute for Materials Science)
Date: August 6, 2019
Title: Spectroscopic study of High-Tc cuprates and ARPES beamline at SSRL
Speaker: Dr. Makoto Hashimoto (SSRL, SLAC National Accelerator Laboratory)
Date: August 21, 2019
Title: State-of-the-art ARPES by third-generation synchrotron light source
Speaker: Dr. Hideaki Iwasawa (Graduate School of Science, Hiroshima University)
Date: September 30, 2019
Title: Novel X-ray Imaging Optics for Coherent X-ray Light Sources
Speaker: Dr. Takashi Kimura (Graduate School of Engineering, The University of Tokyo)
Date: October 11, 2019
Title: Development of hardware and software for observation of material ‘in action’ by single
crystal structure analysis
Speaker: Dr. Manabu Hoshino (Institute of Physical and Chemical Research)
Date: October 16, 2019
Title: Image type multichannel spin polarimeter and related studies
Speaker: Dr. Shan Qiao (Shanghai institute of microsystem and information technology,
Chinese academy of sciences, P. R. China)
Date: March 24, 2020
Title: Development of soft X-ray microscope using Wolter mirrors and its future prospects
Speaker: Dr. Satoru Egawa (Graduate School of Engineering, The University of Tokyo)

SOFT X-RAY EMISSION SPECTROSCOPY STUDY OF A CATHODE
MATERIAL FOR LI-ION BATTERIES IN COMBINATION WITH
FULL-MULTIPLET CALCULATION
Daisuke Asakura1,2, Yusuke Nanba1,#, Eiji Hosono1,2, Hideharu Niwa3,4,†, Hisao Kiuchi5,$, Jun
Miyawaki3,4, and Yoshihisa Harada2,3,4
1

Research Institute for Energy Conservation, National Institute of Advanced Industrial Science andTechnology (AIST)2

AIST-UTokyo Advanced Operando-Measurement Technology Open Innovation Laboratory, AIST

3

Synchrotron Radiation Laboratory, The Institute for Solid State Physics, The University of Tokyo
4

Synchrotron Radiation Research Organization, The University of Tokyo
5

Department of Applied Chemistry, The University of Tokyo

Present affiliations: #Shinshu University, †University of Tsukuba, $Kyoto University

Enhancement of the energy-density and power-density of electrode materials is one of theimportant subjects in the research field of Li-ion batteries (LIBs). To improve theperformances of electrode materials, the redox mechanisms due to Li-ion extraction/insertion
should be clarified. In this study, high-energy-resolution soft x-ray emission spectroscopy(XES) was applied to understand changes in the electronic structure of LiMn2O4 cathode [1]by Li-ion extraction/insertion[2]. The Mn 2p-3d-2p resonant XES spectra were analyzed by
the configuration-interaction full-multiplet (CIFM) calculation, which reproduced both dd andcharge-transfer (CT) excitations.
The LiMn2O4 sample was fabricated by a sol-gel method. For electrochemical experiments,
LiMn2O4 (75 wt%), acetylene black (20 wt%), and polytetrafluoroethylene (5 wt%) wereground into a paste. Lithium metal was used as the counter and reference electrodes, and 1M
ethylene carbonate (EC)/diethyl carbonate (DEC) solutions of LiClO4 were used aselectrolytes for a three-electrode beaker cell. We prepared three samples for initial (before
charging), charged, and discharged states by constant-current charge-discharge experimentswith a current density of 10 mA/g. The corresponding voltages for the three states were 3.2,
4.3 and 3.2 V vs. Li/Li+, respectively. Mn 2p-3d-2p resonant XES measurements for theLiMn2O4 samples were carried out at BL07LSU in SPring-8 using a high resolution XES
spectrometer, HORNET[3]. The total energy resolution for XES was set to E/E = 3200 at
the Mn L3 edge. All the XES measurements were carried out at room temperature in a highvacuum. The CIFM calculation for the electronic-structure analysis of Mn atoms in LiMn2O4
was carried out by assuming an octahedral MnO6 cluster in which the Mn 3d and O 2p
orbitals and the charge-transfer effect were taken into account[4].
The resonant XES spectra taken with an excitation energy of 642.0 eV corresponding to a
Mn L3-edge absorption peak showed drastic changes between the initial and charged states(Fig. 1). The initial state should be attributed to a mixed valence state of Mn3+/Mn4+. For the
initial state, all the Mn3+ should be oxidized to Mn4+. Fo
r the discharged state, the Mn ions should partially be reduced to Mn3+. Between the initialand discharged state, the small changes of the ratio of Mn3+/Mn4+ was detected by the use ofresonant XES.
To confirm the changes and separately understand the Mn3+ and Mn4+states, we analyzed
the XES spectra by using the CIFM calculation (Fig. 2), while the Mn4+ state was regarded as
unchanged during charge-discharge. Both the oxidation states have strong ligand-to-metal CT
effect. In particular for the Mn4+ (charged) state, the CT energy  was highly negative,
resulting in the dominant d4L and d5L2 charge transfer configurations. The present results

Fig. 1. Mn 2p-3d-2p resonant XES spectra
for LiMn2O4 at the initial, charged anddischarged states[2].

Fig. 2. CIFM calculation for (a) Mn4+ withthe charged state and (b) Mn3+ states with adifference spectrum between the initial andcharged states[2].

indicate possible reasons for the capacity fading in LiMn2O4: instability of thecrystal-structure during charge-discharge and Jahn-Teller distortion at the Mn3+ site.
In summary, we measured Mn 2p-3d-2p resonant XES for LiMn2O4. Detailed informationof the electronic-structure change such as the strong CT effect was obtained. In the near future,
we will perform O 1s-2p-1s XES measurement to further understand the redox reaction.

REFERENCES[1] For example, M. M. Thackeray, Prog. Solid State Chem. 25, 1 (1997).
[2] D. Asakura et al., Phys. Chem. Chem. Phys. 21, 18363 (2019).
[3] Y. Harada et al., Rev. Sci. Instrum. 83, 013116 (2012).
[4] For example, D. Asakura et al., J. Phys. Chem. Lett. 5, 4008 (2014).

DEVELOPMENT OF ALL-SOLID-STATE LI-ION-BATTERY CELLFOR OPERANDO SOFT X-RAY EMISSION SPECTROCOSPY
Daisuke Asakura1,2, Eiji Hosono1,2, Kosuke Yamazoe3,4, Jun Miyawaki3,4, and Yoshihisa
Harada2,3,4
1

Research Institute for Energy Conservation, National Institute of Advanced Industrial Science andTechnology (AIST)2

AIST-UTokyo Advanced Operando-Measurement Technology Open Innovation Laboratory, AIST

3

Synchrotron Radiation Laboratory, The Institute for Solid State Physics, The University of Tokyo
4

Synchrotron Radiation Research Organization, The University of Tokyo

Development of high-performance lithium-ion battery (LIB) for electric vehicles is a one of
the important research subjects to realize a sustainable low-carbon society. However, thetrade-off relationships among the performances such as energy density, power performance,
cycle performance, and safety property make the development difficult. Moreover, thecombination of the materials among the cathode, anode, and electrolyte is crucial for the
improvement of total performances of LIB. Thus, comprehensive researches anddevelopments are highly important. In addition to the materials design, cutting-edge analyses
are inevitable because understanding of the redox reactions and structural/electronic
properties for the materials in LIBs is necessary to design/create innovative materials for
LIBs.
Synchrotron radiation X-ray spectroscopy is a powerful technique to understand the crystal
structure and electronic structure in LIB materials. We have been studying thecharge-discharge mechanisms in LIBs based on the electronic-structure analysis by using soft
X-ray spectroscopy at HORNET, BL07LSU. Especially, the redox reactions of electrodematerials have been investigated by operando soft x-ray emission spectroscopy under the
charge-discharge operation [1,2]. The operando cell consists of special electrode chip directlygrown on Si3N4 thin-film window, counter electrode and electrolyte solution. On the other
hand, all-solid-state LIB with solid-state electrolyte is of particular importance. We havealready developed an all-solid-state-type operando cell for operando photoemission
spectromicroscopy at 3DnanoESCA, BL07LSU [3,4].
In this study, we tried to install the all-solid-state-type operando cell for the HORNET
chamber to perform operando soft x-ray emission spectroscopy for the all-solid-state LIBs. A
special transfer vessel was developed to transfer the operando cell from a glove box filledwith Ar gas to vacuum chamber without air exposure, because the operando cell includes a
Li-metal counter electrode. The transfer vessel was fixed to the HORNET chamber and theoperando cell was moved to the beam position (Figs. 1(a) and 1(b)).
The working electrode of the operando cell was LiCoO2, a cathode material, which was
single crystalline particles. The particles were mixed with a binder as a slurry and fixed on acarbon micro grid with an Au mesh (Fig. 1(d)). Then, the working electrode was stacked withthe solid-state electrolyte sheets and Li-metal counter electrode as shown in Fig. 1(d).
Figure 1(c) shows the Co L3-edge soft X-ray emission spectra for different 4 positions on theworking electrode. For all the points, the intensity was too small, indicating that there were
few LiCoO2 particles. Because the size of the particles (m order) is comparable with thebeam size, it was difficult to find the positions of the small number of the particles. The
density of the particles on the Au mesh should be much higher. On the other hand, we
succeeded in the electrochemical charge-discharge using the operando cell and detecting thesignals of Co L3-edge soft X-ray absorption by the total-electron yield detection mode.
In summary, we installed an all-solid-state-type operando cell for the HORNET chamber.
The intensity of the Co L3-edge soft X-ray emission spectra for the LiCoO2 working electrodewas too weak due to the small number of the m-order particles. In the near future, we will
increase the number of the particles by modifying the cell structure in order to perform soft
X-ray emission measurement for the all-solid-state-type operando cell.

Fig.1. (a) A picture of the transfer vessel fixed with the HORNET chamber. (b) A picture of
the inserted all-solid-state-type operando cell into the main chamber taken via a viewport. (c)Co L3-edge soft X-ray emission spectra for different 4 positions on the working electrode. (d)A schematic picture of the all-solid-state-type operando cell [3,4].

REFERENCES[1] D. Asakura et al., Electrochem. Commun. 50, 93 (2015).
[2] D. Asakura et al., Phys. Chem. Chem. Phys. 21, 26351 (2019).
[3] K. Akada et al., J. Electron Spectrosc. Relat. Phenom. 233, 64 (2019).
[4] K. Akada et al., Sci. Rep. 9, 12452 (2019).

VERIFICATION OF TWO-CARRIER STATE
IN THE CUPRATE SUPERCONDUCTOR Pr1.4-xLa0.6CexCuO4
1

Kenji Ishii1, Shun Asano2, Masaki Fujita2,
Kohei Yamagami3, Jun Miyawaki3, Yoshihisa Harada3

Synchrotron Radiation Research Center, National Institutes for Quantum and Radiological Scienceand Technology
2
Intsitute for Materials Research, Tohoku University
3
Synchrotron Radiation Laboratory, The Institute for Solid State Physics, The University of Tokyo

High-transition-temperature superconductivity in the cuprates occurs when either holes orelectrons are doped into parent antiferromagnetic Mott insulators. Dope holes occupy theoxygen 2p orbitals predominantly. In contrast, electrons are doped into the Cu 3d orbital but
doping evolution of the electronic states of the “nominally” electron-doped cuprates is not sosimple. Most electron-doped cuprate superconductors have the chemical formula RE2 −xCexCuO4 (RE = La, Pr, Nd, Sm, Eu) and electrons are doped via partial substitution of trivalent
RE with tetravalent Ce. At low Ce concentration (x), Hall and Seebeck coefficients, which areoften used to determine the sign and number of carriers, are negative as expected. However, thecoefficients change the sign in the temperature dependence in the intermediate concentrationand the sign becomes positive in entire measured temperature at high concentration (x > 0.155
for RE = Pr and x > 0.135 for RE = La) [1-3]. Furthermore, reconstruction of the Fermi surface
with increasing the Ce concentration is observed in ARPES [4] and quantum oscillation [5].
Recently, it was reported in a transport study [6] that superconductivity in the nominallyelectron-doped cuprates emerges under the coexistence of holes and electrons and a relation
between the superfluid density of holes and the superconducting transition temperature agrees
with that of hole-doped cuprates. This result indicates that the superconductivity in thenominally electron-doped cuprates is driven by the holes rather than the electrons and a unified
mechanism of the superconductivity exists irrespective of the type of carries.
In this study, we aim to verify the two-carrier states in the nominally electron-doped cuprates,
especially to prove the existence of the holes by means of x-ray absorption spectroscopy (XAS)
and resonant inelastic x-ray scattering (RIXS) at the oxygen K-edge. In the hole-doped cuprates,
O K-edge XAS gave a direct evidence of the holes in the O 2p orbitals [7]; a peak at the preedge is ascribed to the hole state. In addition, we recently demonstrated that O K-edge RIXS is
sensitive to the charge excitations of the doped holes [8].
The experiment was performed using the HORNET spectrometer at BL07LSU of SPring-8.
Total energy resolution of RIXS was 150 meV. For the present study, we selected Pr1.4xLa0.6CexCuO4 (PLCCO), in which superconductivity is observed in wide Ce concentration (x).
We prepared single crystals of superconducting x = 0.16 and non-superconducting x = 0.08.
The crystals of were cleaved in the air just before the measurement and s-polarized x-rays were
irradiated on the ab-plane of the crystal. The ac-plane was parallel to the horizontal scattering
plane and momentum transfer in the CuO2 plane (q) was scanned by rotating the crystal along
the vertical b-axis. Scattering angle (2θ) was kept at 135˚ and temperature of the crystals was
about 15 K.
The inset of Fig. 1 shows an XAS spectrum of PLCCO x = 0.16. The peak at 528.8 eVoriginates from the absorption to the Cu 3d upper Hubbard band (UHB) hybridized with the O
2p states. In the hole doped cuprates, absorption to the O 2p hole states appears at lower energyof the UHB peak but we cannot find any trace of the hole states in PLCCO. The main panel ofFig. 1 shows incident energy (Ei) dependence across the UHB peak in XAS as indicated byvertical bars in the inset. A Raman feature at 0.4 eV is observed at Ei ≤ 528.7 eV and it graduallycharges to fluorescence at higher Ei.

Figure 2 shows momentum dependence of RIXS
measured at Ei = 528.5 eV. In the four spectra fromthe bottom, lines in light color present raw data andelastic scattering is subtracted for the lines in deepcolor. The subtracted spectra are overlaid at the top.
Most of the spectral weight below 1eV isindependent of the momentum and two-magnon
excitation is a possible origin. In stark contrast,
spectral weight in O K-edge RIXS exhibits large
momentum dependence in hole-doped cuprates [8].
Though it is tiny, a momentum-dependent
component is discernible in Fig. 2. Because thecomponent follows the momentum dependence ofthe charge excitations observed in Cu K- and L3edge RIXS [9,10], it is reasonably ascribed to thesame charge origin through the hybridization
between Cu 3d and O 2p orbitals. Regarding thedoping dependence, the momentum-independent
spectral weight is similarly observed, intensity ofthe momentum-dependent component is weaker in
x = 0.08.
In summary, present XAS and RIXS results of
PLCCO are comprehensible without assuming theexistence of the holes in the O 2p orbitals. Even if
the holes exist in the nominally electron-doped
cuprates, their orbital character is qualitatively
different from that of the O 2p holes in hole-doped
cuprates.

FIG. 1: Incident energy dependence of O Kedge RIXS in PLCCO x = 0.16. Inset shows
XAS at the edge.

REFERENCES[1] Y. Dagan et al., Phys. Rev. Lett. 92, 167001 (2004).
[2] P. Li et al., Phys. Rev. B 75, 020506 (2007).
[3] R. L. Greene et al, Annu. Rev. Condens. Matter Phys.
11, 213 (2020).
[4] N. P. Armitage et al., Phys. Rev. Lett. 88, 257001(2002).
[5] T. Helm et al., Phys. Rev. B 92, 094501 (2015).
[6] Y. Li et al., Sci. Adv. 5, eaap7349 (2019).
[7] C. T. Chen et al., Phys. Rev. Lett. 66 ,104 (1991).
[8] K. Ishii et al., Phys. Rev. B 96 ,115148 (2017).
[9] K. Ishii et al., Phys. Rev. Lett. 94 ,207003 (2005).
[10] K. Ishii et al., Nat. Commun. 5, 3714 (2014).

FIG. 2: Momentum dependence of O K-edge
RIXS in PLCCO x = 0.16.

SPIN-POLARIZED ELECTRONIC STATE OF SINGLE CRYSTAL
Co2MnSi HEUSLER ALLOY PROBED BY RESONANT INELASTIC
SOFT X-RAY SCATTERING (SX-RIXS) IN MAGNETIC FIELD
Rie Y. Umetsu1, Hidenori Fujiwara2, Jun Miyawaki3,4, Kohei Nishimoto2, Toshiyuki
Kashiuchi2, Akira Sekiyama2, Akinori Irizawa5 Yoshihisa Harada3,4 and Shigemasa Suga5
1

Institute for Materials Research (IMR), Tohoku University
Graduate School of Engineering Science, Osaka University, Osaka, Japan
3
The Institute for Solid State Physics (ISSP), The University of Tokyo, Japan
4
Synchrotron Radiation Research Organization, The University of Tokyo, Japan
5
Institute of Scientific & Industrial Research, Osaka University, Osaka, Japan
2

Introduction
It has been predicted from theoretical calculations that some of Mn- and Co-based Heusler
alloys are half-metallic ferro- or ferri-magnets with the spin polarization of 100 % aroundthe Fermi energy (EF) [1-3]. If the electrons are completely polarized around EF, it is very
efficient as a magnetic electrode in various spin dependent devices in the field of spintronics.
In the aspect of the fundamental investigations for half-metallic materials, it has been
considered how to show the real evidence of the characteristic electronic structure. Attempts
have been made worldwide to observe the detailed electronic state of Heusler alloys byphotoelectron spectroscopy. Although a magnetic field must be applied in order to make asample into a single domain state, the photoelectron orbit is greatly affected by the magnetic
field. In our previous work, we have first studied the detailed electronic structure andmagnetic properties of a single crystal Mn2VAl by SX-RIXS measurements in magnetic field[4]. The RIXS is a bulk sensitive photon-in and photon-out spectroscopy, and very powerful
to investigate such as d-d excitations for open shell 3d orbitals and magnetic excitations forspin systems as well as 2p-3d transitions in element- and symmetry-specific ways. In thisstudy, we performed the RIXS experiments for single crystal of Co2MnSi Heusler alloy,
which was predicted to be a half-metallic ferromagnet.
Experimentsa) Sample preparation
A single crystal of Co2MnSi was prepared by the Bridgman method after preparing amaster alloy by arc melting in Ar gas atmosphere, and subjected to homogenizing heat
treatment at 1373 K. The sample composition identified by an electron probe microanalyzer is
Co:50.0, Mn:25.9, Si:24.1 at.%. Crystal orientation was checked by the Laue method and thespecimens were cut out in the stripe form with the length of 6 mm along the <100> with about
1 × 1 mm2 cross section. Magnetic properties were investigated with SQUID magnetometerand the spontaneous magnetization at 5 K was 142.2 emu/g (= 5.1 μB/f.u.), being comparable
to the literature [5].
b) RIXS experiment

RIXS experiments were performed using a high-resolution soft X-ray emission
spectrometer installed at the end of the BL07LSU of SPring-8. The specimen with the (001)plane was fractured in a glove box with Ar gas atmosphere, and then transferred into theanalyzer chamber without exposure to the air. The RIXS spectra for Co and Mn 2p core
excitation were measured with use of the right and left helicity circularly polarized light atroom temperature. A permanent magnet with the field of 0.25 T was installed in the chamber
and arranged as the X-ray incidence direction and the magnetic field direction become parallel[6].

Results
Figures indicate RIXS spectra obtained in parallel (μ+) and antiparallel (μ−) configurations
between the photon helicity and the magnetic field at incoming photon energy, hvin of 779.0
eV (left figure) and 640.5 eV (right figure) for the Co and Mn-2p3/2 edges, respectively. These
photon energies correspond to the peak positions of the Co and Mn-2p3/2 absorption spectra
for the Co2MnSi. Intensity peaks without any energy loss are the elastic component, and theother peaks in the larger energy loss region are the so-called fluorescence peaks. As shown in
the figures, clear fluorescence components were observed and their circular polarization
dependence (MCD = μ+ − μ−) was also confirmed. This RIXS-MCD signals originate fromthe spin-selective dipole-transition from mj = ± 3/2 which is separated by the Zeeman splitting
due to the effective magnetic field of 3d electron states [4]. It was found that the positive andnegative MCD fluorescence components in both Co and Mn have a large contribution offluorescence from mj = +3/2 and −3/2, respectively, suggesting that the valence band
electronic structures of Co2MnSi is highly spin-polarized.

References[1] R.A. de Groot, F.M. Mueller, P.G.van Engen, K.H.J. Buschow, Phys. Rev. Lett. 50 (1983)
2024-2027.
[2] J.J. Kübler, A.R. Williams, C.B. Sommers, Phys. Rev. B 28 (1983) 1745-1755.
[3] S. Ishida, S. Akazawa, Y. Kubo, J. Ishida, J. Phys. F 12 (1982) 1111-1122.
[4] R.Y. Umetsu, H. Fujiwara, K. Nagai, Y. Nakatani, M. Kawada, A. Sekiyama, F. Kuroda, H. Fujii,
T. Oguchi, Y. Harada, J. Miyawaki, S. Suga, Phys. Rev. B 99 (2019) 134414.
[5] R.Y. Umetsu, K. Kobayashi, A. Fujita, R. Kainuma, K. Ishida, Scripta Mater. 58 (2008) 723-726[6] J. Miyawaki, S. Suga, H. Fujiwara, H. Niwa, H. Kiuchi, Y. Harada, J. Synchro. Rad. 24 (2017)449-455.

Figs. RIXS spectra recorded for parallel (μ+) and antiparallel (μ−) configurations of thephoton helicity at Co-2p3/2 with incident photon energy, hvin of 779.0 eV (left figure)and at Mn-2p3/2 of 640.5 eV (right figure). Black lines indicate MCD (=μ+ − μ−).

MULTIORBITAL BOND FORMATION FOR STABLEOXYGEN-REDOX REACTION IN BATTERY ELECTRODES
Masashi Okubo
Department of Chemical System Engineering, School of Engineering, The University of Tokyo

The development of advanced electrochemical energy storage devices has been an active
research field because it offers sustainability prospects, for instance, reduction of fossil fuelreliance by realizing an electric vehicle range of 500 km per charge or integrating renewableenergy sources to an electrical grid. Market-leading lithium-ion batteries (LIBs) efficiently
store energy by lithium-ion (de)intercalation associated with redox reactions. However, theirenergy density approaches the theoretical limit, in part owing to the small capacity of thepositive electrode materials, which severely obstructs the wide deployment of LIBs.
Therefore, it is important to find an alternative battery chemistry that can exceed the existing
positive-electrode capacity limits.
The use of the extra redox reactions of oxygen in addition to conventional transition-metal
redox reactions is an attractive way to increase the capacity of transition-metal oxides.7 In
general, most oxygen 2p orbitals in conventional electrode materials LiMO2 (M: transition
metal) form σ-type bonds with axial M 3d orbitals (eg orbitals in an Oh symmetry). As theatomic energy level of oxygen 2p orbitals is usually lower than that of M eg orbitals, theσ-type bonding orbitals are predominantly from oxygen 2p orbitals. Therefore, it is difficult tooxidize oxygen 2p orbitals that have a σ-type bonding character. However, research groups
led by Bruce and Ceder postulated that oxygen in Li-excess transition-metal oxides
Li1+xM1−xO2 (Fig. 1a) intrinsically have localized (‘orphaned’) 2p orbital along the Li−O−Li
axis without the σ-type bonding character, which can contribute to an additional
oxygen-redox capacity. Indeed, large capacities exceeding the capacity limit of M redox
reactions have been reported for many Li-excess transition-metal oxides including
Li2MnO3-LiMO2 solid solution, layered honeycomb Li2IrO3, random rock salt
Li1.2Ti0.4Mn0.4O2, three-dimensional β-Li2IrO3, and anti-fluorite Li5FeO4. However, despite
the theoretical adequacy of the proposed ‘orphaned’ oxygen 2p orbital hypothesis, theexistence of the localized oxygen 2p orbitals in Li-excess transition-metal oxides has not beenverified
experimentally.
Furthermore,
although localized,
the oxygen 2p
orbital along the
Li−O−Li axis stillhasa
π-typeinteraction
withthe M t2g orbital,
which should play
an essential role inoxygen-redox
reactions. It is
importanttoFigure 1. Resonant inelastic X-ray scattering (RIXS) spectra for delocalized and localizedemphasize that the oxygen 2p orbitals. (a) Crystal structure of Li-excess layered transition-metal oxides
rigid-band model, Li[LixM1−x]O2 (M: transition metal). (b) π-type localized occupied/unoccupied states consisting*which
was of oxygen 2p and M t2g orbitals (b1/b1 bands in a C2v-symmetry OM2Li4 cluster). (c) σ-type
delocalized occupied/unoccupied states consisting of oxygen 2p and M eg orbitals (3a1/3a1* andfrequently used for 3b2/3b2* bands in a C2v-symmetry OM2Li4 cluster).

consideration of oxygen-redox electrodes, cannot qualitatively discuss M−O interaction (e.g.,
covalency), because it only considers a bandwidth and a band center energy of each band.
Therefore, for a thorough understanding of the reversible oxygen-redox reactions occurring in
Li-excess transition-metal oxides, it is necessary to clarify the nature of the π-type bonds
during the oxygen-redox reactions from the molecular orbital viewpoint. In this work, we usedoxygen K-edge resonant inelastic X-ray scattering (RIXS) spectroscopy combined with abinitio computations to demonstrate (σ + π) multiorbital bond formation as a firm criterion forstable oxygen-redox reactions.
Oxygen K-edge RIXS spectroscopy was used to examine the existence of π-type localized
oxygen 2p orbitals in Li2MnO3, a standard Li-excess layered transition-metal oxide. Oxygen
K-edge RIXS spectroscopy records the inelastic scattering during core excitation–relaxation(1s→2p→1s), in which the energy loss corresponds to valence excitation between occupied
and unoccupied 2p bands. RIXS spectroscopy has a probing depth of approximately 100 nm(bulk sensitive). Importantly, when occupied/unoccupied oxygen 2p bands have a localized
character with weak orbital hybridization, the inelastic scattering becomes Raman-like andhas constant energy loss regardless of the incident photon energy (Fig. 1b). Conversely,
delocalized occupied/unoccupied oxygen 2p bands with a large orbital hybridization gives
fluorescence-like inelastic scattering with variable energy loss depending on the incident
photon energy (Fig. 1c). Therefore, oxygen K-edge RIXS spectroscopy allows us to diagnose
localized or delocalized oxygen 2p bands.
The ab initio calculations using HSE06 hybrid functional for Li2MnO3 indicates that a
localized oxygen 2p orbital exists along the Li−O−Li axis with an orbital energy ofapproximately −0.9 to 0.0 eV versus the Fermi energy (Fig. 2a). The RIXS spectra and thecorresponding second differential RIXS map (Fig. 2b and 2c) exhibit two broad
fluorescence-like peaks, which could be related to the valence excitation between the σ-typewide oxygen 2p bands strongly hybridized with axial Mn eg orbitals (Fig. 1c). Based on theC2v symmetry of an OMn2Li4 cluster, these bands are labelled as 3a1/3b2 bands. More
importantly, the RIXS spectra show an intense Raman-like peak with constant energy loss ofapproximately 2.5 eV. This Raman-like peak clearly indicates the existence of the π-type
narrow oxygen 2p bands weakly hybridized with Mn t2g orbitals (b1/b1* bands based on the
C2v-symmetry labelling, Fig. 1b). It is important to note that the Raman-like feature is not thesimple projection of the d-d excitation of Mn L-edge, because O K-edge XAS and RIXS
involve charge-transfer processes through O-Mn interactions. In contrast, for a non-Li-excess
layered oxide LiNi1/3Co1/3Mn1/3O2, the RIXS spectra show only fluorescence-like peaks withvariable energy loss dependingon the incident photon energy
because most oxygen 2p orbitals
form σ-type wide bands withaxial M 3d orbitals. Therefore,
this is the first experimental
verification of the ‘orphaned’
oxygen 2p orbital hypothesis for
Li-excesstransition-metaloxides, and oxygen K-edge
RIXS spectroscopy is definitely
an effective diagnostic tool to Figure 2. Direct observation of orphaned oxygen 2p orbitals in Li MnO .
2
3
detect electrochemically active (a) Calculated partial density of states for oxygen and manganese in
Li2MnO3. The inset shows the spatial electron density at the energy range
oxygen states.
from −0.9 to 0 eV versus the Fermi energy. (b) Oxygen K-edge RIXS
spectra, and (c) second differential RIXS map for Li2MnO3 with excitation
photon energy from 527 to 533.75 eV.

MULTIORBITAL BOND FORMATION FOR STABLEOXYGEN-REDOX REACTION IN BATTERY ELECTRODES
Masashi Okubo
Department of Chemical System Engineering, School of Engineering, The University of Tokyo

High energy density batteries are a long-standing target for sustainability efforts, but theenergy density of the state-of-the-art lithium-ion batteries is limited in part owing to the small
capacity of positive electrode materials. Although the additional oxygen-redox reaction of
Li-excess transition-metal oxides is an attractive approach to increase the capacity, an
atomic-level understanding of the reaction mechanism has not been established. Here, usingbulk-sensitive resonant inelastic X-ray scattering (RIXS) spectroscopy combined with abinitio computations, we prove the existence of an orphaned oxygen 2p orbital that wastheoretically predicted to play a key role in oxygen-redox reactions.
As a target Li-excess layered transition metal oxide, Li1.2Ni0.13Co0.13Mn0.54O2 was used asan oxygen-redox electrode material. Before measuring the RIXS spectra, the ab initio
calculations using the HSE06 hybrid functional were carried out forLi1.2−xNi0.13Co0.13Mn0.54O2. The calculated electronic structures show that the averaged Badercharges of Ni and Co decrease only during the early stage of Li+ extraction (x < 0.4), whereasthat of oxygen continuously decreases during the whole delithiation process, suggesting theoccurrence of the additional oxygen-redox reactions for x > 0.4. Indeed,
Li1.2Ni0.13Co0.13Mn0.54O2 delivers a large capacity of approximately 250 mAh/gexceeding thetheoretical capacity (123 mAh/g) of the Ni4+/Ni2+ and Co4+/Co3+ redox reactions.
The oxygen K-edge X-ray absorption spectrum (XAS) in a bulk sensitive partial
fluorescence yield mode for the pristine Li1.2Ni0.13Co0.13Mn0.54O2 shows two absorption peaks(528.5 and 531 eV) of 1s core-electron excitation to unoccupied σ-type bands (oxygen 2p−Mneg↓/eg↑, 3a1*/3b2* in C2v) and an unoccupied π-type band (oxygen 2p−Mn t2g↓, b1* in C2v). Thisband picture agrees with the calculated partial density of states (pDOS) for Mn. Uponcharging to 4.4 V vs. Li/Li+, a new XAS shoulder emerged at 527 eV owing to hole
generation in σ-type (oxygen 2p−Ni eg) and π-type (oxygen 2p−Co t2g) bands. After oxygen
oxidation, a new absorption emerged at a relatively high absorption-energy region of
approximately 530.5 eV. Similar spectral changes were also observed during the second cycle.
The XAS signal at 530.5 eV can be ascribed to an unoccupied π-type band (oxygen 2p−Mnt2g↑) resulting from oxygen oxidation. The emergence of the absorption at a relatively highenergy region is reasonable because oxygen oxidation increases the effective nuclear charge,
which lowers the initial energy level of an oxygen 1s core and hence raises the excitationenergy18. Similar changes in the XAS signals by oxidation (the emergence of new absorption
at a high energy region) are often observed for transition-metal K-edge absorption spectra.
The RIXS spectra and the second differential RIXS map of pristine
Li1.2Ni0.13Co0.13Mn0.54O2 (Fig. 1) exhibit a Raman-like peak with constant energy loss
corresponding to a π-type narrow oxygen 2p band (π→π*), in addition to the fluorescence-likepeaks (σ→σ*). The ab initio calculations for pristine Li1.2Ni0.13Co0.13Mn0.54O2 supports theexistence of the localized oxygen 2p orbitals along the Li−O−Li axes with an orbital energy
near the Fermi level. The π→π* RIXS peak intensified after charging to 4.4 V vs. Li/Li+(transition-metal oxidation, Fig. 1b), which indicates accumulation of the localized oxygen 2p
states to the Fermi level. The initial Li+ extraction associated with Co and Ni oxidation gives
an oxygen coordination environment with a −O−Li+ or −O− axis (: Li+ vacancy),
which electrostatically raises the energy level of the localized oxygen 2p orbitals. Indeed, thefact that the energy loss of the Raman-like RIXS peak decreased from 2.5 to 2.0 eV supports
the rise in the energy level of the localized oxygen 2p orbitals. We presume that theaccumulation of the localized oxygen 2p states to the Fermi level should be an essential

preceding process to trigger the oxygen-redox reactions. For example, it is well known thatthe electrochemical properties of Li2MnO3 is poor even though oxygen has a Li+−O−Li+ axis
and hence a localized oxygen 2p orbital. Most likely, as Li2MnO3 cannot exhibit the initial Li+
extraction owing to the electrochemically inactive Mn4+, it is difficult to generate the
−O−Li+ and −O− axes, and the energy level of the localized oxygen 2p orbital remains
too low to undergo oxidation within the electrolyte stability window. Conversely, whenredox-active transition metals are present, such as in Li2MnO3-LiMO2 solid solution, the
−O−Li+ and −O− axes are easily formed by the initial Li+ extraction to raise the energy
level of the localized oxygen 2p orbital, unlocking the redox activity of oxygen. Once theoxygen-redox reaction is triggered, the −O−Li+ and −O− axes are continuously
generated to achieve a large oxygen-redox capacity.
After charging to 4.8 V vs. Li/Li+ (oxygen oxidation, Fig. 1c), the Raman-like π→π* RIXS
peaks disappear, and all the RIXS peaks become fluorescence-like. Importantly, a new intense
fluorescence-like peak emerges at an energy loss of approximately 7.0 eV with an incident
photon energy of 530.5 eV. Recalling that an incident photon of 530.5 eV excited 1s coreelectron to an unoccupied π-type band (oxygen 2p−Mn t2g↑), the inelastic scattering resonancewith the 530.5 eV incident photon includes the valence excitation between the π-type bands(π→π*). Therefore, a large energy loss of approximately 7 eV for this RIXS peak corresponds
to π-type splitting. This large π-type splitting should be induced by the hole generation in the
antibonding π-type band, and makes the Mn−O bonds more bonding (more delocalized).
Therefore, the RIXS peak is observed as fluorescence-like. The enhanced π-type interaction
stabilizes the oxidized oxygen, leading to reversible oxygen-redox reactions. Indeed, theRaman-like RIXS peak of the localized oxygen 2p orbitals was recovered after full discharge(Fig. 1d), and the changes in the RIXS spectra were reversible even during the subsequent
cycle.
To
summarize
thereaction mechanism, the
Li+−O−Li+ coordination in
the pristine state forms
localized oxygen 2p orbital
weakly hybridized with theMn t2g orbital. When theinitial Li+ extraction occurs
in association with M redox
reactions, the −O−Li+ and−O− coordination axes
are generated, which raises
the energy level of thelocalized oxygen 2p statesto the Fermi level. After thisaccumulation,oxygenoxidation
occurstodelocalize the hole within a
Mn−O−Mn
bond.
The
resultinglarge
b1/b1*
splittingstabilizes
the1. Monitoring oxygen 2p orbitals during oxygen-redox reaction ofoxidized oxygen, enabling a Figure
Li1.2Ni0.13Co0.13Mn0.54O2. Oxygen K-edge second differential RIXS map for thereversible
oxygen-redox samples (a) before charge, (b) after transition-metal oxidation, (c) after oxygen
oxidation, and (d) after discharge, with excitation photon energy from 527 toreaction.
533.75 eV. The schematic partial density of states (pDOS) for Mn 3d and oxygen
2p orbitals are shown for comparison.

UNDERSTANDING OF SOLVATION AND DESOLVATION STATE OFACTIVE MATERIALS FOR REDOX-FLOW BATTERIESDERIVED FROM HYDROGEN-BONDING ANALYSIS BY
SOFT X-RAY SPECTROSCOPY
Akihiro Ohira1, Eiji Hosono1,2,3, Daisuke Asakura1,3, Takashi Funaki1, Kosuke Yamazoe4, Jun
Miyawaki4,5, Yoshihisa Harada4,5
1

Research Institute for Energy Conservation, National Institute of Advanced Industrial Science andTechnology (AIST), 2Department of Energy and Environment, Global Zero Emission Research Center,
AIST, 3AIST-UTokyo Advanced Operando-Measurement Technology Open Innovation Laboratory(OPERANDO-OIL), 4Synchrotron Radiation Laboratory, The Institute for Solid State Physics, The
University of Tokyo 5Synchrotron Radiation Research Organization, The University of Tokyo

Power generation systems that use natural energy, such as sunlight and wind, and that do
not generate greenhouse gases have been actively introduced globally. Although they do not
generate carbon dioxide, compared to power sources like thermal and hydroelectric power,
these renewable energy sources provide an unstable power supply that is affected by various
situations. In order to use renewable energy efficiently and smoothly, the generated electricity
must be stored and discharged as required; thus, a storage battery (secondary battery) that canadjust the constantly fluctuating output over short and long cycles is significantly desired.
Large scale storage batteries are famous for lithium ion batteries, sodium-sulfur batteries, leadstorage batteries and redox flow batteries.[1-3] Lead storage batteries have been used asbatteries for automobiles and motorcycles for a long time and have abundant achievements as
small storage batteries. Lithium ion batteries also have many achievements as small batteries.
Sodium-sulfur batteries have an extremely high energy density and are widely used in load
leveling applications in substations due to their high charge and discharge efficiency.
Redox flow batteries (RFBs) have been successfully developed by NASA around 1974.[4]At the beginning of development, the Fe/Cr system was mainstream,[5] but it was not put into
practical use due to the disadvantage that mixing of the positive/negative electrolyte throughthe membrane leads to a decrease in capacity of the battery. In around 1984, Kazacos et al.
has developed a vanadium-based RFB (VRFB).[6] Since that time, the awareness of RFBs
have increased and research and development has been accelerating.
The significant issue in studying RFBs is to correctly evaluate the reactivity of electrodes
and active materials in addition to newly active material search and clarify material properties
necessary for improving battery performance.
In addition to the search for new active materials, it is important to precisely understand thereactivity of the electrodes and active materials. In particular, the state of solvation anddesolvation associated with the electrochemical reaction between active material and solvent
has not been well-understood at all, and a detailed understanding of the dynamics ofsolvation/desolvation at the interface between the active material and the electrode is required.
This will lead to a dramatic increase in solubility and redox activity, which will lead to therealization of high-performance RFBs. In this study, to obtain a guideline for the design of theinterface between the active material and the electrode, which is indispensable for therealization of a high-performance RFB, the hydration state of water-soluble active materials
was investigated by X-ray absorption and emission spectroscopy.
A dendrimer molecule with a viologen unit [7] dissolved in methanol was cast on the SiN
window and after drying the membrane, the cell was assembled and mounted in a chamber.
Water vapor was introduced while bubbling with nitrogen gas to control the relative humidity
in the cell. We observed changes in the O1s soft x-ray emission spectra of oxygen upon
hydration and dehydration of the dendrimer molecules.

Intensity (arb. units)

Figure 1 shows the change in
the O1s XES spectrum of water
XES hv=550eV
35% RHin dendrimer molecules during
1b1’ 1b1’’
25% RHthe hydration and dehydration
12%
RH
H
processes. With increasing
2O
Re 35% RH
humidity, emission spectra
Re 25% RH
similar to those of bulk water
Re 13% RH
were obtained. In addition, inthe spectrum of the dehydration
process (the change from wet todry state indicated by the signRe), 1b' is relatively small, andthe change is not reversible
when comparing the each
spectra of the hydration anddehydration
processes.
To
irreversible change in the
spectra, optical microscopy
observations of the cast films
before
and
after
the
measurements were performed.
515
520
525
530
Since the film disappeared in
Photon energy (eV)the area irradiated by X-rays, it
is inferred that the film was Figure 1 Change in the O1s XES spectrum of water
dissolved during the hydration in dendrimer molecules under humidity control.
process.
In the future, we plan to immobilize the molecules on the substrate to check the changes inhydration and dehydration.

REFERENCES[1] B. Zakeri. S. Syri, Renewable and Sustainable Energy Reviews, 42, 569 (2015).
[2] F. Shi, Reactor and Process Design in Sustainable Energy Technology, Elsevier
Amsterdam (2016).
[3] B. Dunn, H. Kamath, J.-M. Tarascon, Science, 334, 928 (2011).
[4] L. H. Theller, NASA TM X-71540, 1 (1974).
[5] R. F. Gahn, N. Hagedorn, NASA Tech Briefs February, 42 (1987).
[6] M. S. Kazacos, M. Rychcik, R. G. Robin, A. G. Fane, J. Electrochem. Soc., 133, 1057
(1986).
[7] A. Ohira, T. Funaki et al. ACS Appl. Energy Mater., 3, 4377 (2020)

PHOTOEXCITED CARRIER DYNAMICS IN LEAD SULFIDEQUANTUM DOT PHOTOVOLTAIC CELL
Kenichi Ozawa a,b, Susumu Yamamoto c, Iwao Matsuda da

Department of Chemistry, Tokyo Institute of Technology, b Institute of Materials Structure Science,High Energy Accelerator Research Organization, c Institute of Multidisciplinary Research for
Advanced Materials, Tohoku University, d The Institute for Solid State Physics, The University ofTokyo

INTRODUCTION
For realizing a sustainable society, uses of solar energy are essential. Photovoltaic (PV)devices are one of solutions to utilize the solar energy. It has been recognized that there is the
Shockley-Queisser limit, which is the theoretical maximum light-electricity conversion
efficiency of a PV cell using a single p/n junction. The limit is 34% at a band gap of 1.34 eV[1]. One of techniques to exceed this limit is tandemaization of multiple p/n junctions
composed of different materials to cover several photon energies. Another solution is the useof quantum dots (QDs) in a light-receiving layer. The QDs allow to absorb lights of various
wavelengths owing to the formation of quantum-well states. One of advantages of the QD PV
cells is that the absorption band can be controlled simply by changing the QD size so that the
production cost can be kept low in comparison with the multijunction PV cells.
Preceding studies on the QD PV cells have mainly focused on synthesis methods of various
kinds of QDs as well as their optical properties. On the other hand, accumulation ofknowledge of photoexcited carrier dynamics in the QD PV cells is still insufficient. In thepresent study, therefore, we shed the light on the dynamics of carriers generated in the QDs
by using time-resolved soft X-ray photoelectron spectroscopy (TRXPS). TRXPS has been
proved to be a powerful tool to clarify the carrier dynamics in model organic PV cells [2, 3].

RESULTS AND DISCUSSION
Fig. 1 shows S 2p and Pb 4f spectra of
the 3-nm PbS QD film. Four peaks areobserved in the S 2p spectrum; two ofthem originate from the spin-orbit splitting
S 2p levels of the S atoms in the PbS QD

Intensity (arb. units)

EXPERIMENTAL
The QDs employed in this work were lead sulfide (PbS) nanoparticles with average sizes of
3 and 9 nm (Quantum Solutions). PbS QD films were prepared under ambient conditions by
spin-coating the PbS QD solutions on 0.05 and 0.5 wt% Nb-doped rutile TiO2(110) substrates.
Since PbS and TiO2 are p-type and n-type semiconductors, respectively, a p/n junction isrealized at the PbS/TiO2 interface. The prepared QD films were treated with
3-mercaptopropionic acid (3-MPA) by dropping a 3-MPA solution onto the films and then byremoving the solution. This procedure allowed to replace a capping layer of the PbS QD fromoriginal oleic acid to 3-MPA.
The TRXPS measurements were carried out at BL07LSU of SPring-8 in the H operation
mode [4]. Visible (807 nm) and ultraviolet (404 nm) lasers with a repetition rate of 208 kHz
were used to generate the photoexcited
carriers in the PbS QD films, and the
S 2p
PbS
Pb 4f
excited states were monitored throughsynchrotron-radiation excited Pb 4f and S
2p peaks of PbS and Ti 2p peak of TiO2.
3−MPA
165

160

Binding Energy (eV)

145

140

135

Binding Energy (eV)FIGURE 1. S 2p and Pb 4f core-level peaks of the
3-nm PbS QD film on TiO2(110). The photon energy
was 370 eV.

Peak Shift (meV)Intensity (arb. units)

film, and the other two arise from 3-MPA.
(a)
(b)
150
The lower and higher binding-energy
S 2p
Pb 4f7/2
Pb 4f7/2
components are associated with PbS and100
3-MPA, respectively. This assignment isrationalized because (1) the PbS/3-MPA
50intensity ratio is higher in the film
pump off
pump on (800 nm)
composed of the 9-nm QDs than the 3-nm
0
164
162
160 139 138 137
1 10 100 1000
Binding Energy (eV) Binding Energy (eV)Delay Time (ns)QDs and (2) the low binding-energy
component is transiently shifted upon laser
pulse irradiation (this will be described FIGURE 2. (a) Comparison of the S 2p and Pb 4f7/2
peaks of 3-nm PbS QD with and without 800-nm
below). The Pb 4f7/2 and 4f5/2 peaks have a laser pulse irradiation. The photon energy was 370
tailing towards higher binding energies eV. The spectra with laser irradiation were acquired(Fig. 1). This suggest an existence of at a delay time of 1 ns. (b) Change in the magnitude
oxidation species such as PbO. Although of the Pb 4f7/2 peak shift as a function of a delay time.
the PbS QD films were prepared andtreated under ambient conditions, oxidation of the PbS QD is limited because of a relatively
low intensity of the tail structure.
As the PbS QD films are irradiated by either visible or ultraviolet laser, the PbS-originatedpeaks move to the higher binding energies. Fig. 2a compares the S 2p and Pb 4f7/2 spectra of
the 3-nm PbS QD film acquired with and without the visible laser (807 nm = 1.54 eV). Except
for the S 2p component of 3-MPA, all peaks move by ~100 meV upon laser irradiation. The
direction of the peak shift can be interpreted by a loss of an excited electron in the PbS QD by
charge transfer from PbS to TiO2.
Fig. 2b shows that the magnitude of the laser-induced shift of the PbS peaks diminisheswith a delay time. This suggests that electron-hole recombination should proceed with time if
the magnitude of the shift is proportional to the number of the PbS cations. It is apparent thatthe decay curve can be reproduced neither by a biexponential function nor by a triexponentialfunction. This is in contrast to decay curves obtained from model organic PV cells [2], where
the curves are reproduced by a biexponential function. A gradual decrease of the peak shift in
the present case implies that there should be several PbS cations with different lifetimes. A
certain distribution of the QD size, though the average size is 3 nm in the case of Fig. 2b, may
be responsible for a wide range of the lifetime.
A point to be noted is that the peak shift remains even after 4.8 μs, the longest delay time
examined in the present study. A long lifetime of the excited state of PbS, i.e., the cationicstate of PbS, is one of advantages for the efficient light-electricity conversion because there isan enough time for the free electrons and holes to be transported to the electrodes before they
are quenched via electron-hole recombination. Thus, the PbS QD/TiO2 system can be a goodcandidate for a PV cell with high efficiency.
ACKNOWLEDGEMENTThe TRXPS study was conducted using the facilities of the Synchrotron Radiation Research
Organization, The University of Tokyo (Proposal No. 2019B7460). A financial support from
MEXT, Japan (KAKENHI 16H06027) is appreciated.
REFERENCES[1] S. Rühle, Sol. Energy 130, 139−147 (2016).
[2] K. Ozawa et al., J. Phys. Chem. C 123, 4388−4395 (2019).
[3] F. Roth et al., Phys. Rev. B 99, 020303(R) (2019).
[4] S. Yamamoto et al., J. Synchrotron Radiat. 21, 352−365 (2014).

Photo-induced valence transition dynamics of SmS studied by time-resolved
x-ray absorption spectroscopy
H. WatanabeA, T. NakamuraA, Y. ShibataA, K. YamagamiB, Y. HirataC, K. IkedaB, Y. ZhangD,
H. WadatiD, K. ImuraE, H. S. SuzukiB, N. K. SatoE, S. KimuraAA
Osaka University, BISSP. The University of Tokyo, CNational Defense Academy,
D
University of Hyogo, ENagoya University
Samarium monosulfide (SmS) is known to show a phase transition (namely BG transition)from a black insulator phase to a valence-fluctuating golden metal phase by applying pressure
and/or elemental substitution (yttrium doping) [1]. Although the research has a long history forabout 50 years, the origin of the BG transition has not been revealed yet. One possibility of theorigin is due to an instability of excitonic states, namely excitonic instability [2]. If so, whenthe exciton states are produced by photo-irradiation, a phase transition like the BG transition is
supposed to occur. So far, we have reported the existence of a metastable state of yttrium-doped
SmS accompanied by high carrier density by photo-irradiation [3]. However, it has not been
revealed that the metastable state is related to the valence transition. In this study, to clarify thevalence change of Sm ions by photo-irradiation, we measured the temporal structure of the Sm
3d5/2 XAS after the photo-irradiation.
The time-resolved pump-probe XAS experiment was performed at BL07LSU of SPring-8
using the H mode (11/29-filling + 1-bunch). The Sm 3d5/2 core level absorption peak at about1080 eV was used to evaluate the ratio between Sm2+ and Sm3+ ions. A Ti:sapphire laser (pulse
width: 50 fs, photon energy: 1.55 eV, reputation rate: 1 kHz) synchronized with the synchrotron
radiation x-ray (pulse width: 50 ps) was used as the pump light.
Figure 1 shows the time evolution of the signal intensity of 1077 eV (Sm2+ 3d5/2 core level)
before and after the one-laser-pulse irradiation at 0 ps, but no significant change was observed
after the photo-irradiation. The same result was obtained at the Sm3+ 3d5/2 peak at 1080 eV.
These suggest that the spectral change due to the one-pulse irradiation was less than theexperimental accuracy of about 1 % estimated from the variation in strength before the pumppulse irradiation (t < 0 ps) as shown in Fig. 1. According to our time-resolved reflectancemeasurements, a 20-% change of the reflectivity was observed just after the photo-irradiation,
but the change was relaxed to less than 1 % within 1 ps. Therefore, no spectral change in theFig. 1. Time development of the peak intensity at
1077 eV before and after photo-irradiation at thetime of 0 ps.

Fig. 2. XAS spectra of the Sm 3d5/2 core level
before (red), just after (green) and 10 minutes
after (blue) the photo-irradiation (PI). The spectra
were normalized by the peak intensity at 1077 eV.

XAS result by one laser pulse isconsistent with our previous timeresolved reflectance measurement.
Next, the time evolution of the Sm
3d5/2 XAS spectrum after many lightpulses irradiation was measured. Figure
2 shows the Sm 3d5/2 core absorption
spectra before (red), immediately after(green) and 10 minutes after (blue) thephoto-irradiation for 1 hour at atemperature of 58 K. The absorption
shape obviously changes by timesuggesting the temporal change of themean valence. Also, combined with notemporal change by one pulse excitation, Fig. 3. Time evolution of the mean valence before andthe valence change by one pulse is too after one-hour laser pulse irradiation (PI). The meansmall to be observed and it is pilled up by valence is obtained by the ratio between the 1077-eV andmany pulses because it does not relax 1080-eV peaks.
completely before the next pulse after 1
ms.
Before the photo-irradiation, not only the peak at 1077 eV (mainly originating from a Sm2+
component) but also the peak at 1080 eV (mainly from Sm3+) was observed suggesting theintermediate valence of Sm ions. From Figure 2, combined with core level calculations, themean valence can be roughly evaluated from the ratio of peak intensity at 1077 eV and 1080eV as shown in Fig. 3. The mean valence before the photo-irradiation was evaluated as 2.40.
Just after the photo-irradiation shown by blue circles, the intensity of the Sm3+ peak increased
compared to that of Sm2+ suggesting the mean valence of Sm increased to 2.51 by many laser
pulses. After 10 minutes, the mean valence of Sm decreased to 2.47, but the mean valence does
not come back to the original value even after 1 hour. The origin of the remained higher valencecannot be explained by a very long relaxation time because surface oxidation also makes avalence change from Sm2+ to Sm3+. However, since the decrease of the mean valence after 10
min is the opposite change to the oxidation, it suggests the relaxation from Sm3+ to Sm2+ without
photo-irradiation. This implies that the photo-excited Sm3+ intermediate state decays to theSm2+ ground state by a very long relaxation time for several 10 min. The detailed relaxation
dynamics is reported in a separate report [4].
REFERENCES[1] K. Matsubayashi et al., J. Phys. Soc. Jpn. 76, 064601 (2007).
[2] T. Mizuno et al., J. Phys. Soc. Jpn. 77, 113704 (2008).
[3] R. Ikeda et al., J. Phys.: Conf. Ser. 1220, 012005 (2019).
[4] H. Watanabe et al., ACTIVITY REPORT OF SYNCHROTRON RADIATION LABORATORY 2019,
ISSP, The University of Tokyo.

Relaxation dynamics of Photo-induced valence transition of SmS
studied by time-resolved x-ray absorption spectroscopy
H. WatanabeA, T. NakamuraA, Y. ShibataA, K. YamagamiB, Y. HirataC, K. IkedaB, Y. ZhangD,
H. WadatiD, K. ImuraE, H. S. SuzukiB, N. K. SatoE, S. KimuraAA
Osaka University, BISSP. The University of Tokyo, CNational Defense Academy,
D
University of Hyogo, ENagoya University
Samarium monosulfide (SmS) is known to show a phase transition (BG transition) from a
black insulator phase to a valence-fluctuating golden metal phase by applying pressure and/or
elemental substitution (yttrium doping) [1]. We have reported the existence of a metastable stateof yttrium-doped SmS accompanied with high carrier density by photo-irradiation [2] and thevalence transition from Sm2+ to Sm3+ could be occurred by photo-irradiation at 58 K by thetime-resolved x-ray absorption spectroscopy (trXAS) [3]. In the trXAS, the valence change of
Sm could not be observed by one-pulse excitation, but after the many pulse irradiation, thevalence finally changes from 2.40 to 2.51. This suggests that the photo-excited metastable state
has a very longer life time than the interval of the pump pulse (1 ms) and it is piled up by themulti-pulse excitation, and finally, the photo-excited state could be observed. The origin of themetastable state is related to the pressure-induced BG transition and recently observed nonlinear conduction phenomena of black-SmS [4].
The origin of the metastable state, as well as the valence transition by the photo-irradiation,
has not been clarified yet. One possibility is the BCS-to-BEC transition of the photo-excited
exciton state, namely “excitonium” [5]. The excitonium is expected to have carriers, so theother experiment to detect the time-dependence of carrier density, band structure, and lattice
constants must be combined with trXAS. In this study, to evaluate the lifetime of the photoexcited state, we measured the relaxation dynamics after multi-pulse excitation.
The time-resolved pump-probe XAS experiment was performed at BL07LSU of SPring-8
using the H mode (11/29-filling + 1-bunch). The Sm 3d5/2 core level absorption peak at about1080 eV was used to evaluate the ratio between Sm2+ and Sm3+ ions. A Ti:sapphire laser (pulse
width: 50 fs, photon energy: 1.55
eV, reputation rate: 1 kHz)synchronized with the synchrotron
radiation x-ray (pulse width: 50 ps)
was used as the pump light.
Figure 1 shows the temporal
structure of the 1080-eV peak after
the 5-min (about 300,000 laser
pulse) laser irradiation. The valenceof the photo-excited state has been
evaluated as about 2.5 [3], which issmaller than that of the golden
phase (2.6-2.7) induced by pressure[6]. So, the consistency between thephoto-excited state and the golden
phase is still unclear but it isobviously different from that of the Fig. 1. Temporal structure of the 1080-eV peak at 52.7 K.
divalent black phase. The photo- The time dependence can be fitted by an exponential decay
excited state might be a new curve with a lifetime of 1400 s.

intermediate state. The decay curve can be fitted by an exponential relaxation with the a lifetimeof 1400 s (about 23 min), i.e., the photo-excited intermediate state has a very long time. To
clarify the origin of the photo-excited state, a further trXAS and other time-resolved
experiments are needed.
REFERENCES[1] K. Matsubayashi et al., J. Phys. Soc. Jpn. 76, 064601 (2007).
[2] R. Ikeda et al., J. Phys.: Conf. Ser. 1220, 012005 (2019).
[3] H.Watanabe et al., ACTIVITY REPORT OF SYNCHROTRON RADIATION LABORATORY 2019,
ISSP, The University of Tokyo.
[4] H. Ando et al., JPS Conf. Proc. 30, 011132 (2020).
[5] B. I. Halperin and T. M. Rice, Rev. Mod. Phys. 40, 755 (1968).
[6] K. Imura et al., JPS Conf. Proc. 30, 011131 (2020).

LOW-TEMPERATURE METHANE OXIDATIVE COUPLING ON
MODIFIED PD MODEL CATALYSTS STUDIED BY OPERANDOAMBIENT-PRESSURE XPS
Takanori Koitaya,† Susumu Yamamoto,‡ Iwao Matsuda,‡ Jun Yoshinobu‡ and Toshihiko
Yokoyama††

Department of Materials Molecular Science, Institute for Molecular Science‡

The Institute for Solid State Physics, The University of Tokyo

Direct observation of heterogeneous catalysts under operation condition is important forunderstanding reaction mechanism and nature of active sites on the catalysts. Various operando
methods have been developed to investigate surface chemistry on heterogeneous catalysts. We
have investigated reactions of inert molecules, such as carbon dioxide and methane, using an
ambient-pressure XPS (AP-XPS) system at SPring-8 soft X-ray beam line BL07LSU [1, 2].
The XPS measurements can be currently performed in the presence of reactant gases up to 2000
Pa using synchrotron radiation or a conventional X-ray source. In the gas environment, thesample can be heated up to around 900 K to investigate thermal reaction on the surface of
catalysts.
In this study, oxidation of methane on the Pd(110) surface was studied by AP-XPS and QMS.
Fig. 1 shows QMS spectra of reaction products and sample temperature of the Pd(110) surface
in the presence of 2.0 mbar oxygen and 5.6 mbar methane. As shown in Fig. 1, reactivity of thePd(110) sample was very low, when the sample was heating from room temperature to 615 K
in the presence of the feed gases. However, the amount of the products increased significantly
after heating to 780 K.
Chemical states of the surface during the reaction were investigated by operando AP-XPS.
Fig. 2 shows AP-XPS spectra measured at sample temperature of 615 K. Before heating to 780
K, the surface of the sample was oxidized, and PdO thin film was formed as indicated by Pd 3d
and O 1s spectra. In addition, peaks of adsorbed species were observed in O 1s and C 1s, which
can be attributed to oxidized carbon species (COx). Since the surface before heating to 780 K
is less reactive, the observed adsorbate is not an important reaction intermediate, but a spectatoror poisoning species. After the activation process, Pd 3d and O 1s spectra shows that the sample
was still (partially) oxidized. In O 1s and C 1s spectra, gas-phase peaks of the produced species(water and CO2) were observed, indicating that this surface is much more reactive. Note thatthe adsorbed COx was not observed after the activation process. This indicates that coverageof the intermediates is negligible during the methane oxidation reaction. Therefore, the ratelimiting step for the methane oxidation is dissociative adsorption of methane. Once theformation of CH3 species occurs, following reactions, including desorption of products, proceed
at much faster rates compared to the initial methane dissociation.

(II)

(I)

Figure 1. QMS spectra of oxidation products (water, CO, CO2) on the Pd(110) surface in thepresence of 2.0 mbar oxygen and 5.6 mbar methane. Sample temperature is also shown in the
figures.

(I)

(II)

Figure 2. AP-XPS spectra measured at region (I) and (II) indicated in Fig. 1. During themeasurement, the Pd(110) sample was kept at 615 K in the presence of 2.0 mbar oxygen and
5.6 mbar methane gases.

REFERENCES[1] Koitaya, T. et al., Top. Catal. 59, 526-531 (2016).
[2] Koitaya, T. et al., e-J. Surf. Sci. Nanotech. 17, 169-178 (2019).

X-ray Spectral Imaging Analysis of Interface States at p+-WSe2/n-MoS2
Hetero Junctions in TFET Device Structure
Naoka Nagamura1,2, Keigo Nakamura3, Hayami Kawamoto3, Naoki Higashitarumi3,
Keishi Akada4, and Kosuke Nagashio3
1

National Institute for Materials Science (NIMS), Tsukuba, Ibaraki, Japan.
PRESTO, Japan Science and Technology Agency, Honcho, Saitama, Japan.
3
Department of Materials Engineering, The University of Tokyo, Hongo, Tokyo, Japan.
4
Institute for Solid State Physics, The University of Tokyo, Kashiwa, Chiba, Japan.
2

Reducing power consumption in semiconductor devices is an imminent challenge, but thepresent field effect transistors (FETs) require at least 60 mV of gate voltage to increase thecurrent by one order of magnitude at room temperature. Tunnel FETs can overcome thislimitation because of the band-to-band tunnelling (BTBT).
Two-dimensional materials are promising for TFETs. Recently, using p+-WSe2 source
doped by charge transfer from a WOx surface oxide layer, we succeeded in observing a clear
BTBT in the stable TFET system of p+-WSe2/n-MoS2[1].
In this study, we systematically study the local bandalignment in the p+-WSe2/MoS2 TFET device structure asshown in Fig. 1 by scanning photoelectron emission
microscopic(SPEM)
measurementsusingathree-dimensional nanoscale electron- spectroscopy
chemical analysis (3D nano-ESCA) system installed at
BL07LSU of SPring-8.
2D materials used in the sample are fabricated by
Fig. 1 Schematic image of a
exfoliation of bulk crystals. WOx side of WSe2 film is p+-WSe2/MoS2 TFET device
transferred onto h-BN on a SiO2/n+-Si substrate after structure
forming WSe2/WOx by O3 annealing. Then, thep+-WSe2/MoS2 heterostructures were fabricated via dry transfer method using PDMS under
the alignment system. Ni/Au was deposited as source/drain electrodes by EB lithography.
Figure 2a shows an intensity
mapping image for the Mo 3d 5/2
peak for the 3L-n-MoS2/p+-WSe2
heterostructure on h-BN. The
position of the heterostructure isclearly identified. Figure 2b
shows the pinpoint core-level
spectra for W 4f 5/2 and 4f 7/2
peaks recorded at points of (i)p+-WSe2/h-BN
and
(ii)
+
3L-n-MoS2/p -WSe2/h-BN. W 4f
peaks for (ii) p+-WSe2 in contact Fig. 2 (a) Photoelectron intensity mapping image for the Mo 3dwith 3L-n-MoS2 shifts to the 5/2 peak for the 3L-n-MoS2/p+-WSe2 heterostructure on h-BN.
higher binding energy side than (b) Pinpoint core-level spectra for W 4f 5/2 and 4f 7/2 peaks
that for (i) p+-WSe2 directly on recorded at points (i) and (ii).
h-BN. When p+-WSe2 is dopedwith electron, EF in p+-WSe2 increases, resulting in the higher binding energy of W 4f peaks.
Therefore, this data supports the p-doping reduction in WSe2[2]. The mapping of the EF
modulation in p+-WSe2 by the top gate is obtained by high-throughput peak fitting analysis
using machine learning techniques[3].

We also worked on chemical characterization of monolayer tin monosulfide (SnS), a
two-dimensional group IV monochalcogenides[4]. The special distribution of different phases,
SnS, Sn2S3, and SnS2, around the SnS single crystal nanosheet grown by the physical vapor
deposition on a mica substrate has been clarified[5].
REFERENCES[1] J. He et al. Adv. Electron. Mater. 4, 1800207 (2018).
[2] K. Nakamura et al., submitted.
[3] T. Matsumura et al. Sci. Technol. Adv. Mater. 20, 733 (2019).
[4] N. Higashitarumi et al. Nature Commun. 11, 2428 (2020).
[5] H. Kawamoto et al., submitted.

SOFT X-RAY ADSORPTION AND EMISSION SPECTROSCOPY OF
THE INTERMEDIATE WATER IN BIOCOMPATIBLE POLLYMERS
Shin-nosuke Nishimura1, Keishi Akada3, Daiki Murakami1,2, Kosuke Yamazoe3, Tomoya
Ueda2, Jun Miyawaki3, Yoshihisa Harada3, Masaru Tanaka1,2
1Institute of Materials Chemistry and Engineering, Kyushu University,
2
Graduate School of Engineering, Kyushu University,
3
Institute for Solid State Physics, The University of TokyoINTRODUCTION
Recently, significant efforts have been applied toward the design of polymer biomaterials
which have central roles in regenerative medicine and tissue engineering application. Water
definitely exists on an interface between material and biotissue, of which hydration stateessentially affects for biocompatibility [1]. Hydrated water around polymer, as represented
by poly(2-methoxyethyl acrylate) (PMEA), is classified into three types, non-freezing water(NFW), intermediate water (IW), free water (FW) in ascending order of the intensity ofinteraction with polymer chain [2]. Among them, the IW has a key role on the affinity ofpolymer with an organism. However, it has not been clarified what it is in a chemical
structure of polymer that works on formation of the IW. As the first step to systematize
relationship between the IW and polymer structure, we evaluated a hydration process on the
PMEA brush depending on humidity and succeeded to detect the IW by soft x-ray
absorption/emission spectroscopy (XAS/XES), whereas origin of the IW is still unclear [3, 4].
In this study, we designed and prepared the cyclic PMEA and linear poly(ethylene glycol)
monomethyl ether (mPEG) brushes to examine effect of a polymer end and polymer species,
respectively (Figure 1). The hydration process on these polymer brushes was evaluated by
XAS/XES measurement using the same method as our previous reports.
EXPERIMENTAL
We prepared the cyclic PMEA andlinear mPEG brushes on gold-coated
SiC substrate by grafting-to method
using sulfur-gold interaction (Figure 1).
Thiol-terminated PMEA at both end
wassynthesized
via
reversible
addition-fragmentation chain transfer(RAFT) polymerization and subsequentaminolysis.
The obtained polymer
was treated with iron(III) chloride
under high-diluted condition, resulting Figure 1. Chemical structure of cyclic PMEA and linear
in an objective cyclic PMEA. The PEG brushes used in this study. These polymer brushes
number average molecular weight (Mn) were prepared by grafting-to method.
and polydispersity index (Ð) of cyclic PMEA were estimated to be 76000 g mol-1 and 1.06
from size exclusion chromatography measurement, respectively. A commercially available
product was used for thiol-terminated mPEG (Mn = 10000 g mol-1, Ð = 1.13). The
gold-cated SiC substrates were soaked in methanol solutions of these polymers. The
substrates were washed with methanol and dried in vacuo to give objective polymer brushes.
Grafting density of the cyclic PMEA and linear was determined to be 0.048 and 0.11 chains
nm-2 by quartz crystal microbalance measurement, respectively.
XAS/XES measurement was performed at BL07LSU HORNET station in SPring-8. The
humid air with controlled humidity was flowed on the samples to change the water contents inthe brush samples

RESULTS AND DISCUSSION
In the XES spectrum of bulk liquid water, two significant peaks are basically observed at ca.
525.6 eV and 526.8 eV corresponding to ordered hydrogen bonds (tetrahedral coordination)and distorted hydrogen bonds, respectively [5]. Figure 2 shows XES spectra of the cyclic
PMEA and linear mPEG brushes under the humidity control. In the low humidity range
below a relative humidity of 65%, broad profiles peaked at the tetrahedrally-coordinatedcomponent dominates along with a sharp peak at ca. 527.1 eV. Above a relative humidity of
75%, increase of the peak at 526.5 eV was observed from the both spectra. We reported that
IW was formed on the linear PMEA brush at a relative humidity of 75% [4]. The hydration
behaviors obtained from this experiment were similar to that of our previous study, indicatingthat the IW existed on the cyclic PMEA and linear PEG brushes. Interestingly, formation of
the IW on the cyclic PMEA brush significantly increased compared with that on the linear
PMEA brush even though these polymer brushes had the same monomer unit and chemicalcomponent. This result suggested that the cyclic PMEA brush formed nano-scaled space
based on its loop structure and promoted adsorption of interfacial water by confinement effect.
There is a possibility that the cyclic PMEA brush shows higher biocompatibility than thelinear PMEA.
In this study, we successfully carried out to trace hydration process of the interfacial water
including IW onto the cyclic PMEA and linear mPEG brushes by XAS/XES measurementwith humidity control. State of polymer chain on the outermost of surface might has a great
effect on structure of the interfacial water. Although this challenging experiment make it
increasingly clear that the origin of IW and the relationship of chemical structure with IW
formation, the knowledge still is not enough to accomplish the ultimate design of biomaterial.
Further investigation will be performed in our future work in order to clarify the essential
feature of IW and the role on biocompatibility.

Figure 2. XES spectra of cyclic PMEA and linear mPEG brushes under the humidity control.

REFERENCES[1] T. Tsuruta, J. Biomater. Sci. Polym. Ed., 2010, 21, 1831-1848.
[2] M. Tanaka et al., Polym. Int., 2000, 49, 1709-1713.
[3] D. Murakami et al., Activity Report of Synchrotron Radiation Laboratory 2018, 2019, 68-69.
[4] D. Murakami et al., Activity Report of Synchrotron Radiation Laboratory 2018, 2019, 70-71.
[5] T. Tokushima et al., Chem. Phys. Lett., 2008, 460, 387-400.

DEVELOPMENT OF TIME-RESOLVED X-RAY PHOTOELECTRONDIFFRACTION FOR THE OBSERVATION OF THE STRUCTURAL
DYNAMICS OF EPITAXIAL SILICENE
Kouichi Hayashi1,2, Artoni Kevin R. Ang1, Koji Kimura1, Yuichiro Fukatsu1,
Susumu Yamamoto3, Iwao Matsuda3, Takahiro Yonezawa4, Antoine Fleurence4,
Yukiko Yamada-Takamura4
1

Department of Physical Science and Engineering, Nagoya Institute of Technology, Nagoya, Japan
Frontier Research Institute for Materials Science, Nagoya Institute of Technology, Nagoya, Japan
3
Institute of Solid State Physics, The University of Tokyo, Kashiwanoha, Chiba, Japan
4
School of Materials Science, Japan Advanced Institute of Science and Technology, Ishikawa, Japan
2

Introduction
Over the past half-century, photoelectron spectroscopy (PES) has become one of the mostimportant experiments in physics, chemistry and material science. Among the related techniques,
X-ray photoelectron diffraction (XPD) has grown into a powerful technique in studying surface
structures. Since the first reported XPD experiments by Siegbahn and Fadley in the 1970’s, XPD has
been used to investigate the structures of surface reconstructions, adsorbates, interfaces and thin films.
Through the years, PES has evolved significantly alongside the rapid developments in electron
spectrometers, synchrotron radiation facilities, laser light sources, and other related technologies.
Typical PES experiments can now resolve nm-resolution spatial information, time-resolved and evenspin-resolved information. Taking advantage of these developments, new dimensions of space-, timeand spin- can incorporated into XPD experiments.
In this work, we demonstrate, for the first time,
time-resolved X-ray photoelectron diffraction (trXPD) of an
epitaxial silicene layer using a time-resolved two-dimensional
angle-resolved time-of-flight (2DARTOF) system. The
evolution of the diffraction rings (DR) and the forward
focusing peaks (FFP) after laser irradiation can be used to
extract atomic resolution structural information about thesurface structure (Fig. 1). The 2DARTOF system in BL07LSU
is well suited for this experiment because of its high energy
and angular resolutions, and most importantly, its
Fig. 1. Schematic diagram of thesimultaneously recording of the photoemission intensity as atime-resolved XPD experiment.
function of θx, θy, and E.
Experiment and data processing
The experiments were performed using an epitaxial silicene sample grown on a ZrB2 thin film [1].
It has been shown that this epitaxial silicene layer has minimal electronic interactions with thesubstrate, and that the silicene layer buckled easily, resulting in a mixed sp2/sp3 hybridization that canbe tuned to control its electronic properties [2,3]. XPS studies have shown that the Si atoms occupy atleast 3 chemically different sites, and that the surface structure consists of a planar surface structurewith an adatom [3,4]. This sample was chosen because of its relatively simple surface structure, andthe current interest in silicene for next generation electronic device applications.
The sample was prepared in situ by the spontaneous segregation during the annealing of theZrB2/Si(111) substrate at 800°C [1]. LEED and the Si 2p core-level spectra confirms the formation ofa well-defined silicene layer.
The photoemission experiments were done using a photon energy of hν = 300 eV. The ARTOF
detector was set to simultaneously measure the photoemission intensity with an acceptance angle of±13°, within the energy range 97-103 eV. To obtain a larger section of the hologram pattern, thesample’s orientation was scanned along the [1120] direction. For the time-resolved experiments, a 1.5
eV, 208 kHz laser was used.

Compared to other electron detector
systems used in XPD, the 2DARTOF
system has a major advantage in that it cansimultaneously measure I(θx, θy, E) (Fig.
2a). To extract the angle-integrated Si 2p
spectra, the raw data is integrated along
both the θx and θy directions. To get the
XPD spectra, the raw data is integrated
within a small energy region centredaround the Si 2p binding energy (Fig. 2c).
However, this will contain XPD signal
from Si atoms at all sites. To separate the
XPD patterns from the Si atoms occupying
different sites, peak fitting of the spectra ateach (θx, θy) pixel is done [5].
The extracted XPD patterns are thennormalized by a background image Fig. 2. a) Schematic diagram of the raw data obtained by theobtained by integrating the raw 3D data 2DARTOF system, b) a typical Si 2p spectra from a (θx, θy)within a narrow energy region in the low pixel, and c) a raw 2D cut taken at the Si 2p binding energy.
binding energy side.
Results
Figure 3a shows the “planar” structuralmodel of epitaxial silicene showing the 3
distinct sites occupied by Si atoms. From thepeak fitting of the angle integrated Si 2p
spectra (Fig. 3b), the 2 main peak components
corresponding to the Si atom at the SiA and SiB
sites were resolved. The simulated andexperimental XPD patterns extracted for thesepeak components are shown in Fig. 3c) and3d), where the DR and FFPs in the simulation
are well resolved in the experiment.
Epitaxial silicene consists mostly of
in-plane scatterers, and thus the XPD patterns
are dominated by these DRs. In XPD patternof SiB, a strong FFP at θ~48° is observed,
which corresponds to the direction of the SiC
atom from the SiB emitter. This confirms thatthe 2DARTOF system can be a viable system
for XPD measurements.
Fig. 3. a) “Planar” model for epitaxial silicene, b) the Si
Conclusion
2p core level spectra measured from the sample, andIn this work, we have demonstrated for simulated and experimental XPD patterns from the c) Si A
the first time that the 2DARTOF system is and d) SiB component.
well-suited in XPD experiments. The
simultaneous measurement of I(θx, θy, E), makes acquisition of XPD patterns faster, opening up the
possibility of time-resolved XPD measurements. In our future work, we will obtain the trXPD patternsof epitaxial silicene in the pump-probe scheme.
REFERENCES[1] A. Fleurence, et al., Phys. Rev. Lett. 108, 245501 (2012).
[2] R. Friedlein and Y. Yamada-Takamura. J. Phys. Condens. Matter 27, 203201 (2015).
[3] C.C. Lee, et al., Phys. Rev. B, 90, 075422 (2014).
[4] R. Friedlein, et al., J. Chem. Phys. 140, 184704 (2014).
[5] K. Tsuitsui, et al., Nano Lett. 17, 7533 (2017).

TIME-RESOLVED X-RAY PHOTOELECTRON DIFFRACTIONSTUDY OF EPITAXIAL SILICENE
Kouichi Hayashi1,2, Artoni Kevin R. Ang1, Koji Kimura1, Yuichiro Fukatsu1,
Susumu Yamamoto3, Iwao Matsuda3, Takahiro Yonezawa4, Antoine Fleurence4,
Yukiko Yamada-Takamura4
1

Department of Physical Science and Engineering, Nagoya Institute of Technology, Nagoya, Japan
Frontier Research Institute for Materials Science, Nagoya Institute of Technology, Nagoya, Japan
3
Institute of Solid State Physics, The University of Tokyo, Kashiwanoha, Chiba, Japan
4
School of Materials Science, Japan Advanced Institute of Science and Technology, Ishikawa, Japan
2

Introduction
Silicene is a monolayer of Si atoms arranged in a honeycomb lattice that exhibits massless Diracfermions, similar to its carbon based counterpart in graphene. The development of silicene based
devices has seen significant interest recently because of the expected remarkable electronic properties,
stronger spin-orbit coupling of Si, and the ease of adapting such technologies into the already Si-based
manufacturing infrastructure. Until recently, silicene could only be synthesized on conductive
substrates, resulting in electronic interactions with the substrate. It was recently demonstrated thatsilicene with minimal substrate interactions can be grown on a ZrB2/Si(111) substrate through thesurface segregation of Si atoms[1]. This silicene layer was found to buckle easily, resulting in sp 2/sp3
hybridization that can tailored to tune its electronic properties [1,2]. Photoemission studies haveshown that the Si atoms occupy at least 3 chemically different sites on the silicene layer (Fig. 1a) [1,3].
In this study, we investigate the local structures of epitaxial silicene and the structural dynamics
induced by laser irradiation.
Experiment
The sample was prepared in situ by the spontaneous segregation during the annealing of theZrB2/Si(111) substrate at 800°C [1]. The ZrB2/Si(111) substrate was prepared elsewhere, and degassedin the prior to sample preparation. The silicene surface was confirmed by LEED and the core-level
spectra.
The photoemission experiments were done using a photon energy of hν = 300 eV. The ARTOF
detector was set to simultaneously measure the photoemission intensity with an acceptance angle of±13°, within the energy range 97-103 eV. To
obtain a larger section of the hologram
pattern, the sample’s orientation was scannedalong the [1120] direction. For thetime-resolved experiments, a 1.5 eV, 208 kHz
laser was used.
Results
The LEED and Si 2p spectra confirmed
the formation of a well-defined silicene layer.
From the peak fitting of the Si 2p spectra (Fig.
1b), the 2 main peak components of silicene
were resolved. These 2 peaks correspond to
the Si atoms occupying the SiA and SiB sites
in Fig. 1a [2,3]. The XPD patterns of these 2
peak components were extracted by peakfitting [5], and are shown in the righthemispheres of Fig. 1c and 1d. The
corresponding simulations are shown in theleft hemispheres. Some diffraction rings (DR)and forward focusing peaks (FFP) in the Fig. 1. a) “Planar” model for epitaxial silicene, b) the Si
simulations are well reproduced in the 2p core level spectra measured from the sample, andsimulated and experimental XPD patterns from the c) Si A
experiment.
and d) SiB component.

In this sample, most of the neighboring
scatterers are in-plane, thus the XPD patterns
are dominated by these DRs. Meanwhile, theXPD pattern of SiB shows a strong FFP at
θ~48°, which corresponds to the direction of
the Si atom at the SiC site from the emitter at
SiB.
In the time-resolved experiments, we
focused on this FFP at θ~48° to observe anychanges in the surface structure. At -100 ns,
the FFP is observed as a strong peak (marked
with the black circle in Fig. 2a). At later delay
times (Figs. 2b, 2c, 2d), the FFP is first seen
decreasing in peak intensity and shifting tolower θ (0.1 and 0.5 ns), before shifting tohigher θ and recovering its intensity (1.0 ns).
At 1.0 ns, the shift to higher polar angles
corresponds to an in-plane expansion of thesilicene layer, which could be explained bylaser-induced thermal expansion.
Meanwhile, at 0.1 and 0.5 ns, the decreasein FFP peak intensity suggests an intermediate
structure that occurs before the onset ofthermal equilibrium between the photoexcitedelectrons and phonons. This could be due tobond softening due to the increased
anti-bonding character of the excited charge
distribution. At higher laser fluence, it has
been reported that this can even result in bondbreaking and the formation of disordered state,a non-thermal melting of the surface [6,7].
Fig. 2. The time-resolved XPD patterns of the SiB
While the details of the structural changes
component taken at various delay times: a) -100ns, b) 0.1
observed in silicene is still not clear, this workns, c) 0.5 ns and d) 1.0 ns, and d) the line profiles taken
has demonstrated that time-resolved XPD
across the FFP.
experiments are now feasible and can be used
to probe the structural dynamics of surfaces. Analysis of the evolution of the diffraction rings andforward focusing peaks in the XPD patterns can provide element sensitive, atomic resolution structure
information about surfaces. Further experiments and first principles calculations are needed tocompletely understand the evolution of the epitaxial silicene structure after laser-induced heating.
REFERENCES[1] A. Fleurence, et al., Phys. Rev. Lett. 108, 245501 (2012).
[2] R. Friedlein and Y. Yamada-Takamura. J. Phys. Condens. Matter 27, 203201 (2015).
[3] C.C. Lee, et al., Phys. Rev. B, 90, 075422 (2014).
[4] R. Friedlein, et al., J. Chem. Phys. 140, 184704 (2014).
[5] K. Tsuitsui, et al., Nano Lett. 17, 7533 (2017).
[6] K.J. Gaffney, et al., Phys. Rev. Lett. 95, 125701 (2005).
[7] M. Harb et al. Phys. Rev. Lett., 100, 155504 (2008).

ELECTRIC STRUCTURE OF WATER IN THE VICINITY OF MODELORGANIC SURFACES BY X-RAY EMISSION SPECTROSCOPY
Tomohiro Hayashi
Department of Materials Science and Engineering, School of Materials and Chemical Technology
Tokyo Institute of Technology
JST-PRESTO

Introduction
Self-assembled
monolayers (SAMs)
have been employed
as model organic
surfaces since the1980s (Figure 1). Justby changing theterminal groups of themolecules, we caneasily control thephysicochemical
properties of thesurfaces. In a field of Figure 1 illustration of self-assembled monolayers (SAMs) ofbiomaterial
and alkanethiols on gold.
biosensing, SAMs areoften employed as scaffolding layers for protein and cells or passivating layer preventing nonspecific adsorption of non-target molecules. 1 Recently, water-mediated force has been foundto play an important role to govern the interaction of materials with protein molecules and cells.
2-5
However, the properties of the interfacial water, in particular, hydrogen bonding state andthe mechanism underlying the water-mediated force are still matters of intense debate. 1 In thiswork, we attempt to answer these questions by using X-ray emission spectroscopy (XES),
which provides a detailed picture of the hydrogen bonding state of water.

Experimental
In this work, we employed a measurement
setup shown in Fig. 2. SAMs were formed on
the backside of the window. In this work, weused 8 types of SAMs. They are hydrophobic(metyl-terminated:
C8),hydrophilic(hydroxy-terminated: OH), positively charged(trimethylamine-terminated:
TMA),
negatively charged (sulfuric acid-terminated:SA), and non-fouling [oligoethylene glycol:
EG and sulfobetaine: SB]. The humidity ischanged to vary the amount of water
condensed on the SAMs. The energy of theincident X-ray was fixed at 550 eV.

Figure 2 schematic illustration of themeasurement system田先生と の共同研究)
吸着水の厚さ : 8 n m
Results and discussion
We confirmed that the thickness of thewater strongly depended on the humidity
in the chamber. From the quartz crystal
microbalance (QCM), the thickness ofwater at relative humidity at 28 degree in
Celcius is about 0.5 to 1 nm.
Figure 3 shows XE spectra of water
adsorbed on the SAMs. For watermolecules in the vicinity of OH- and C8SAMs, the tetrahedral-coordinated water(ice-like water) is concentrated only near
the interface (with a film thickness of lessthan 0.5 nm) and steeply changes to thestructure of bulk water. In contrast, it was
found that water molecules with hydrogenbonding in the distorted tetrahedral
coordination was dominant for nonfouling SAMs (Fig. 3). Therefore, thedistorted tetrahedral-coordinated waterlayer is key to the anti-adhesion properties,
in agreement with our previous findings.

T. To k u sh im a , e t a l. C h em Ph ysREFERENCES
Lett 2 0 0 8 , 4 6 0 , 3 8 7 .

(1) Hayashi, T.; Hara, M., Nonfouling selfassembled monolayers: mechanisms
underlying protein and cell resistance.
Current Physical Chemistry 2011, 1, 90Figure 3 X-ray emission spectra of water
98.
(2) Chang, R.; Asatyas, S.; Lkhamsuren, adsorbed on SAMs.
G.; Hirohara, M.; Mondarte, E. A. Q.;
Suthiwanich, K.; Sekine, T.; Hayashi, T., Water near bioinert self-assembled monolayers. Polym.
J. (Tokyo, Jpn.) 2018, 50, 563-571.
(3) Kanayama, N.; Sekine, T.; Ozasa, K.; Kishi, S.; Nyu, T.; Hayashi, T.; Maeda, M., TerminalSpecific Interaction between Double-Stranded DNA Layers: Colloidal Dispersion Behavior andSurface Force. Langmuir 2016, 32, 13296-13304.
(4) Sekine, T.; Tanaka, Y.; Sato, C.; Tanaka, M.; Hayashi, T., Evaluation of Factors To
Determine Platelet Compatibility by Using Self-Assembled Monolayers with a Chemical
Gradient. Langmuir 2015, 31, 7100-5.
(5) Tanaka, M.; Hayashi, T.; Morita, S., The roles of water molecules at the biointerface of
medical polymers. Polym. J. (Tokyo, Jpn.) 2013, 45, 701-710.

基に大き く 依存する
面体水素結合状態から ひずんだ状態)の割合が低い。

ま っ て おり 、 よ り 詳細な スペク ト ルの門家と 連携し たい。

MEASUREMENT OF THE THICKNESS DEPENDENCE OF THE
MAGNET-OPTICAL CONSTANTS OF METAL FE USING FAST
POLARIZATION SWITCHING
Yasuyuki Hirata1, Keisuke Ikeda1, Kohei Yamagami1, Yujun Zhang2, Hiroki Wadati2,
Souliman El Moussaoui1, Iwao Matsuda1
1

Synchrotron Radiation Laboratory, Institute for Solid State Physics, University of Tokyo
2

Graduate School of Material Science, University of Hyogo

Magneto-optical effect is quite important not only for its use in devices such as
magneto-optical storage devices, but also as a probe for unraveling the spin and orbital statesof electrons in magnetic materials, therefore researchers of magnetism have measured
magneto-optical spectrum at various wavelengths. In particular, the measurement of resonant
X-ray magnetic circular dichroism (R-XMCD) and resonant X-ray magneto-optical Kerr
effect (R-XMOKE) using soft X-ray are powerful measurement methods that can selectively
observe the magnetic behavior of each 3d-transition-metal element, which is a main player ofmagnetism. However, despite the fact that the fine structure of the spectrum is important foranalysis, R-XMCD for materials with a small magnetic signal has a difficulty on improving
accuracy because it requires subtraction between two absorption/reflection spectra for theincident light with different polarizations. R-MOKE measurement is free from this problem,
but it has an experimental limitation that it takes a long time for measurement because it
requires an ellipsometry scan for each measurement point.
Taking advantage of the characteristics of the BL07LSU light source where high-speed
polarization switching is available [1,2], from the 2016A period we have performed the
R-MOKE measurement using the optical delay modulation method [3], and measured themagneto-optical constant of the Fe thin film (30 nm thickness) [4]. With this method, the realpart (corresponding to Kerr rotation angle) and the imaginary part (corresponding toellipticity) of the off-diagonal component of the permittivity can be simultaneously measured
independently in a short time, without Kramers-Kronig transformation analysis. Although theobtained permittivity of Fe thin film was generally in good agreement with the result of thefirst-principles calculation, a dip structure at Fe L3 absorption edge in the measured spectrum
could not be reproduced from the calculation. It is very important to distinguishexperimentally whether the dip structure is intrinsic in Fe metal, comes from interference withthe thin film [5], or has another origin. In this study, we performed an R-MOKE measurement
on Fe thin films with different film thickness by the optical delay modulation method using
polarization switching, and clarified the film thickness dependence of the magneto-optical
spectrum. The dip structure was hardly reproduced, which indicates it is not intrinsic in themagneto-optical spectrum of Fe metal. No systematic film thickness dependence wasobserved in the permittivity spectrum.
We prepared two Fe thin films with different thickness (30 nm thick and 15 nm thick) on
MgO substrate, and capped with Ru layer (5 nm thick) to prevent surface oxidization. The
reflection spectroscopy measurement with grazing geometry (incident angle of 80 degree) was
performed at Fe L3 and L2 edge under the in-plane magnetic field of 0.3 T applied bysuperconducting coil magnet. The polarization of the incident light was continuously
modulated between left circular polarization and right circular polarization with the frequencyof 13 Hz. The reflected light was analyzed with ellipsometer at both p-polarization ands-polarization geometry. The 13 Hz component of analyzed signal corresponded to ellipticity,and the 26 Hz component of analyzed signal corresponded to Kerr rotation angle.

Fig. 1: Kerr rotation angle, ellipticity, and off-diagonal permittivity  of Fe film with 30
nm thick (left panel) and Fe film with 15 nm thick (right panel). Solid line (dashed line)
is obtained from p-polarization (s-polarization) reflection measurement, respectively.

Figure 1 shows the measured spectra of Kerr rotation angle, ellipticity, and off-diagonal
permittivity of Fe film (30 nm thick) and Fe film (15 nm thick). Off-diagonal permittivity wasderived from Kerr rotation angle, ellipticity, and refraction index of Fe [6]. Under bothp-polarization and s-polarization reflection geometry, obtained off-diagonal permittivity
spectra show good agreement with theoretical calculation. We cannot find any systematic
dependency on thickness in the spectra of the 30 nm thick film and the 15 nm thick film, anda large dip structure observed in previous experiment has almost disappeared. Our resultclearly shows the dip structure is neither real magnetic signal of Fe nor originating from an
optical interference within the film. The capping layers of the samples are different between
previous measurement (Cu 2 nm thick and Ta 2 nm thick) and ours (Ru 5 nm thick), but it isless likely that the dip structure comes from the capping layer because Cu and Ta do not haveany optical structure at the energy around Fe L-edge. One possible origin of the dip structure
is surface oxidization of the sample which may be caused by x-ray radiation.
In conclusion, we have observed the thickness dependence of off-diagonal permittivity of Fe
thin film at Fe L-edge by the optical delay modulation method. No systematic dependency onthickness was found out, and the dip structure in spectrum observed in previous experiment
was shown not to be intrinsic.
REFERENCES[1] S. Yamamoto et al., J. Synchrotron Radiat. 21, 352 (2014).
[2] I. Matsuda et al., Nucl. Instrum. Methods Phys. Res., 767, 296 (2014).
[3] K. Sato, Jpn. J. Appl. Phys. 20, 2403 (1981).
[4] Y. Kubota et al., Phys. Rev. B 96, 214417 (2017).
[5] S. Valencia et al., Physica B 345, 189 (2004).
[6] H.-C. Mertins et al., J. Magn. Magn. Mater. 240, 451 (2002).

Creation of multi-dimensional operando photoelectron spectroscopy (I)Selection of industry-government-academia collaborative model sample
Hirokazu Fukidome1, Takamasa Kamogawa1, Naoka Nagamura2,3, Masaharu Oshima4,
Masato Kotsugi5, and Issei Watanabe6
1Research Institute of Electrical Communication, Tohoku University, Sendai, Miyagi, Japan.
2
National Institute for Materials Science (NIMS), Tsukuba, Ibaraki, Japan.
3
PRESTO, Japan Science and Technology Agency, Honcho, Saitama, Japan.
4
Institute for Solid State Physics, The University of Tokyo, Kashiwa, Chiba, Japan.
5Tokyo University of Science, Katsushika, Tokyo, Japan.
6
National Institute of Information and Communications Technology (NICT), Koganei, Tokyo, Japan.

Virus and environmental issues
threaten the sustainability of mankind.
To balance the suppression of virus
infection and active socio-economic
activities,
weshoulddevelopinformation technologies enabling ahigh telepresence communications. For
thispurpose,thewireless
communication using THz frequencies
is needed, indicating 100 times
speeding up, compared to the current Fig. 1 A schematic view of the high telepresence
wireless communication, so-called 5G.
communication
Two-dimensional (2D) materials suchas graphene and ultrathin semiconductor devices
using AlGaN/GaN [1] and InGaAs heterointerfaces
is promsing for THz devices for high speed wirelesscommunication, 6G and more. This due to excellent
electronic properties, such as giant carrier mobility
and ultrahigh saturation velocity, and suppression of
so-called short-channel effects arising from theultrathinness of the channles. With the advance ofminiaturization and integration of devices, theimportance of interfaces further increases. Especially,
2D materials are sensitive to interface environment,so many environmental parameters drive theelectronic and chemical properties of 2D materials.
Moreover, better spatial and time resolution isrequiredfortheadvanceddevice analysis. Then, we attempt to realize
multi-dimensional
operando
photoelectron
spectroscopy using machine learning analysis
Fig. 2 Angle resolved Photoelectron
techniques in the viewpoint of Measurement Informatic.
At first, we performed depth profiling of III-V spectroscopy of Ga 3d peak for thecompound semiconductor heteroepitaxial layers by GaAs heteroepitaxial layers.
ARPES measurements. Maximum Entropy Method isapplied to enable sub nm order analysis (Fig. 2)[2].
In-plane 2D spectral mapping is also carried out for InP high electron mobility transistor(HEMT) device structures. Present in-plane spatial resolution of 3D nano-ESCA is ~70 nm,

which is deteriorated bymechanicalvibrations,
thermal drift, charging
effect, and so on. To
overcome the limit of spatial
resolution and make itpossible to observe objects
in the size of several nms,
we adopt sparse modelling
approach for the spectral
intensity mapping datasets.
Fig. 3(a) is a low-resolution Fig. 3 (a) Photoelectron intensity mapping image for the Au 4f peak forimage taken by coarse steps the InP-HEMT device structure. Low-resolution image by downin short measurement time. sampling. (b) Super-resolution image using sparce modeling.
Using
this
kind
of
down-sampling mappings, super-resolution image processing by sparse modelling [3] wasperformed as shown in Fig. 2(b). Effective spatial resolution seems to be improved by thismachine learning approach.
Our final goal is to achieve below 10 nm-resolution with the aid of machine learning, suchas super-resolution. Then, we will plan to execute industry-government-academia
collaboration that utilize operando 3D nano-ESCA to develop novel next-generation
communication devices necessary for balancing the suppression of virus infection and active
socio-economic activities.

REFERENCES[1] H. Fukidome et al. Sci. Rep.8, 13268 (2018).
[2] K. Horiba et al. Rev. Sci. Instrum. 82, 113701 (2011).
[3] T. Kato et al. Neurocomputing 240, 115 (2017).

OPERAND MEASUREMENT OF ALL SOLID-STATE LI-ION
BATTERYS BY PHOTOELECTRON MICROSPECTROSCOPY
Eiji Hosonoa,b,c, Wenxiong Zhangd, Daisuke Asakurab,c, Naoka Nagamurae,
Masaharu Oshimaf, Yoshihisa Haradac,d, faNational Institute of Advanced Industrial Science and Technology (AIST), Global Zero
Emission Research Center, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japanb
National Institute of Advanced Industrial Science and Technology (AIST), Research Institutefor Energy Conservation, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japanc
AIST-UTokyo Advanced Operando-Measurement Technology Open Innovation Laboratory,
5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8565, Japand
Institute for Solid State Physics (ISSP), The University of Tokyo,
5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8581, Japane
National Institute for Materials Science, Research Center for Advanced Measurement andCharacterization, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japanf
Synchrotron Radiation Research Organization, The University of Tokyo, 7-3-1
Hongo, Bunkyo-ku, Tokyo, 113-8656, Japan

Development of high-performance lithium-ion battery (LIB) for stationary type and vehicle isan important research topic. The total development of LIB is very difficult, because
performances such as energy density, power performance, cycle performance, and safety
property are in the trade-off relationships. Thus, comprehensive researches and developments
are highly important to development of LIB. Detailed understanding of the redox reactions by
Li-insertion/extraction, structural change, electronic state change for the materials in LIBs is
important to develop innovative materials for LIBs. In these days, cutting-edge analyses
methods are necessary in addition to the innovative materials development.

Figure 1 XPS images of LiCoO2

To truly understand LIBs, synchrotron radiation soft X-ray spectroscopy is a usefultechnique with elemental selectivity, valence selectivity and orbital selectivity. We have been
studying the charge-discharge mechanisms of LIBs based on the electronic-structure analysis
by soft X-ray spectroscopy. In addition, operando measurement is very important because
LIB is a high voltage device. Even deconstruction of LIB in the glovebox filled by inert Argas, the potential control of active material is difficult. Operando measurement system in soft
X-ray beamline is our core-technique. We reported the operando soft x-ray emission
spectroscopy under the charge-discharge operation [1,2].
Furthermore, microspectroscopy is another important point because the redox reaction bycharge-discharge reaction influenced by the position like a facet of an electrode material andthe interfaces between the electrode materials and electrolytes. Spectroscopy with spatial
resolution is of particular importance.
We reported operando photoemission spectromicroscopy of single crystalline Li4Ti5O12 (ananode material) [2] and LiCoO2 (an cathode materials) [3] by 3DnanoESCA [4] withall-solid-state LIB. The soft X-ray photoemission spectromicroscopy experiments have been
conducted at BL07LSU of SPring-8.
Here, we measured single crystalline LiCoO2 cathode material with clear facet. Figure 1
shows images of XPS mapping image of single crystalline LiCoO2. From the clear image ofeach facet, resonant XPS was conducted.
ACKNOWLEDGEMENT
This work was conducted under the international joint research program for innovative energy
technology by METI, Japan.
REFERENCES[1] D. Asakura, E. Hosono, H. Niwa, H. Kiuchi, J. Miyawaki, Y. Nanba, M. Okubo, H.
Matsuda, H.S. Zhou, M. Oshima, and Y. Harada, “Operando Soft X-ray Emission
Spectroscopy of LiMn2O4 Thin Film Involving Li-ion Extraction/insertion Reaction”,
Electrochem. Commun., 50, 93 (2015).
[2] D. Asakura, Y. Nanba, M. Okubo, H. Niwa, H. Kiuchi, J.Miyawaki, M. Oshima, E.
Hosono, Y. Harada, “Operando soft X-ray emission spectroscopy of the Fe2O3 anode toobserve the conversion reaction”, Physical Chemistry Chemical Physics, 21, 26351 (2019).
[3] K. Akada, T. Sudayama, D. Asakura, H. Kitaura, N. Nagamura, K. Horiba, M. Oshima, E.
Hosono, Y. Harada, “Operando measurement of single crystalline Li4Ti5O12 withoctahedral-like morphology by microscopic X-ray photoelectron spectroscopy”, J. Electron
Spectrosc. Relat. Phenom., 233, 64 (2019).
[4] K. Akada, T. Sudayama, D. Asakura, H. Kitaura, N. Nagamura, K. Horiba, M. Oshima, E.
Hosono, Y. Harada, “Microscopic photoelectron analysis of single crystalline LiCoO2
particles during the charge-discharge in an all solid-state lithium ion battery”, Scientific
Reports, 9, 12452 (2019).
[5] K. Horiba, Y. Nakamura, N. Nagamura, S. Toyoda, H. Kumigashira, M. Oshima, K.
Amemiya, Y. Senba, H. Ohashi, Scanning photoelectron microscope for nanoscale
three-dimensional spatial-resolved electron spectroscopy for chemical analysis. Rev. Sci.
Instrum. 82, 113701 (2011).

MAPPING MEASURMENT OF SINGLE CRYSALLINE LITHIUMMETAL OXIDE BY ALL SOLID-STATE LI-ION BATTERYS
Eiji Hosonoa,b,c, Wenxiong Zhangd, Daisuke Asakurab,c, Naoka Nagamurae,
Masaharu Oshimaf, Yoshihisa Haradac,d, faNational Institute of Advanced Industrial Science and Technology (AIST), Global Zero
Emission Research Center, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japanb
National Institute of Advanced Industrial Science and Technology (AIST), Research Institutefor Energy Conservation, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japanc
AIST-UTokyo Advanced Operando-Measurement Technology Open Innovation Laboratory,
5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8565, Japand
Institute for Solid State Physics (ISSP), The University of Tokyo,
5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8581, Japane
National Institute for Materials Science, Research Center for Advanced Measurement andCharacterization, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japanf
Synchrotron Radiation Research Organization, The University of Tokyo, 7-3-1
Hongo, Bunkyo-ku, Tokyo, 113-8656, Japan

Many researchers have studied development of clean energy devices for the realization of alow-carbon society. There is a great need to develop innovative clean energy devices. In
particular, high performance LIBs are important not only for electric vehicles, but also for
power source of next-generation semiconductor devices for "Beyond 5G", which will enablehigh-speed, high-capacity data communication in the IoT society.
In response to these high societal needs, LIBs with higher performance than the current state
are required. In recent years, research and development of all-solid-state LIBs have been
focused for innovative improvement of the performances of LIBs. It is necessary to establish
design guidelines based on fundamental science because a lot of technical issues remainunresolved in the all-solid-state LIBs. Thus, understanding the charge-discharge mechanismof LIBs is important to improve their properties.
In addition, operando measurement under the control of potential is important for detail andprecise understanding because LIBs are high voltage device. Even deconstruction of LIB in
the glovebox filled by inert Ar gas, the potential maintaining of active material is difficult.
Recently Akada and Hosono et al. have successfully introduced operando soft X-ray
photoelectron spectroscopy as a novel method to understand the LIBs during
Li-intercalation/deintercalation process in LIB cathode and anode materials [1, 2]. These areresults of photoelectron microscopy with spatially resolved information by 3D nanoESCA
station at BL07LSU of SPring-8 [3]. Microspectroscopy is another important point in addition
to operando measurement because the redox reaction by charge-discharge reaction affected by
the position like a facet of an electrode material and the interfaces between the electrodematerials and solid-state electrolytes. Spectroscopy with spatial resolution is important
techniques.
Figure 1 shows the intensity mapping of Ni 3p and Mn 3s of LixNi0.5Mn1.5O4. Here, lithium
ratio is controlled by the cell using liquid electrolyte. We obtained intensity mapping. The
next step is operando measurement by all-solid-state LIBs. We will create oprerando
measurement system and conduct XPS mapping measurement.

Ni3p

Ni3p

Mn3s

Mn3s

Figure 1 Intensity mapping of Ni 3p and Mn 3s of LixNi0.5Mn1.5O4.

ACKNOWLEDGEMENT
This work is partially conducted on the basis of the International joint research program forinnovative energy technology by Ministry of Economy, Trade and Industry, Japan.
REFERENCES[1] K. Akada, T. Sudayama, D. Asakura, H. Kitaura, N. Nagamura, K. Horiba, M. Oshima, E.
Hosono, Y. Harada, “Operando measurement of single crystalline Li4Ti5O12 withoctahedral-like morphology by microscopic X-ray photoelectron spectroscopy”, J. Electron
Spectrosc. Relat. Phenom., 233, 64 (2019).
[2] K. Akada, T. Sudayama, D. Asakura, H. Kitaura, N. Nagamura, K. Horiba, M. Oshima, E.
Hosono, Y. Harada, “Microscopic photoelectron analysis of single crystalline LiCoO2
particles during the charge-discharge in an all solid-state lithium ion battery”, Scientific
Reports, 9, 12452 (2019).
[3] K. Horiba, Y. Nakamura, N. Nagamura, S. Toyoda, H. Kumigashira, M. Oshima, K.
Amemiya, Y. Senba, H. Ohashi, Scanning photoelectron microscope for nanoscale
three-dimensional spatial-resolved electron spectroscopy for chemical analysis. Rev. Sci.
Instrum. 82, 113701 (2011).

Resonant inelastic soft x-ray diffraction of nanoparticles
J. Miyawaki1, T. Yachi2, S. Maki2, K. Kanie2 and Y. Harada1
1

2

Institute for Solid State Physics, The University of Tokyo
Institute of Multidisciplinary Research for Advanced Materials, Tohoku University

The excitation energy dependence of soft X-ray emission spectra (XES) was first measured
for TiN in 1988 [1]. Four years later, Ma et al. proposed that the excitation energy
dependence could be interpreted by the theory of resonant inelastic X-ray scattering (RIXS),
and band dispersions of electronic state were observed [2]. This indicates that the absorptionand emission-processes are described by a single scattering theory, and the momentum isconserved during that process. In other words, when a core electron with momentum k is
excited to the conduction band and an electron at the valence band is decayed to the core hole,the emissions from the different cores could interfere with each other, and Bragg-like
diffraction results in the emissions of the same momentum k. These facts suggest that a moregeneral Bragg’s law, qin = qout + kh − ke + G is valid, in addition to the energy conservation
law in RIXS, hνin = hνout + Eh – Ee, where hνin (qin) and hνout (qout) are the energy (momentum)of the incident and scattered X-ray, Eh (kh) and Ee (ke) are the energy (momentum) of the holein the valence band and the electron in the conduction band at the final state, and G is thereciprocal vector of the crystal. When the excitation and scattered energies are the same, themomentum conservation law gives qin = qout + G, which is widely accepted as Brass’s law.
The more general Bragg’s law qin = qout + kh − ke + G (the momentum conservation law in
RIXS) has been validated by the RIXS studies of the band dispersions in SiC [3], graphite,diamond [4], Si [5], and LiBC [6]. Recent observations of the dispersions of orbitons,
magnons, and phonons also confirmed the validity of the principle [7].
In the above discussion, the Bragg diffraction and RIXS were treated as independent
processes. However, if the Bragg diffraction and RIXS occur simultaneously, the intensity of
RIXS is expected to increase assisted by the Bragg diffraction. We named this method
resonant inelastic X-ray diffraction (RIXD). In the case of nanoparticles, the momentum
dispersion of the elementary excitations is small, and the diffraction from the nanoparticles,that is, small angle X-ray scattering (SAXS), is expected to increase the intensity of RIXS.
Therefore, we measured soft X-ray SAXS of FePt nanoparticles by a RIXS spectrometer anddiscussed the feasibility of RIXD by comparing the SAXS profile obtained by the elastic
scattering and the momentum dependence of RIXS.
The measurement of SAXS and RIXD were conducted at HORNET end-station ofSPring-8 BL07LUS using θ-2θ scan system. In this scan system, RIXS spectra can bemeasured at an arbitrary scattering angle 2θ in the range between 45° and 135°, and SAXS
and RIXD profiles can obtained from momentum dependence of the intensity of the elastic
and inelastic peaks, respectively. The measured sample was FePt nanoparticles. The average
particle size and standard deviation were 2.7 nm and 0.37 nm, respectively.
Figure 1 shows the RIXS spectra of the FePt nanoparticle measured at 708.1 eV,
which is the peak top of Fe L3-edge X-ray absorption spectra. A SAXS profile was obtainedby integrating the intensity of the elastic peak (Fig. 2). A peak was observed in the SAXS
profile at around θ=31°. In the hard X-ray SAXS, the SAXS profile is observed at small
angles (typically 0.1–10°), but in the case of soft X-ray SAXS, SAXS profile is observed atlarger angle, since the wave length of the soft X-ray is longer than that of hard X-ray. The
obtained profile is well reproduced by a simulation with the particle size of 2.73 nm andstandard deviation of 0.37 nm, and thus the obtained q-dependence of the elastic peak was
confirmed to be SAXS. Furthermore, the momentum dependence of the inelastic scattering
indicated by a1, a2, a3, and a123 in Fig. 1 was calculated, and the increase of the intensity ofa3 peak was found at the same scattering angle of the SAXS peak, indicating the observationof RIXD. In the future, we will measure the particle size and incident energy dependences ofvarious samples to further establish RIXD and elucidate its mechanism.

Fig. 1 RIXS spectra of FePt nanoparticle measured at 708.1 eV and 2θ=66°.

Fig. 2 The momentum dependence of the elastic peak intensity in RIXS spectra of FePt nanoparticle.
Red circle and blue line correspond to the experimental result and simulation, respectively.

REFERENCES[1] J. E. Rubensson, et al., Phys. Rev. Lett. 60, 1759 (1988).
[2] Y. Ma, et al., Phys. Rev. Lett. 69, 2598 (1992).
[3] J. Luning, et al., Phys. Rev. B 56, 13147 (1997).
[4] A. V. Sokolov, et al., J. Phys. Condes. Matter 15, 2081 (2003).
[5] A. V. Sokolov, et al., J. Electron Spectrosc. 137, 591 (2004).
[6] P. F. Karimov, et al., J. Phys. Condens. Matter 16, 5137 (2004).
[7] L. J. P. Ament, et al., Rev. Mod. Phys. 83, 705 (2011).

Development of X-ray Polarization Precession Spectroscopy
J. Miyawaki, Y. Hirata†, K. Yamagami‡, I. Matsuda, and Y. Harada
Institute for Solid State Physics, The University of Tokyo
Magnonics is a next-generation technology, that utilizes a collective excitation of electron
spin (magnon or spin wave), and is an alternative technology to electronics, which utilizes
electrons for information transfer and computation. Compared with electron- and spin-based
electronics and spintronics, magnonics does not suffer from Joule heat. Magnon has longer
mean free path, which is advantageous for long-distance information transfer. Magnon has awave nature, which offers a possibility of quantum computation. Thus, researchers are trying
to use high-frequency terahertz magnons to speed up magnonic devices and observe magnons
injected to magnonics materials to understand the operating mechanism of devices [1–6].
For the magnon observation, ferromagnetic resonance (FMR), Brillouin lightscattering (BLS), inelastic neutron scattering (INS), spin-polarized electron energy loss
spectroscopy (SPEELS), and time-resolved spectroscopy have mainly been used. FMR andBLS observe magnons only at q≈0 nm−1 and cannot observe propagating magnons at
q≠0 nm−1. INS and SPEELS can observe at larger q, but INS cannot observe higher energy
magnons and require large sample volume, and SPEELS is surface sensitive. Therefore, INS
and SPEELS is not suitable for operando experiments. Thus, a new technique to observe
propagating magnons in operando condition has been explored. In this study, we developed
X-ray polarization precession spectroscopy (XPPS), which is a new method to observe
dispersion of magnons by X-ray with high energy resolution to understand the properties ofmagnon in devices.
XPPS was proposed as a new technique to measure the frequency of spin waves byobserving a rotational/angular Doppler effect in the polarization of scattered X-ray induced byspin waves in the sample via magnetic linear dichroism [7]. Since magnetic linear dichroismis employed in XPPS, resonant condition is required to measure XPPS signal. The
measurement principle and experimental setup are shown in Fig. 1(a). The scattered X-ray isirradiated to a multilayer polarizer, the intensity of the reflected X-ray is spatially resolved by
CCD, and the period of the rotation of the polarization can be detected. The energy resolutionof XPPS reaches 50 GHz (0.2 meV), which is very high energy resolution for X-ray based
spectroscopy to obtain the energy of magnons. Thus, XPPS has a promising technique for
operando experiment of magnonic devices.
XPPS experiments were conducted at free-port end-station of SPring-8 BL07LSU
using superconducting magnet chamber equipped with a polarization analyser (Fig. 2). The
linear vertical polarization was used for the incident X-ray, and the multilayer polarizer of thepolarization analyser was positioned orthogonal to the electronic vector of the incident X-ray.
The position dependence of the reflected X-ray intensity was measured by CCD.
Figure 3 shows a XPPS result of Y3Fe5O12 (YIG) and Pt. 705.0 eV and 709.6 eV
correspond pre-edge and the peak-top of Fe L3-edge X-ray absorption spectra of YIG. YIG
showed the difference between pre- and post-edge of Fe L3-edge, while Pt, which is
non-magnetic, showed no difference. In YIG, the difference spectra showed the intensity
modulation with a period of ~100 μm. This period corresponds the magnon energy of~5.5 meV, which matches the energy obtained by inelastic neutron scatterings [8]. Thus, we
succeeded in detecting XPPS signal and extracting the energy of magnon.
†Present Address: Department of Applied Physics, National Defense Academy‡Present

Address: Quantum Materials Science Unit, Okinawa Institute of Science and Technology
Graduate University

(b)

(a)

FIG. 1. (a) The measurement principle and (b)experimental setup of XPPS. Linearly polarized
X-ray of a resonant energy is irradiated to asample that possesses a spin wave of frequency
Ω, and the precessional motion of frequency 2Ω
is induced to the polarization vector of thescattered X-ray. The multilayer polarizer reflects
the vertically polarized component of thescattered X-ray, and thus spatially resolved
reflection intensity gives the intensity modulation
due to the precession in the scattered X-ray.

FIG. 2. XPPS of Pt and YIG at Fe L3-edge.
Spatially resolved reflection intensity of thescattered X-ray from the multilayer polarizer
was measured at pre- (red) and post-edge(blue) of Fe L3-edge. Since the intensity
profiles by the pre-edge excitation has noresonant effect, the signal can be regarded as a
background affected by the surface of thesample and the polarizer.

REFERENCES[1] S. Neusser et al., Adv. Mater. 21, 2927 (2009).
[2] Y. Kajiwara et al., Nature 464, 262 (2010).
[3] V. V. Kruglyak et al., J. Phys. D: Appl. Phys. 43, 264001 (2010).
[4] A. V. Chumak et al., Nat. Phys. 11, 453 (2015).
[5] H. Yu et al., Nat. Comm. 7, 11255 (2016).
[6] C. Du et al., Science 357, 195 (2017).
[7] R. Röhlsberger, Phys. Rev. Lett. 112, 117205 (2014).
[8] J. S. Plant, J. Phys. C: solid State Phys. 10 4805 (1977).

Resonant Inelastic Soft X-ray Scattering of Y3Fe5O12
J. Miyawaki and Y. Harada
Institute for Solid State Physics, The University of Tokyo

Further development of the information society requires innovation in information technology,
and faster, compact, and low energy-consumption devices have been pursued. One of the next
generation technologies to realize this goal is magnonics, which utilizes magnons (spinwaves), that is, a collective excitation of spins instead of electrons for the transmission andprocessing of information. Since no electronic current is induced when the magnon
propagates through the magnetic material, there is no energy dissipation due to Joule heating.
There is a very wide frequency of magnons known, and if higher frequency magnons can beused, ultra-fast devices could be created. Currently, fundamental research is in progress torealize actual devices. Higher-frequency terahertz magnons have been investigated to speed
up the devices, and the magnons injected in the magnonic materials has been investigated tounderstand the behaviour of the magnons in the device and efficiently utilize magnons.
Magnons have been observed by using ferromagnetic resonance, Brillouin light scattering,inelastic neutron scattering, spin-polarized electron energy loss spectroscopy, andtime-resolved spectroscopy. However, these experimental methods are not suitable forobserving high-energy magnons at larger q, and the development of a new method to observea propagating magnons is urgently needed. In this study, we focus on X-ray spectroscopy forthe observation of the high-frequency propagating magnons and apply resonant inelastic soft
X-ray scattering (SX-RIXS) to magnonic materials to elucidate the behaviour of thehigh-energy magnon.
A Y3Fe5O12(YIG)(111) single crystal was measured, which is very promising for
magnonics due to its very low damping and long mean free path of magnon. X-ray absorption
spectra (XAS) and RIXS were measured at HORNET end-station of SPring-8 BL07LSU. All
the measurements were performed at room temperature. XAS was measured in partial
fluorescence yield mode using a silicon drift detector, and Fe L3-edge inverse partial
fluorescence yield (IPFY) XAS was obtained from O K emission. The energy resolution of
RIXS at Fe L-edge was set at 175 meV. A momentum-resolved RIXS was obtained by θ-2θscan of 2θ=45–135° and 9° step. During the θ-2θ scan, an elastic peak was measured at each2θ from Au with σ polarization. For RIXS of YIG, π polarization was used.
Fe L3-edge XAS of YIG is shown in Fig. 1(a). The spectral shape of XAS is good
agreement of YIG, having two characters of Fe3+ in octahedral (Oh) and tetrahedral (Td)symmetry [1,2]. Figure 1(b) shows the incident photon energy dependence of RIXS spectra of
YIG at θ=45° and 2θ=90°. In addition to dd excitations at ~1.3 eV and ~1.8 eV, which mainly
originate from Oh Fe3+, dd excitations around 2.5 eV were observed, which would be assigned
for Td Fe3+. The peak width at around loss energy of 0 eV is clearly larger than the energy
resolution, indicating that there is a contribution of inelastic scattering. For the
momentum-resolved RIXS, #7 (710.1 eV) was chosen since the contribution of inelastic
scattering was largest among the measured energies. Figure 2 shows RIXS spectra near theelastic peak obtained by θ-2θ scan. The peak at each 2θ was analyzed by a peak fitting of two
gaussian profiles: One is the elastic peak, ant the other is for inelastic scattering. The
experimental results (red circles) were well reproduced by the two gaussian profiles. Since theenergy resolution was set at 175 meV, it was difficult to identify low energy magnons below
100 meV. The inelastic peaks (green lines) at ~110 meV showed a weak dispersion. Theenergy scale of the inelastic peaks ranges from ~110 meV to ~130 meV. Inelastic neutron
scattering measurements and a theoretical calculation revealed that YIG possessed magnon
dispersions up to 100 meV [3,4]. The energy scale of the observed dispersion is higher thanthat of magnons of YIG, indicating that the observed inelastic scattering would be assigned tobimagnon. The identification of the bimagnon requires a complete picture of the magnetic
excitations in YIG. Further analysis will be conducted to understand RIXS spectra and thedispersion of the inelastic peak near the elastic peak.

FIG. 1. Fe L3-edge (a) XAS and (b)RIXS spectra of YIG(111) single
crystal. The numbers labeled in (a)indicate the incident photon energies
used to measure RIXS spectra in (b).

FIG. 2. Fe L3-edge RIXS spectra of
YIG measured at different θ-2θ angles.
Red circles are experimental results,
and black, green, and blue lines
correspond to the fitting results ofelastic peaks, inelastic peaks, and sumof two peaks and background. Angles
labeled on the spectra indicate 2θ
angle.

REFERENCES[1] F. M. F. de Groot et al., J. Phys. Chem. B 109, 20761 (2005).
[2] H. B. Vasili et al., Phys. Rev. B 96, 014433 (2017).
[3] J. S. Plant, J. Phys. C: Solid State Phys. 10, 4805 (1977).
[4] A. J. Princep et al., npj Quantum Materials 2, 63 (2017).

Development of Coherent Soft X-ray Diffraction Imaging Technique for
Observation of Ultra-fast Magnetic Dynamics
Yuichi Yamasaki1,2,3, Kohei Yamamoto4,5, Yuichi Yokoyama1,6, Yasuyuki Hirata4,
Hiroki Wadati4,7, and Taka-hisa Arima3,81

Research and Services Division of Materials Data and Integrated System (MaDIS),
National Institute for Materials Science (NIMS)
2PRESTO, Japan Science and Technology Agency (JST)3

4
5

RIKEN Center for Emergent Matter Science (CEMS)

The Institute for Solid State Physics, University of Tokyo

Department of Materials Molecular Science, Institute for Molecular Science
6
7
8Japan Synchrotron Radiation Institute (JASRI)

Graduate School of Material Science, University of Hyogo

Department of Advanced Materials Science, University of Tokyo

Introduction:
In recent years, topological magnetic structures, such as magnetic skyrmions, have been
actively studied. Magnetic skyrmions, nanometer-scale vortex-like magnetic structures, canbe controlled by external stimuli, such as electric field and pressure, and thus are expected to
be a promising candidate for next-generation spintronics device materials. To understand the
dynamics of magnetic skyrmions, it is important to observe them at high spatial and temporal
resolutions. Resonant soft X-ray scattering is also known to be a suitable technique forobserving such magnetic skyrmion dynamics because of its high sensitivity in detecting spinpolarization [1-3]. Therefore, in the present study, we have developed a diffractometer ofsmall-angle resonant soft x-ray magnetic scattering to observe topological magnetic texturesin the undulator beamline BL07LSU, SPring-8. The apparatus is equipped with a vacuum
chamber having a background pressure of < 1×10-5 Pa, a CCD camera (2048x2048 pixel,
Princeton Instrument), a direct beam catcher, and a sample holder. To observe transmitted soft
X-ray scattering, we prepared thin plate sample thinned by FIB method or magnetic thin films
fabricated on Si3N4 substrates.
Results:
We tried to measure resonant soft X-ray small-angle
scattering from magnetic multilayer films andmultiferroic BiFeO3 lamellas. The magnetic films
were fabricated with thickness of several tens
nano-meters and with different compositions,
Ta5[Pt3CoTa2]10, Ta5[Pt3Co1.2Ta2]10 , [Co/Pt]15, and[Co/Pt]6. The thickness of the samples were
sufficiently thin for soft X-rays to penetrate, andabsorption measurements confirmed the presence ofthe sample. A pinhole with a diameter of several
micro-meters was placed on the thin film sample toirradiate a circularly shaped soft X-ray to the sample.
Figure 1 shows resonant soft x-ray diffraction image

Figure 1: Diffraction patternfrom Co/Pt multilayer thin film.

from [Co/Pt]6 observed at the Co L-edge resonance energy. However, the scattering intensity
was weak and the diffraction pattern was not sufficiently accurate to reconstruct the real space
image from the obtained coherent diffraction pattern. This is probably due to the small
perpendicular magnetization component in the magnetic multilayer film. The other samples
were damaged during the experiment and no significant data could be obtained.
The multiferroic iron oxide BiFeO3 is known to have a ferroelectric polarization, which
gives rise a long-period cycloidal magnetic structure with a wave-length of 62 nm. The
competition between ferromagnetic interaction and the Dzyaloshinskii-Moriya interactions
between the Fe ions results in the non-collinear magnetic structure. We performed
small-angle scattering experiments at the L-absorption edge of Fe on the lamella samples toobserve the magnetic scattering derived from thecycloid magnetic structure. Figure 2 shows diffraction
patterns from the lamella sample of BiFeO3. Magnetic
scattering was observed with superimposing on theconcentric diffraction pattern from pinhole. In theprevious study, six magnetic scatters were observed
for the (110) surface sample, whereas in the present
experiment, magnetic scattering was found to bebroad in the azimuthal direction. It is assumed that themagnetic structure may be different due to the
influence of FIB processing. To confirm whether themagnetic scattering observed in the lamella sample isintrinsic or not, a follow-up experiment comparing
with other FIB processed samples is required in the
Figure 2: Diffraction pattern
future.
from BiFeO3 lamella sample.
Conclusion:
In this study, we developed the equipment for resonant soft x-ray small angle scattering andsucceeded in observing the magnetic scattering from the magnetic thin films and multiferroic BiFeO3
lamella sample. As a next step, we aim to observe temporal variations of the magnetic structure in real
space by combining the coherent soft x-ray diffraction imaging technique with time-resolved
pump-probe measurement.

References[1] “Dynamical Process of Skyrmion-Helical Magnetic Transformation in Chiral-Lattice Magnet FeGe
as Probed by Small-Angle Resonant Soft X-ray Scattering”, Y. Yamasaki, D. Morikawa, T. Honda,
H. Nakao, Y. Murakami, N. Kanazawa, M. Kawasaki, T. Arima, and Y. Tokura, Phys. Rev. B 92,
220421(R) (2015)[2] “Coherent Resonant Soft X-ray Scattering Study of Magnetic Textures in FeGe”, Victor Ukleev, Y.
Yamasaki, Daisuke Morikawa, Naoya Kanazawa, Yoshihiro Okamura, Hironori Nakao, Yoshinori
Tokura, Taka-hisa Arima, Quantum Beam Science. 2 [1] 3 (2018)[3] “Element-specific soft x-ray spectroscopy, scattering, and imaging studies of the skyrmion-hosting
compound Co8Zn8Mn4”, V. Ukleev, Y. Yamasaki, D. Morikawa, K. Karube, K. Shibata, Y.
Tokunaga, Y. Okamura, K. Amemiya, M. Valvidares, H. Nakao, Y. Taguchi, Y. Tokura, T. Arima,
Physical Review B. 99 [14] 144408 (2019)

REVERSIBLE AND IRREVERSIBLE ANIONIC REDOX FOR
HIGH-CAPACITY BATTERY MATERIALS
Naoaki Yabuuchi1 and Daisuke Asakura2
1

Department of Chemistry and Life Science, Yokohama National University
Research Institute for Energy Conservation, National Institute of Advanced Industrial Science andTechnology

2

Voltage / V

To realize the sustainable energy development, the electrification of automobiles has
progressed in the past several years. This movement reduces the dependence on fossil fuels
used for internal combustion engine technology together with accelerating the use ofrenewable energy, including solar, wind, and biomass. In the past three decades, technologyof rechargeable lithium batteries, i.e., lithium-ion batteries, has been highly sophisticated, andcurrently Ni-based layered materials dominate the market of lithium-ion batteries for electricvehicles. However, further increase in energy density of lithium-ion batteries is required toextend cruising distances of electric vehicles. In the past few years, many lithium-excess
metal oxides with layered and disordered rocksalt structures, Li2MnO3-based electrodematerials have been studied as potential high capacity electrode materials. However, Mn ionsin Li2MnO3 (Li1.33Mn0.67O2) cannot be oxidized beyond the tetravalent limit, and therefore,
anionic species, oxide ions, donate electrons on Li extraction. Nevertheless, low reversibilityof anionic redox for pure Li2MnO3 is experimentally observed, leading to insufficient
reversibility as electrode materials. To increase in the reversibility of anionic redox, thesubstitution of metal ions with higher ionic characters with oxide ions, for instance, Nb5+
(Li3NbO4) and Ti4+ (Li2TiO3), has been proposed. For the Li-excess metal oxides, the charge
compensation process on Li extraction is achieved by electron donation from oxide ions, but
detailed reaction mechanisms remain as a debatable subject, e.g., dimerization of oxygen andformation of holes in oxygen.
In this study, a binary system of Li2TiO3–LiMnO2 is examined as electrode materials for Li
storage applications, and its reaction mechanism is studied by resonant inelastic X-ray
scattering (RIXS) study. In this binary system, nanosized Li7/6Ti1/3Mn1/2O2 has been preparedby mechanical milling from a mixtureof Li2TiO3 and LiMnO2.[1] TEM
observation reveals that particle size is
reduced to 5 – 20 nm, and these
Li7/6Ti4+1/3Mn3+1/2O2
1 -5
5.0
nanosize particles are agglomerated
for each other, forming secondary
3.0
particles with diameters of 50 – 200
nm
after
milling.
Nanosize
1.0
10 mA g-1 at 25 oC
Li7/6Ti1/3Mn1/2O2
delivers
large
reversible capacities in Li cells, and an
Li7/6Mn4+1/3Mn3+1/2O2
1
5
2
5.0
initial discharge capacity reaches 295
mA h g-1 even at room temperature
3.0(Figure 1). Charge compensation
processesfornanosize
1.0
Li7/6–yTi1/3Mn1/2O2
were
further
examined by O K-edge RIXS study
0
100
200
300
400
(Figure 2a). From the data of Mn-1
Capacity / mAh g
K-edge and L-edge XAS study, Mn
oxidation from a trivalent to Fig. 1. Charge/discharge curves of nanosize
tetravalent state is evidenced on Li7/6Ti1/3Mn1/2O2 at room temperature. The data of
4+3+charge for the slope region of 3 – 4 V Li7/6Mn 1/3Mn 1/2O2 is also shown for comparison.
[1], but no oxidation of Mn ions is
st

th

th

st

nd

found at the plateau region. O K-edge RIXS spectra clearly reveals that a new peak isobserved after charge at 8 eV, which suggests that charge compensation is achieved by the
donation of electron from oxide ions at the plateau region, and the formation of holes byelectrochemical oxidation.[2]To further clarify the role of Ti ions on anionic redox reversibility, a binary system of
Li2MnO3-LiMnO2 is also prepared and tested as electrode materials. Li7/6Mn4+1/3Mn3+1/2O2
was also successfully prepared by mechanical milling from the mixture of Li 2MnO3 and
LiMnO2. This phase is metastable containing Mn4+ and Mn3+ ions and cannot be obtained byaconventional
calcination
process.
Charge/discharge
curvesofnanosize
Li7/6Mn4+1/3Mn3+1/2O2 in a Li cell are also shown in Figure 1. An observed initial discharge
capacity reaches 350 mA h g-1,
which is larger than that of
Li7/6Ti1/3Mn1/2O2. Nevertheless,(a)cyclability is insufficient aselectrode materials, and areversible capacity of ca. 100 mA
h g-1 is lost on initial five cycles.
Much better capacity retention isachieved for Li7/6Ti1/3Mn1/2O2.
Reversibility of anionic redox for
Li7/6Mn4+1/3Mn3+1/2O2 has been
also examined by RIXS study as
shown in Figure 2b. A similar(b)peak with Li7/6Ti1/3Mn1/2O2 at 8eV
isobserved
forLi7/6Mn4+1/3Mn3+1/2O2 after charge.
However, intensity of the peak ismuchhigherfortheLi7/6Ti1/3Mn1/2O2, indicating thatoxygen redox chemistry is clearlystable for the Ti-substituted
sample. This result directly
supports the role of Ti4+ ions,which are effective to stabilize Fig. 2. Comparison of changes in RIXS spectra for nanosize
anionic redox associated with the Li7/6Ti1/3Mn1/2O2
and
Li7/6Mn4+1/3Mn3+1/2O2
oncharge/discharge
processes.
high ionic character and donationof electron to oxide ions.
These findings open a new direction to design high-capacity positive electrode materials
with highly reversible electrochemistry on the basis of anionic redox. Further optimization ofchemistries, compositions, and particle sizes on these electrode materials contributes thedevelopment of advanced high-energy rechargeable Li batteries in the future.
REFERENCES[1] Y. Kobayashi, M. Sawamura, S. Kondo, M. Harada, Y. Noda, M. Nakayama, S. Kobayakawa, W.
Zhao 1, A. Nakao, A.Yasui, H. B.Rajendra, K.Yamanaka, T.Ohta, and chi, Materials Today,
in-press[2] T. Sudayama, K. Uehara, T. Mukai, D. Asakura, X.-M. Shi, A. Tsuchimoto, B. Mortemard de
Boisse, T. Shimada, E. Watanabe, Y. Harada, M. Nakayama, M. Okubo and A. Yamada, Energy &Environmental Sciences, in-press

ELECTRONIC STATE OF INTERMEDIATE WATER IN
BIOCOMPATIBLE POLYMERS INVESTIGATED BY XAS AND XES
Koichi Ozaki1, Jumpei Yahiro1, Masaru Nakada1, Masahiro Kunisu1,
Kosuke Yamazoe2, Jun Miyawaki2 and Yoshihisa Harada2
1
Toray Research Center, Inc.,
2
The Institute for Solid State Physics, The University of Tokyo
Introduction
Poly (vinyl-pyrrolidone) (PVP) and Poly (ethylene-glycol) (PEG) have been widely used forvarious biomedical applications because of their excellent biocompatibility. Biocompatibility
is one of the important factors for polymers used for biomedical applications. It is consideredthat adsorbed water on the polymer surface contributes to expression of biocompatibility.
Water molecules existing near the biocompatible polymer could be classified into three types;
free water, intermediate water and non-freezing water [1]. In particular, the intermediate water
has an important role to control the biocompatibility of polymers. However, the local
structural and dynamical properties of intermediate water are not well understood yet. It has
been reported that water molecules confined in a polyelectrolyte brush form characteristic
local hydrogen-bonded structure [2]. Based on this result, water molecules in the close
vicinity of a biocompatible polymer are expected to form a specific hydrogen-bonded
structure to express biocompatibility. In order to reveal the role of hydrogen-bonded structureof water on biocompatibility, it is necessary to obtain accurate information about thehydrogen-bonded structure of those water molecules. In this study, we examine thehydrogen-bonded structure of water molecules hydrating PVP and PEG molecules in detailusing X-ray absorption spectroscopy (XAS) and X-ray emission spectroscopy (XES).
Experimental
The PVP (Mw=4000-5000) and PEG (Mw=3350 and 20000) were purchased from
Sigma-Aldrich. PVP and PEG aqueous solutions at different concentrations were prepared asmeasurement samples. Absolutely dry PVP (dry-PVP) and PEG (dry-PEG) were alsomeasured. O 1s XAS and XES measurements were performed at the BL07LSU HORNET
station in SPring-8. The measurement of the polymer aqueous solution was performed using asolution cell under ambient pressure by circulating the aqueous solution. The dry samples
were mounted on a Cu sample plate and measured in a high vacuum condition.
Results and Discussion
Figure 1 shows the O 1s XES spectra of dry-PEG(Mw=20000) measured at various scan rates of the Cu
plate. XES measurements were performed at theexcitation energy of 537 eV. A broad peak around 527
eV was observed for the XES spectra with the scan rateof 0.16 and 0 m/s. The shape of the XES spectrachanged gradually with increasing the scan rate,
indicating the presence of X-ray induced degradation of
the polymer during the XES measurement at lower
scan rate. Since the shapes of the XES spectra above
the 5.0 m/s scan rate were almost the same, we set thescan rate of 5.0 m /s to obtain the XES spectra of thedry-PVP and dry-PEG samples.
Figure 2(a) depicts O 1s XES spectra of PVP aqueous
Figure 1. O 1s XES spectra ofsolution at the excitation energy of 550 eV. XES dry-PEG with various scan rate: 7.2,
measurements were performed with a flow rate of 5.0
5.0, 3.2, 1.6, 0.16 and 0 (m /s).
ml/s to avoid the accumulation of radiation damage.

Two 1b1-derived peaks (1b1’ and 1b1’’) were observed in the XES spectra of pure water [3].
The raw XES spectra for aqueous solutions contain X-ray emission signals from oxygens bothin water molecules and in PVP. Subtracting the polymer component, we can obtain the signal
from water molecules. In order to remove the PVP polymer contribution from the raw XES
spectra, we focused on the energy range below 522 eV. In this region, the spectral intensity
seems to well reflect the relative contributions of water and polymer. Thus each contribution
was estimated by performing least-squares fit of the XES spectrum of dry-PVP and pure waterin the energy range below 522 eV. The results after subtraction of the polymer contribution
are shown in the bottom of Figure 2(a). It was found that the 1b1’ peak is dominant in theextracted spectra. The 1b1’-dominant peak profile with a clear shoulder structure around 524.5
eV (possibly assigned as 3a1 derived peak) obtained for the 65 wt% PVP solution is quite
similar to the previously obtained spectrum for water molecules confined in a polyelectrolyte
brush [2]. Therefore water molecules in the 65 wt% PVP solution may exist almost in theform of non-freezing and intermediate water hydrating the PVP polymer. Water molecules inthe 30 wt% PVP solution consist of strong 1b1’ and weak 1b1’’ profiles, which can be
reproduced by the mixture of bulk liquid water as well as non-freezing and intermediate water
hydrating the PVP polymer. Detailed analysis of the XES fraction for the water hydrating PVP
may have rich information
about the hydrogen-bonded
structure of intermediatewater and its relation to thebiocompatibility of PVP.
Figure 2(b) shows the O 1s
XES
spectraofPEG(Mw=3350) aqueous solutions.
Broad peaks at ~526.5 eV and~528.0 eV were observed for
the XES spectra of dry-PEG.
In this case, in addition to theenergy range below 522 eV,
the energy range above 526
eV was also used for thefitting to estimate the PEG
contribution. The results after Figure 2. O 1s XES spectra of (a) PVP and (b) PEG aqueous
removal of the polymer solutions. The spectra of pure water and dried polymer powder
contribution are shown in the were also shown for comparison. Bottom: Spectra with thebottom of Figure 2(b). It polymer contribution subtracted.
should be noted that the extracted XES profiles are close to the spectrum of pure water, incontrast to the PVP results. The present study raises the possibility that a statichydrogen-bonded structure of intermediate water is different between PVP and PEG aqueous
solutions. In contrast, dynamical hydrogen-bonded structure of intermediate water hydrating
PVP and PEG aqueous solutions is similar [4, 5]. Consequently, to understand the behavior ofintermediate water, it is important to understand both the dynamical and the statichydrogen-bonded structures. Several questions remain unanswered at present yet. Futurestudies on the current topic are recommended.
References[1] T. Tsuruta, J. Biomater. Sci. 21 (2010) 1831.
[2] K. Yamazoe et al., Langmuir 33 (2017) 3954.
[3] T. Tokushima et al., Chem. Phys. Lett. 460 (2008) 387.
[4] T. Takei et al., Anal. Sci.26 (2010) 337.
[5] M. Nakada et al., Materialia 12 (2020) 100743.

Present status of the development of time-resolved ambient pressure
X-ray photoelectron spectroscopy system at SPring-8 BL07LSU

Susumu Yamamoto* and Iwao Matsuda
The Institute for Solid State Physics, The University of Tokyo* Present affiliation: Institute of Multidisciplinary Research for Advanced Materials,
Tohoku University

Introduction
Operando spectroscopy, observation of working catalysts under reaction conditions, isessential for our understanding of reaction mechanisms as well as the rational development
guidelines for novel catalysts. Ambient pressure X-ray photoelectron spectroscopy (AP-XPS)
is one of the most powerful operando spectroscopies, which permits the investigation of theelectronic and chemical states of adsorbates and catalysts under gas atmospheres atnear-ambient pressure. However, the typical temporal resolution of AP-XPS has been limited
to ms, and thus the dynamic processes faster than ms could not be studied. A pump-probe
measurement can overcome the limitation of the temporal resolution, in which a sample ispumped by ultrafast optical laser and probed by X-ray. At SPring-8 BL07LSU, we developed
time-resolved X-ray photoelectron spectroscopy (tr-XPS) in ultrahigh vacuum in a
pump-probe fashion [1, 2]. The temporal resolution of tr-XPS is ~50 ps, which is determinedby the pulse width of probe X-ray.
In this study, we aimed to develop a time-resolved ambient pressure X-ray photoelectron
spectroscopy (tr-APXPS) by combining AP-XPS with tr-XPS. tr-APXPS permits monitoring
the time evolution of photocatalytic reactions in real time and under reaction conditions; all
the players in photocatalytic reactions such as photo-excited carriers, adsorbates, substrates
can be investigated by tr-APXPS.

Experimental
The experiments were performed at the soft X-ray undulator beamline BL07LSU at
SPring-8 [3]. AP-XPS experiments were carried out using an AP-XPS apparatus thatcombines a differentially pumped electron analyzer (SPECS, PHOIBOS 150 NAP) with anambient-pressure gas cell [4]. A one-dimensional delay-line detector (DLD) is adopted as adetector of the electron analyzer.
Femtosecond (fs) laser systems were installed at SPring-8 BL07LSU. The fs laser pulses
were synchronized to synchrotron radiation X-ray pulses, and the delay time of fs laser pumppulse relative to X-ray probe pulse can be controlled. There were two different fs laser
systems; high pulse energy and low repetition rate (mJ, 1 kHz) and low pulse energy and highrepetition rate (J, 208 kHz). Optical parametric amplifier (OPA) was installed in both laser
systems, which can change the wavelength of fs laser. In the present study, high pulse energy
and low repetition rate (mJ, 1 kHz) laser system was used.

Results and discussion
In the development of tr-APXPS, we reported the installation of a soft X-ray chopper at
SPring-8 BL07LSU. The soft X-ray chopper was operated at 1 kHz, and reduced therepetition rate of probe X-ray (208 kHz in the H-mode operation of SPring-8) to 1 kHz tomatch that of pump laser. Consequently, time-resolved XPS measurements became possible
with the same experimental setup as in conventional static XPS measurements. In addition,
soft X-ray chopper blocks all the X-ray pulses that are not used in pump-probe measurements.
This decreases unnecessary burden on the sample and the detector. Thus, the photon flux canbe increased by opening slits in X-ray beamline to improve measurement efficiency.
Test tr-APXPS measurements were performed on the naturally oxidized p-type Si(111)(SiO2/p-Si(111)) surface in 1 mbar CO2. The wavelength of pump laser was changed from
800 nm to 400 nm. This was because the small pre-pulses in 800 nm can induce the surface
photovoltage effect of Si. The amplitude of the pre-pulses was much decreased by frequency
doubling to 400 nm. In addition to the energy shift of Si 2p core level due to surface
photovoltage effect, we could detect the energy shift of CO2 gas phase peak in O 1s core level.
The energy shift of gas phase peak in AP-XPS reflects the change of the sample work
function. Notably, the energy shift was different between Si 2p and O 1s core levels. The
present results demonstrate that of tr-APXPS can provide a wealth of information inphoto-excited systems and will be very suited to study various light-induced reactions
including photocatalysts and plasmonic chemical reactions.

REFERENCES
[1]
[2]
[3]
[4]

K. Ozawa, S. Yamamoto, et al., e-J. Surf. Sci. Nanotechnol. 17, 130-147 (2019).
M. Ogawa, S. Yamamoto, et al., Rev. Sci. Instrum. 83, 023109 (2012).
S. Yamamoto et al., J. Synchrotron Rad. 21, 352-365 (2014).
T. Koitaya, S. Yamamoto, et al., e-J. Surf. Sci. Nanotechnol. 17, 169-178 (2019).

Development of time-resolved X-ray absorption and photoelectron
spectroscopy system using time-of-flight electron analyzer
Susumu Yamamoto1,*, Rei Hobara2, Kenichi Ozawa3, and Iwao Matsuda1
1

The Institute for Solid State Physics, The University of Tokyo
2
3

Department of Physics, The University of Tokyo

Department of Chemistry, Tokyo Institute of Technology* Present affiliation: Institute of Multidisciplinary Research for Advanced Materials,
Tohoku University

Introduction
Understanding the dynamics of photoexcited carriers is essential for improving theefficiency of light-energy conversion materials such as photocatalysts and solar cells.
Photoexcited carriers (electrons/holes) generated by light absorption move between an
occupied state and an unoccupied state. Therefore, it is important to observe both occupied
and unoccupied states in order to get the whole picture of the photoexcited carriers dynamics.
However, there has been no experimental method that allows the observation of both
occupied and unoccupied states dynamics.
In this study, we aim to develop a picosecond time-resolved X-ray absorption/photoelectron
spectroscopy (XPS/XAS) system using a time-of-flight (TOF) electron analyzer, and track the
temporal changes of electronic structures in both occupied and unoccupied states. In addition,
the TOF electron analyzer has a unique feature that all kinetic energy electrons can bedetected and energy-resolved by the detector. In the case of a hemispherical electron analyzer,
only electrons with the fixed kinetic energy determined by electrostatic lenses are detected by
the detector. Taking advantage of this unique feature of TOF electron analyzer, we canmeasure XAS spectra with different probing depths at once. The time-resolved XAS spectrausing TOF analyzer will allow the temporal evolution of excited electronic states along thedepth direction.
Experimental
The experiments were performed using a TOF electron analyzer (VG Scienta, ARTOF 10k)at the soft X-ray undulator beamline BL07LSU at SPring-8 [1]. We developed a picosecond
time-resolved XPS system using the TOF electron analyzer, and studied the relaxation
dynamics of photoexcited carriers on various surfaces and interfaces [2]. In the present study,
thus, we aim to extend the application of the TOF electron analyzer to XAS measurements.
We developed the software program that can control both TOF electron analyzer and the soft
X-ray beamline (undulator and monochromator) to measure XAS spectra. The sample studied
in this research was the rutile TiO2(110) surface. The sample surface was cleaned by Ar ion
sputtering (1.0 kV, 1x10-4 Pa Ar, 10 min), and annealing in an oxygen atmosphere (920 K in
1x10-4 Pa O2). The surface order was confirmed by low energy electron diffraction (LEED).

Results and discussion
Figure 1 shows the Ti L-edge XAS spectrum of the clean TiO2(110) surface measured using
TOF electron analyzer. The XAS spectrum in total electron yield (TEY) was obtained byintegrating the whole TOF region indicated by the blue lines in Fig. 1(a). The obtained XAS
spectrum in Fig. 1(b) is in good agreement with the TEY-XAS spectrum measured with thesample current. This confirms the successful measurement of XAS spectra using TOF
electron analyzer. By changing the TOF window for the integration, we can change theprobing depth. For example, we could obtain the surface sensitive XAS spectra if the TOF
window is tuned to Auger electrons. Now we plan to perform XAS experiments on the sample
that has a distribution of chemical states in the depth direction (e.g., ice H2O thin films
adsorbed on the TiO2 surface).

Figure 1. Ti L-edge XAS spectrum of the clean TiO2(110) surface measured using TOF
electron analyzer. (a) TOF spectra as a function of photon energy (b) XAS spectrum obtainedby integrating the TOF region indicated by the blue lines in the panel (a).
REFERENCES[1][2]

S. Yamamoto et al., J. Synchrotron Rad. 21, 352-365 (2014).
S. Yamamoto et al., J. Phys. Soc. Jpn. 82, 021003 (2013).

SOFT X-RAY ABSORPTION SPECTROSCOPY OF Ca2RuO4
UNDER ELECTRIC FIELD
Daiki Ootsuki1, Yukie Takasuka1, Ayuko Hishikawa1,
Tatsuhiro Ishida1, Teppei Yoshida1, Kosuke Yamazoe2, Tomoyuki Tsukada2,
Jun Miyawaki2, Yoshihisa Harada2,
3Chanchal Sow , Shingo Yonezawa3, Yoshiteru Maeno3, and Fumihiko Nakamura4
1

Graduate School of Human and Environmental studies, Kyoto University,
Synchrotron Radiation Laboratory, The Institute for Solid State Physics, The University of Tokyo,
3
Department of Physics, Kyoto University,
4
Department of Education and Creation Engineering, Kurume Institute of Technology

2

The exploration of non-equilibrium phase transition and non-linear conductivity in strongly
correlated electron systems is one of the frontier of condensed matter physics. By applying
electric fields, Mott insulators exhibit the insulator to metal transition with a non-linear
conductivity [1-2], which is so called “Mott breakdown” analogous to dielectric breakdown[3]. Recently, Nakamura et al. have discovered an electric-field induced insulator to metaltransition of Ca2RuO4 with nonlinear conductivity. The threshold electric field Eth of ~40
V/cm is weakest among the Mott insulators [4]. It would be desirable to understand how theelectronic structure is changed by applying electric field and to clarify the origin of the
non-linear conductivity of Ca2RuO4.
The various stimuli such as temperature, chemical substitution, and applied pressure induce
the metal to insulator transition of Ca2RuO4 with a structural phase transition [5-7]. The
structural phase transition affects the orbital configuration of this system through RuO6
octahedra [5]. Actually, the insulating state of Ca2RuO4 is characterized by the crystal field
splitting as well as on-site Coulomb interaction. The recent polarization-dependence ofangle-resolved photoemission spectroscopy combined with dynamical mean-field theory(DMFT) revealed the coexistence of the band insulating state of dxy orbitals originating fromthe crystal-field splitting and the Mott insulating states of half-filled dxz/yz orbitals [8].

Figure 1. O 1s XAS spectra of Ca2RuO4 as a function of (a) angle  and (c) current
density J. Intensities of apical and in-plane components as a function of (b) angle  and(d) current density J.

In this study, we have investigated the electric-filed dependence of the electronic structure
for Ca2RuO4 by means of soft x-ray absorption spectroscopy (XAS) under electric field at
SPring-8 BL07LSU. High quality single crystals of Ca2RuO4 were grown by floating-zone
method. In order to apply the electric field, the gold wires were attached to the terminals bothends of the sample using silver epoxy. The electric field was applied parallel to the ab-plane
and the voltage-current characteristics were measured in situ during the XAS measurements.
The sample temperature is monitored by the directly attached Pt thermometer and controlled
by liquid helium. Although the current flow gives rise to the joule heating effect, the sample
heating kept less than 3 K during all measurements.
Figure 1 (a) shows the angle dependence of the O 1s x-ray XAS spectra take at T = 300 K.
The XAS spectrum strongly depends on the angle . We focus on the intensities at ~528.3 eV
and ~529.1 eV derived from the apical and in-plane oxygen, respectively. To quantify theintensities of the apical and in-plane components, we fitted the intensities derived from apicaland in-plane oxygen with a Gaussian as shown in Fig. 1 (b). The angle dependence of XAS
spectra is basically consistent with the previous XAS study reported by Mizokawa et al [9].
This indicates that the carrier concentration between the Ru 4dxy and 4dyz/zx orbitals is close to
1:1 at T = 300 K. Figures 1 (c) and (d) show the electric field dependence of the XAS spectra
at T = 300 K. Here, we obtained the electric-field dependence of XAS spectra at the normal
incidence. By increasing current density J, theapical intensity decreases, while the in-plane
intensity increases. As shown in Fig. 1 (d), theintensity ratio of the apical and in-planecomponents gradually increases with current
density J.
Figure 2 shows the relationship between the
crystal-field splitting , spin-orbit coupling so,and the intensity ratio of the apical and in-planecomponents of XAS spectra. The solid curve
indicate the theoretical estimation on the basis of
the simple Hamiltonian of Ref. 10. From thetheoretical curve, we obtained the crystal-field
splitting  and spin-orbit coupling so. With
increasing current density, the crystal field
splitting energy increases from the compressed Figure 2. XAS intensity ratio between
RuO6 octahedra to the elongated RuO6 octahedra. the apical and in-plane components.
These results suggest that the electric field The solid line is the model expectationinduces the elongation of the RuO6 octahedra.
of the Hamiltonian of Ref. 10.
REFERENCES[1] Y. Taguchi et al., Phys. Rev. B 62, 7015 (2000).
[2] K. Inagaki et al., J. Phys. Soc. Jpn. 73, 3364-3369 (2004).
[3] T. Oka, Phys. Rev. B 86, 075148 (2012).
[4] F. Nakamura et al., Sci. Rep. 3, 2536 (2013).
[5] O. Friedt et al., Phys. Rev. B 63, 174432 (2001).
[6] F. Nakamura et al., Phys. Rev. B 65, 220402 (2002)[7] S. Nakatsuji and Y. Maeno, Phys. Rev. Lett. 84, 2666–2669 (2000).
[8] D. Sutter et al., Nat. Commun. 8, 15176 (2017).
[9] T. Mizokawa et al., Phys. Rev. Lett. 87, 07702 (2001).
[10] G. Fatuzzo et al., Phys Rev. B 91, 155104 (2015).

OPERANDO XPS STUDY OF CO2 HYDROGENATION ON A SINGLEATOM ALLOY Pd-Cu MODEL CATALYST
J. Yoshinobu, S. Tanaka, K. Mukai, W. Osada, T. Matsuda, S. Nakano, Y. Choi,
S. Yamamoto, and I. Matsuda
The Institute for Solid State Physics, The University of Tokyo
Introduction
Single-atom alloy catalyst (SAAC) is an alloy in which a small amount of functional metal
atoms are deposited and embedded in the surface of a metal substrate. The deposited atoms givea new specific function to the substrate, where the main catalytic activity of the substrate is still
active [1]. When high-resolution XPS measurement is performed using high-intensity
synchrotron radiation, a very small amount (a few percent or less) of embedded atoms on thesubstrate surface can be detected, and their chemical states can be clearly identified.
Furthermore, it is possible to observe the changes in the electronic and chemical states of theadsorbed atoms/molecules as well as the catalyst surface in ambient conditions using NAPXPS. Operando measurments using mass spectrometry and NAP-XPS can elucidate thecatalytic function of SAAC during the reaction.
In this study, we prepared a single-atom alloy catalyst (SAAC) Pd-Cu(111) and investigatedthe hydrogenation of carbon dioxide (CO2) using NAP-XPS. CO2 is one of the greenhouseeffect gases. At present, methanol synthesis from CO2 is performed at high temperature(~500K) and under high pressure using Cu/ZnO catalysts. This reaction is an exothermic
reaction, in which the molar number decreases for the forward direction. Thus, more active andefficient catalysts which can be operated at lower temperature are highly demanded.
Recently, we found the desorptions of hydrogenated species after a large amount of hydrogen
exposure to formate species (HCOO) on a Pd-Cu(111) surface using temperature programmed
desorption. In the present study, we aimed to clarify the hydrogenation reaction process of CO2
on a single-atom alloy catalyst Pd-Cu(111) by NAP-XPS.
Experimental
Experiments were carried out using NAP-XPS system at SPring-8 BL07LSU. The cleanCu(111) was prepared in a UHV preparation chamber of NAP-XPS system. A small amount of
Pd was vapor-deposited on the surface to prepare a single atom alloy model catalyst Pd-Cu(111).
The amount of Pd deposited was estimated to be 0.02 ML (monolayer). In a NAP cell, CO2 (0.9mbar) and H2 (0.5 mbar) were introduced under isothermal conditions near room temperature.
During this process, the model catalyst surface was observed by NAP-XPS. Next, the substratetemperature was raised stepwise by heating under constant pressure of CO2 (0.9 mbar) and H2(0.5 mbar).
Results and discussion
Before the introduction of mixed gases (CO2 + H2), only H2 was gradually introduced in the
NAP cell at 300 K up to 3.0 mbar, and the model catalyst surface was investigated by NAPXPS; the core-level spectra of Cu 2p, Cu LVV Auger, Cu 3p, and Pd 3d were measured.
However, any peak shifts due to hydrogen adsorption and an appearance of new peaks were notobserved under the present conditions. Even if the dissociative adsorption of hydrogen occurs,hydrogen is thermally desorbed at room temperature, i.e., a sufficient amount of hydrogen is
not adsorbed on the surface.
CO2 (0.9 mbar) and H2 (0.5 mbar) were introduced into the NAP cell at 300 K, and Pd3d,
C1s, and O1s were observed by AP-XPS (see Fig. 1). Based on our previous study [2], major
surface species were attributed to CO3 species; a small amount of atomic oxygen were also
observed. In the case of Pd3d, a small peak was newly observed on the high binding energy
side. This may be assigned to the Pd atoms bonded to atomic oxygen species.
When heating the sample from 300K to 500K, the C1s and O1s peaks originating from CO3
species were observed up to 375K but almost disappeared above 390K. The O1s peak attributed
to atomic oxygen almost disappeared above 375K. On the other hand, the peak derived fromcarbon species (~284 eV) gradually increased with heating.
The present study suggests that the dissociation of CO2 and the formation of CO3 and O
species occur at 300 K (disproportionation). However, no experimental evidence was detected
to indicate hydrogenation of CO2 at the present experimental conditions. Therefore, in order toachieve a sufficient amount of adsorbed hydrogen atoms at the surface to induce hydrogenation,
lower temperature and/or higher H2 pressure may be needed.

-

Figure 1 A series of XPS spectra of C 1s, O 1s and Pd 3d of the Pd-Cu(111) surface
under CO2 (0.9 mbar) and H2 (0.5 mbar) at 300 K and subsequent heating.
REFERENCES[1] G. Giannakakis et al., Accounts of Chemical Research, 52, 237 (2018).
[2] T. Koitaya et al., Topics in Catalysis, 59, 526 (2016).

AP-XPS STUDY OF ELECTRONIC STATES OF METAL NANOPARTICLES AND METAL HYDRIDE COMPLEX
J. Yoshinobu1, W. Osada1, Y. Choi1, S. Tanaka1, K. Mukai1,
T. Koitaya2, S. Yamamoto3, and I. Matsuda1
1
The Institute for Solid State Physics, The University of Tokyo
2
Institute for Molecular Science
3
Institute of Multidisciplinary Research for Advanced Materials, Tohoku University
Introduction
When a hydrogen atom is bonded to metal, the chemical state of hydrogen can be classified
into atomic hydrogen (H), proton (H+) or hydride (H-), or an intermediate state between them;it can easily change, depending on its surrounding environment. The chemical state of metal
which is bonded to hydrogen also changes accordingly. By utilizing various hydrogen species
on a metal surface, a new reaction path can be developed for "highly selective", "verydifficult" and/or "highly efficient" processes. One might think that a metal is “reduced” when
it is bonded to hydrogen. However, when neutral hydrogen takes an electron from metal andit becomes hydride (H-), the metal is now “oxidized”. Using high-resolution X-ray
photoelectron spectroscopy (XPS), the chemical state of metal can be elucidated at each site,
based on the chemical shift of its core level peak. In the case of surface chemistry, we cantrace the chemical states of surface and bulk during the processes including hydrogen
adsorption, absorption and desorption. Although the formal charges of hydrogen and metal
have been estimated based on valency in a hydrogen-containing metal complex, one canexperimentally elucidate the chemical state of a metal atom using XPS. In this study, weplanned to perform ambient pressure (AP) XPS measurements of metal-alloy nanoparticles
and multinuclear metal complexes in the presence of hydrogen.
Experimental
In this beam time, we carried out XPS measurements of Pt nano particles and Pt75Pd25 nanoparticles synthesized by Yamauchi group at Kyushu University, and multinuclear Ti
complexes including Ti4N4 [C5Me4SiMe3]4 synthesized by Shima group at RIKEN (hereafter
abbreviated as Ti4N4 complex).
(A) In the case of Pt and Pt75Pd25 nano particles, these nano particles were deposited on a
Au foil. First, we measured Pt 4f and Pd 3d XPS in vacuum at room temperature. Next, thesample was heated to 250 °C in vacuum, and Pt 4f and Pd 3d XPS spectra were measured in
vacuum at room temperature. Finally, the sample was heated under H2 (20 mbar) at 250 °C
for 20 minutes, and XPS of Pt 4f and Pd 3d measurements were performed at room
temperature.
(B) In the case of Ti4N4 complex, a small amount of Ti4N4 complex was dissolved indegassed n-hexane in an Ar glove box and 5 μL of this solution was dropped on a PdCu alloy
substrate. After evaporating and drying the sample solvent, the sample was carefully moved
from the glove box to the load-lock chamber of the AP-XPS system using a vacuum transfer
device. First, we measured XPS spectra of Ti 2p, C 1s, O 1s and N 1s in vacuum at room
temperature. Next, H2 gas (5 mbar) was introduced and subsequently evacuated; during thisprocess AP-XPS measurement was carried out for Ti 2p, C 1s, O 1s and N 1s.
Results and discussion(A) XPS spectra of Pt nano particles and Pt75Pd25 nano particles during the above processes
were obtained. When comparing the XPS spectra of the as-installed sample and the heatedsample at 250°C, we confirmed that the O 1s intensity was decreased after heating, but aremarkable change in the spectral shapes of Pt 4f and Pd 3d was not observed. Even after
heating at 250°C under H2 pressure (20 mbar) for 20 minutes, no significant change wasobserved in Pt 4f and Pd 3d XPS in vacuum. Thus, the significant surface segregation of Pd
atoms in the alloy nano particle was not observed under the present hydrogen pressure andheating conditions.
(B) Our previous experiments suggested that the present Ti4N4 complex reacts with oxygen
and water, and it is easily oxidized. In this experiment, we carefully prepared the sample andtransferred it to the AP-XPS chamber as described above. Figure 1 shows Ti 2p, C 1s, O 1s andN 1s XPS spectra of Ti4N4 complex on a PdCu substrate. Ti 2p3/2 and N1s peaks were observed
at 456.8 eV and 397.2 eV, respectively, indicating that Ti takes a tetravalent state and N atoms
are bonded to the metal. Next, we also carried out AP-XPS measurements during the H2
exposure and pumping processes. Upon H2 exposure, we found the disappearance of N 1s peak,the increase of O1s and the peak shift of Ti 2p3/2 to 459.6eV. These results indicate that there
is no nitrogen in the complex and it was oxidized. We assume that upon H2 exposure the partialpressure of water vapor was increased in the AP cell by some reactions between hydrogen andthe inner wall of a stainless-steel AP cell, and thus produced water reacts with the complex.
For future work, we need to control the gases in the AP cell to eliminate the possibility ofoxidation of metal complex.

Figure 1 Ti 2p, C 1s, O 1s and N 1s XPS spectra of Ti4N4 complex deposited on a PdCu

substrate. XPS measurements were carried out in vacuum at room temperature (hn =680 eV).

DYNAMICS OF PHOTOCATALYTIC MATERIALS FOR WATERSPLITTING REACTION STUDIED BY TRANSIENT X-RAY
ABSORPTION SPECTROSCOPY
Yohei Uemura,1 Ahmed Ismail,1 Hebatalla Elnaggar,1 Federica Frati,1 Hiroki Wadati,2,3
Yasuyuki Hirata,2 Yujun Zhang,2 Kohei Yamagami,2 Susumu Yamamoto,2 Iwao Matsuda,2Frank de Groot1
1

Inorganic Chemistry and Catalysis, Debye Institute for Nanomaterials Science, Utrecht University,
Universiteitslaan 99, 3584 CG Utrecht, The Netherlands
2Institute for Solid State Physics, University of Tokyo, Kashiwa, Chiba 277-8581, Japan
3
Graduate School of Material Science, University of Hyogo, Kamigori, Hyogo 678-1297, Japan

Introduction
From the viewpoint of building a sustainable society, developing a renewable energy
resource is an important subject in the modern science and technologies. Water splitting reaction
where water molecules (H2O) are decomposed into hydrogen (H2) and oxygen (O2) is expected
to be a key reaction to achieve a renewable energy process. Photocatalysts and photoelectrodes
play a central role in the water splitting reaction utilizing sunlight. Semiconductors such as TiO2,
-Fe2O3 is typical photocatalysts/photoelectrodes. They have been developed for several
decades and new materials are still explored to enhance its catalytic efficiency for our daily use.
Fundamental studies on the mechanisms of photocatalysts/photoelectrodes have been based ontheoretical calculations, electrochemical methods and spectroscopic studies. X-ray absorption
spectroscopy (XAS) is a new trend to study the excited states of photocatalytic materials.
XAS can address local symmetries and electronic states of a specific element in the material
under investigation and give complementary information which is not provided from optical
spectroscopies. In order to understand dynamics of excited states of photocatalysts andphotoelectrodes, we measured thin films of hematite (-Fe2O3) and copper tungstate (CuWO4).
-Fe2O3 and CuWO4 are have been studied as photoelectrodes for solar-assisted water splitting
reaction. We have already measured fast carrier dynamics of both the samples based on
femtosecond XAS measurements in PAL-XFEL. 1 We focused on how the photoexcited states
are progressed in hundreds of picoseconds. Here, we will present the result of the transient XAS
study on the photoexcited state of -Fe2O3, which has been recently published in ref. 1.
Experimental details
Pump probe XAS experiments were performed at BL07LSU, SPring-8. An illustration of
the setup employed in this study is displayed in Figure 1 (a) and the details of the setup isdescribed in ref. 2. The repetition of the excitation laser was ~1 kHz and its wavelength was~400 nm which was generated by doubling the fundamental wavelength of a Ti:Sapphire laser(~800 nm) using a BBO crystal. The samples were thin films of -Fe2O3 and CuWO4 depositedon an Indium-Tin-oxide-coated glass substrate. The thickness of the samples was ~ 50 nm.
Results
Transient XAS measurements by the sample current: In our experiments, an x-ray chopper
developed in SPring-8 was employed in order to select x-ray synchronised with an excitation
laser.3 In the previous experiments 2, the signals from the x-ray pulses synchronised with an
excitation laser were accumulated by using electric gates. Although the x-ray signals can be
efficiently by this method, a fast-response detector (response time: nanoseconds) is needed tobe employed. The x-ray chopper can isolate the x-rays synchronised with the laser and only theselected x-rays hit a sample. As the consequence, a relatively slow-responce detector (~miliseconds) could be employed. In our beamtime, we tried to measure transient XAS bymeasuring the sample current in addition to counting photoelectrons from the sample. transient
XAS of the -Fe2O3 sample was measured by the sample current.

Although XAS spectra witha good signal-to-noise ratio (S/N)
was obtained by measuring thesample current, we did not confirm
transient signals measured by the
sample current. It would because
the excitation laser affected thesample current signals, which
could be smeared out transient
signals.
However,thesynchronization of the x-ray pulses
was achieved and obtained XAS
spectra with a good S/N. It would Figure 1 (a) an illustration of the experimental setup at
be possible to measure XAS with BL07LSU, (b) the transient Fe LIII edge XAS spectra ofa transmitting geometry by hematite (c) the kinetic trace at 709.6 eV
employing the x-ray chopper,
which should extend the choice of the samples to be measured. It would be possible todistinguish transient signals from bulk and from surface of a sample when the sample current
and PEY are employed at the same time.
The transient XAS of -Fe2O3: Transient Fe LIII XAS of hematite was measured by a partial
electron yield (PEY) detection. The fluence of the excitation laser was set to 10 mJ/cm2, which
is below the damage threshold of the sample. The transient XAS at a delay time of 80 ps isshown in the central panel of Figure 1. There are three distinctive transient features foundaround 708.6, 709.6 and 710.2 eV. The transient feature around 709.6 eV was also observed by
the transient experiments in PAL-XFEL, which appeared just after the sample was excited by
the laser.1 The transient feature originates from the creation of Fe(II) species compared by thetheoretical calculations. The transient features seen around 708.6 eV and 710.2 eV were notobserved at faster delay time (< 10 ps) in PAL-XFEL. These new features could come fromlocal structural changes which happens between 10 ps and 80 ps after the photoexcitation. Small
polaron creation in a hematite thin film was observed around a delay time of 1 ps after
photoexcitation by femtosecond transient extreme ultraviolet spectroscopic experiments4.
Further local structural changes could happen in later delay time. Considering the time rangeof water splitting reaction, the excited state of hematite should be important to understand thedriving force of the water splitting reaction. We have also measured transient XUV spectra ofseveral hematite samples by a delay time of 1 ns. We would like to combine the transient Fe LIII
XAS data with the XUV data to understand more details of photoexcited states of hematite.
REFERENCES
1.
Ismail, A. S. M., et al., Direct Observation of the Electronic States of Photoexcited Hematitewith Ultrafast 2p3d X-Ray Absorption Spectroscopy and Resonant Inelastic X-Ray Scattering. Phys.
Chem. Chem. Phys. 2020, 22, 2685-2692.
2.
Takubo, K., et al., Capturing Ultrafast Magnetic Dynamics by Time-Resolved Soft X-Ray
Magnetic Circular Dichroism. Appl. Phys. Lett. 2017, 110, 162401.
3.
Osawa, H.; Ohkochi, T.; Fujisawa, M.; Kimura, S.; Kinoshita, T., Development of Optical
Choppers for Time-Resolved Measurements at Soft X-Ray Synchrotron Radiation Beamlines. Journalof Synchrotron Radiation 2017, 24, 560-565.
4.
Carneiro, L. M.; Cushing, S. K.; Liu, C.; Su, Y.; Yang, P.; Alivisatos, A. P.; Leone, S. R.,
Excitation-Wavelength-Dependent Small Polaron Trapping of Photoexcited Carriers in Α-Fe2o3. Nat.
Mater. 2017, 16, 819-825.

INVESTIGATION OF THE ELECTRONIC STRUCTURE OF COBALTOXIDE SPINEL ON ELECTRODES FOR WATER OXIDATIONCATALYSTS USING CO 2P3D RIXS
Minmin Chen,a Kosuke Yamazoe,b,c Jun Miyawaki,b,c Yoshihisa Harada,b,c Serena DeBeer,aOlaf Rüdigeraa

Max Planck Institute for Chemical Energy Conversion, Mülheim an der Ruhr, GermanybInstitute for Solid State Physics, The University of Tokyo, Kashiwa, Chiba 277-8581, Japanc

Synchrotron Radiation Research Organization, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku,
Tokyo 113-8656, Japan

Development of new anode materials based on earth abundant transition metals for water
splitting is strategically important for industrial and large-scale applications. Water splitting
has been considered one of the most promising strategies for energy conversion and storage.1
Here, we study Co3O4 spinel on electrode surfaces by spectroelectrochemistry. The spinels aresynthesized as nanoparticles of around 10 nm that form aggregates of 100-300 nm diameter.
The materials are drop cast on the electrode surface, in this case a gold-coated SiN window.
In order to stabilize the small catalyst particles (particle size 300 nm) on the electrode surface,
Nafion is used as a binder. Unfortunately, Nafion showed extreme X-ray sensitivity during
our experiments. Being a fluorine-rich polymer, the F K-edge at 690 eV absorbed the X-rays
and damaged the polymer structure, which resulted in a simultaneous decrease of the Co
intensity.
The use of Fumion as a binder
solves this problem. Fumion
does not contain any F atoms in
its structure and showed no
detectable
photodegradation
duringthespectroelectrochemical
experiments. Nevertheless, we
did not detect any significant
changes on the 2p3d RIXS or at
the Co L-edge XAS with theapplied potential (Figure 1).
In order to understand the
2p3d RIXS of the spinel system,
we also measured spectra from
Figure 1. Co L-edge XAS (top-left) and 2p3d RIXS spectra of an
Al2CoO4, Co2ZnO4 and CoO aselectrode prepared by drop casting Co3O4 nanoparticles andpowder samples. The Al2CoO4 is
Fumion as a binder. The conditions are indicated on the figure, theelectrolyte was KOH 0.1 M.
a spinel, where the Co3+ at the
Oh sites has been substituted
3+2+with Al atoms. Similarly, on the Co2ZnO4, the Td Co has been substituted with Zn2+ ions.
CoO has a NaCl-like structure with all the Co2+ ions in a Oh geometry. The total electron yield
Co L-edge XAS shown in Figure 2 (left), correspond to what would be expected for a Co3+
HS in the case of the Co2ZnO4. The lower L3 edge position for the CoO and Al2CoO4 is also
in agreement with a lower oxidation state of the cobalt and the different fine structure may
reflect the different multiplet structure resulting from a Td vs an Oh coordination geometry.
The Al2CoO4 L3 edge shows a broad peak around 785 eV that at this point we suspect is anartefact. The RIXS spectra of the CoO show a well-defined feature at 0.9 eV and a second
feature that appears at higher excitation energies at 2 eV. The Al2CoO4 has 2 features at 0.5
and 0.9 eV. At higher incident energy an additional band appears at 2 eV. The Co2ZnO4 does

not show any well-defined bands, only a peak a 1.8 eV and at higher excitation energies(780-784 eV, not shown) a broad band was observed. The 2p3d RIXS spectra have been
compared to the powder Co3O4 measured at Sextants (Figure 2, right). The spectra have beennormalized to the elastic peak, except for the spectra at 779 where the elastic peak was quite
weak. At low excitation energies there is a band at 0.75 eV, most likely resulting from theCo2+ sites on the spinel, while as the excitation energy is increased the most prominent
features at around 2 eV could be assigned to the Co3+ sites, demonstrating the potential of
2p3d RIXS to study such materials. Multiplet calculations are planned for the near future to
analyse these results in detail.

Figure 2. Co L-edge XAS (TEY) from powder samples (left) and 2p3d RIXS (right) of the same samples atthree different incident energies. The spectra of Co3O4 powder measured at SOLEIL Sextants beamline wasadded to the figure as a comparison.

No significant changes were observed for the Co3O4 particles under different applied
electrode potentials. This could be a limitation of the experimental setup used in soft X-ray
measurements. Electrochemistry takes place at the interface with the electrolyte. If particle
films are too thick and the changes with the applied potential take place far from the electrodesurface, the X-ray beam may not penetrate enough on the film to observe any changes.
Another possibility is that the particle conductivity is too small, as a result, only a minor
fraction of the film may be under potential control. Consequently, we may be measuring all
the time the bulk of the particles, which remain unresponsive to the applied potential (seeFigure 3). We have repeated thesemeasurements at the Co K-edge. When using
hard X-rays, we probe the whole film.2
Similarly to what we have observed on thesoft X-ray regime, we could not detectsignificant changes on the spectral signatureswith the applied potential. This suggests, thatthe particles may be isolating, and that only aminor fraction is responding to the potential.
Figure 3. Schematic representation of the particles
We are currently investigating how to
on an electrode and the attenuation lengths at
producehomogeneous
Co3O4
different energies for X-ray. The arrows on the side
spinel-structured
films
with
controlledof the figure are scaled to the represented particle
size (assuming particles of around 150 nm inthicknesses on electrodes to see if we candiameter).
overcome this limitation in future
experiments.
References
1.
2.

T. Grewe, M. Meggouh and H. Tuysuz, Chem.-Asian J., 2016, 11, 22-42.
H. J. King, M. Fournier, S. A. Bonke, E. Seeman, M. Chatti, A. N. Jumabekov, B. Johannessen, P.
Kappen, A. N. Simonov and R. K. Hocking, The Journal of Physical Chemistry C, 2019, 123,
28533-28549.

INVESTIGATION OF THE ELECTRONIC STRUCTURE OF
ELECTRODEPOSITED COBALT OXIDE FILMS ON ELECTRODESFOR WATER OXIDATION CATALYSTS USING CO 2P3D RIXS
Minmin Chen,a Kosuke Yamazoe,b,c Jun Miyawaki,b,c Yoshihisa Harada,b,c Serena DeBeer,aOlaf Rüdigeraa

Max Planck Institute for Chemical Energy Conversion, Mülheim an der Ruhr, GermanybInstitute for Solid State Physics, The University of Tokyo, Kashiwa, Chiba 277-8581, Japanc

Synchrotron Radiation Research Organization, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku,
Tokyo 113-8656, Japan

Development of new anode materials based on earth abundant transition metals for water
splitting is strategically important for industrial and large-scale applications. Water splitting
has been considered one of the most promising strategies for energy conversion and storage.1
Cobalt oxide films electrodeposited from Co2+ salts in neutral pH buffered solutions showhigh catalytic activity at low overpotential in mild environments, combined with extended
stability as a result of a self-healing mechanism.2 Although this catalyst has been studied overthe last years using different spectroscopic techniques (including operando Co K-edge XAS)
and EPR, most of the studies presented so far, did not employ ultra-pure electrolytes and it
has recently been shown that trace amounts of iron dramatically improve the catalytic
performance of such materials.3 At this point, the role of iron in catalyst activation is still not
fully understood.
We measured the Co L-edge XAS while we were electrodepositing Co oxide on theelectrode from a solution of Co(NO3)2 0.5 mM in phosphate buffer solution (PBS) at pH 7.0.
In Figure 1, one can see that the electrode did not contain any Co. Once the cell was filledwith the Co(NO3)2 solution, but kept at open circuit potential (OCP), we can already detect acobalt signal from the Co in solution. As we apply a potential of 0.9 V (vs Ag/AgCl), theintensity of the Co L-edge increases. After the second 60 s potential pulse, the intensity does
not increase any further, indicating that either the film does not grow anymore, or that the film
is too thick and the growth goes beyond the penetration limit of the X-rays. After theelectrodeposition, the electrolyte was switched to PBS without cobalt. Unfortunately, the Co
signal rapidly disappears upon changing the electrolyte. Even if the potential was kept high tostabilize higher oxidation states on the film. If the electrolyte was switched to KOH 0.1 M,
the films were more stable so that we could measure spectra at different applied potentials asa result of the lower solubility of Co at high pH.

Figure 1. Co L-edge XAS of an electrode where Co was electrodeposited. On the left, the spectra measured
during electrodeposition are shown. Measured in PBS pH 7.0 and in the presence of Co(NO3)2 0.5 mM insolution. On the right figure, the L-edges measured at different applied electrode potentials in KOH 0.1 M.

Interestingly, the Co L-edge shifts to higher energy at 0.7 V (vs Ag/AgCl), with a maximum
intensity for the L3 at 782 eV, similar to what was measured for Co2ZnO4 (see our 2019activity report on spinel Co3O4), suggesting that the cobalt has been oxidized to Co3+. On theother hand, when the potential was increased and the catalyst started doing water oxidation
the L-edge maximum shifts to lower energy (Figure 1).
The 2p3d RIXS spectra of these electrodeposited Co oxides is less structured than thatmeasured for the spinel systems. The operando measurements showed just a small change at
779 eV excitation energy, where the spectra recorded at the highest potential did not show aseparate feature at 1 eV (Figure 2).

Figure 2. Corresponding 2p3d RIXS spectra measured for the electrodeposited Co oxide film shown in figure
6 at 3 different incident energies as indicated on the figure. Measured in KOH 0.1 M.

Operando studies in the soft X-ray regime are extremely challenging. On one hand, theexperimental setup requires a vacuum environment, which makes the design of thespectroelectrochemical cell challenging. Nevertheless, we have successfully designed a cellthat operates on a three-electrode configuration. The cell did not leak during themeasurements.
The electrodeposited films showed a small response to the applied potential. We could
monitor the film growth by following the Co L-edge XAS. Under basic pH conditions, themeasured spectra suggest the formation of Co3+ at applied potentials just before the catalytic
wave starts. Under turnover conditions, the L3 edge shifts to lower energy, suggesting a Co2+
configuration. This may be because the rate-limiting step is the oxidation of Co2+, and once
this state is formed, water oxidation takes place at a fast rate, and the higher valence statescannot be observed on the time scale of the experiment. This contrasts with what we havemeasured using in situ EPR for the same catalyst, where a Co4+ signal was detected.4
Nevertheless, EPR measurements are not real operando experiments, since the measurement
requires low temperatures (10K). The freezing process may slow down the catalytic cycle andallow the detection of Co4+.
REFERENCES
1.
2.
3.
4.

T. Grewe, M. Meggouh and H. Tuysuz, Chem.-Asian J., 2016, 11, 22-42.
C. Costentin and D. G. Nocera, Proceedings of the National Academy of Sciences, 2017, 114,
13380-13384.
F. Song, L. Bai, A. Moysiadou, S. Lee, C. Hu, L. Liardet and X. Hu, J. Am. Chem. Soc., 2018, 140,
7748-7759.
Y. Kutin, N. Cox, W. Lubitz, A. Schnegg and O. Rüdiger, catalysts, 2019, 9, 926.

SPIN-POLARIZED SURFACE STATES ON
MAX PHASE COMPOUND V2ALC
Takahiro Ito1,2, Masashi Ikemoto2, Damir Pinek3, Koichiro Yaji4,
Shik Shin4, and Thierry Ouisse3
1

Nagoya University Synchrotron radiation Research center (NUSR), Nagoya University, Nagoya
464-8603, Japan
2

Graduate School of Engineering, Nagoya University, Nagoya 464-8603, Japan
3

4

Grenoble Alpes, CNRS, Grenoble INP, LMGP, F-38000 Grenoble, FranceInstitute for Solid State Physics, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba
277-8581, JapanMAX phases (Mn+1AXn, where M is an early transition metal, A belongs to groups 13-15
and X is either C or N, n = 1 - 3) have recently attracted much attention due to their possible
application to the production of a new class of two-dimensional (2D) systems called MXenes[1]. However, the bulk electronic structure of MAX phases has been studied mostly throughab initio, DFT calculations, mainly due to a lack of single crystalline samples. We have
performed angle-resolved photoemission spectroscopy (ARPES) on several MAX phasesingle crystals to directly investigate the electronic structure of these systems [2,3]. Amongthe MAX phases, V2AlC has been expected to be categorized as a high-symmetry pointsemi-metal with crossing point with some Dirac-like properties referred as “Dirac point (DP)”at the Fermi level (EF) along ΓM line together with “nodal-line (NL)” around 0.2 eV from
DFT calculation [3, 4]. From the recent SARPES study of V2AlC with using a quasi continuous - wave laser [5], the existence of the spin-polarized states has been elucidated [6].
Since the observed spin states show the anti-parallel spin splitting along the tangential axis tothe larger electron pocket around the Γ point, Rashba effect [7] or the formation of thespin-polarized “drum-head surface states” have been expected to be possible origins of thespin states. To pursue the origin of the observed spin-states as well as the other spin-states if
exists on V2AlC, we have performed SARPES study with using He I light source, which isavailable for the spin mapping at the wide momentum space.
Figures 1(a) and (b) show the band structure near EF and the constant energy ARPES image
at 0.2 eV, comparing with the DFT band structure along the ΓM (AL) line and the DFT Fermi
surface of ΓKM (AHL) plane, respectively. While the large (small) electron pocket eL (es)seems to be in consistent with the previous ARPES studies [3,6], we have found an apparent
dispersive feature (SS) appears between eL and es bands. Since the observed SS has not been
reproduced by the bulk DFT calculation, and SS dispersive feature shows similarity with thees band with higher binding energy shift, we expect that the origin of SS to be a “surface
state”. It should be noted that the observed SS has never been observed on the previous
ARPES studies with using UV synchrotron from hν = 40 ~ 140 eV [4] or laser-source of hν ~7 eV [6]. We expect possible origin of SS might be so called surface termination effect at thecleaved surface, which could be different from the other previous studies.
In Fig. 1(c), the SARPES spectra at the momentum cut-m1 to m6 and p1 to p6 are shownwith the spin polarization along y-axis (Sy). Here, y-axis corresponds to the tangential
direction to the electron pockets around the Γ point as shown in Fig. 1(b). Around the eL
pocket, Sy− (Sy+) polarization around 50 - 200 meV following a relatively small anti-parallel
spin feature at the lower binding energy has clearly been observed around kx ~ ±0.5 Å-1. The

observed spin-states around the eL pocket show a good consistency with the previous
laser-SARPES study [6]. On the other hand, we have found a sizable spin polarization around
SS. At the cut-m1 and m2 in Fig. 1(c), Sy− polarization has been observed. Though the strong
polarization around NL (0.2 eV at the cut-m4) might be expected to affect to the polarizationin the wide momentum/energy range, appearance of Sy− polarization below 0.1 eV at the
cut-p1 and p2 should ensure the different origin of the spin-states around SS from the ones
around NL (and/or eL pocket). From the detail comparison of the relative peak position of SS
between Sy+ and Sy− SAPES spectra, Sy+ appears at the lower binding energy side at the cut
m1(m2), while it appears at the higher side at p1(p2). Therefore it has been expected that the
SS spin-polarized states might has spin-splitting which is inverted with respect to the Γ point.
This result suggests that the size of spin-splitting at SS will be much smaller than one aroundeL. To understand whether the mechanisms of the formation of spin-states around eL pocket
and SS are the same or not, further ARPES study choosing surface termination as well as
DFT calculation of spin-texture will be performed in our future work.

Fig. 1: (a) Band structure along ΓM line of V2AlC. White solid (dashed) lines are obtained
from DFT calculation along the ΓM (AL) line. (b) Constant energy ARPES image at 0.2 eV of
V2AlC. Purple solid (dashed) lines are Fermi surface on the ΓKM (AHL) plane obtained by the
DFT calculation. The definition of spin polarization axis (Sy) is indicated by black arrow. (c)SARPES spectra (Sy+:△; Sy−:▼) (left) and spin-polarization (right) of V2AlC at the momentum
cut from m1 (p1) to m8 (p6) in Fig. 1 (a).

REFERENCES[1] M. Barsoum, MAX phases (Wiley, Weinheim 2013).
[2] T. Ito et al., Phys. Rev. B 96, 195168 (2017).
[3] D. Pinek et al., Phys. Rev. B 98, 035120 (2018).
[4] T. Zhang et al., Nature 566, 475 (2019).
[5] K. Yaji et al., Rev. Sci. Instrum. 87, 053111 (2016).
[6] T. Ito et al., ACTIVITY REPORT OF SYMCHROTRON RADIATION LABORATORY 2018,
90 (2019).
[7] V. Sunko et al., Nature 549, 492 (2017).

ABSENCE OF HYBRIDIZATION BETWEEN SPIN-POLARIZEDBANDS IN QUASI-ONE-DIMENSIONAL GIANT RASHBA SYSTEM
BI/INAS(110)-(2×1)
Takuto Nakamura1, Yoshiyuki Ohtsubo2,1, , Ayumi Harasawa3, Koichiro Yaji3,
Shik Shin3, Fumio Komori3, Shin-ichi Kimura2,1
1

Department of Physics, graduate School of Science, Osaka University
2
Graduate School of Frontier Bioscience, Osaka University
3
The Institute for Solid State Physics, The University of Tokyo

Surface electronic structure with helical-spin texture thanks to spin-orbit interaction (SOI)
have been extensively studied not only for basic physics but also for applications to spintronic
devices [1-3]. In order to describe such system, a simple spin texture, one band has only one
spin polarization orientation and orbital character, has been widely used in the earlier studies.
However, recent spin and angle-resolved photoelectron spectroscopy (SARPES) and firstprinciples calculation studies have been revealed that the spin texture of these systems are
actually not so simple, but strongly entangled with the orbital components of the surface band
structure [4-6]. Spin inversion and gap opining due to hybridization between spin-polarizedbands has also been reported about 2D Rashba system such as Bi/Ag quantum well [6, 7].
Although such studies have been carried out extensively for 2D systems, its extension to
one-dimensional (1D) or quasi-1D (Q1D) systems were not achieved yet. In this project, weobserved spin-orbital texture on a giant Rashba-type spin-splitting Q1D system
Bi/InAs(110)-(2×1) [8] by using laser-based SARPES equipment [9].
Figure 1 (a) shows spin-integrated ARPES band mapping of the Bi/InAs(110)-(2×1) surface
along Q1D chain direction with p- polarized photons. Two parabolic bands, S1 and S2, with a
top away from the center of the surface Brillouin zone (k = 0 Å-1) were observed near the
Fermi level. ARPES constant energy contour (not shown) indicated the anisotropic, Q1D
character of both S1 and S2. From the in-plane spin-resolved measurement as shown in figure
1 (b), both parabolic bands show clear spin polarization, and the signs of the spinpolarizations are the opposite to each other. These results are consistent with our previous
data [7], suggests the spin splitting due to Rashba-type SOI.
In order to clarify the hybridization effect of the two spin-polarized bands, we extracted theenergy distribution curves (EDCs) from figure 1 (a) and show the results in figure 1 (d). The
peak positions of the S1 (S2) band are indicated by a filled (open) triangles. Although the twobands are crossing each other at k = 0.21 Å-1, no energy gap was observed. Moreover,spin-resolved EDCs as shown in figure 1 (e) indicated no spin-inversion nor interference at
the crossing point (k = 0.21 Å-1). These results suggest that there is no band modulation in
Bi/InAs(110)-(2×1) surface, such as spin flipping or spin gaps opening , due to thehybridization between the spin-polarized bands. This behaviour is different from that of the
known 2D Rashba systems such as Bi/Ag quantum wells [6,7].
We are currently tracking the detailed changes in orbital components and spin polarization
at the initial state in combination with the density-functional-theory calculation, in order tounderstand this lack of hybridization between Rashba-type spin-polarized surface bands.

REFERENCES[1] É. I. Rashba, Sov. Phys.-Solid State 2, 1109 (1960).
[2] M. Z. Hasan and C. L. Kane, Rev. Mod. Phys. 82, 3045 (2010).
[3] A. Manchon et al., Nature Mat. 14, 871 (2015).
[4] K. Yaji et al., Nat. Commun. 8, 14588 (2017).
[5] K. Kuroda et al., Phys. Rev. B 94, 165162 (2016).
[6] R. Noguchi et al., Phys. Rev. B 95, 041111(R) (2017).
[7] H. Bentmann et al., Phys. Rev. Lett. 108, 196801 (2012).
[8] T. Nakamura et al., Phys. Rev. B 98, 075431 (2018).
[9] K. Yaji et al., Rev. Sci. Instrum. 87, 053111 (2016).

Figure1. Surface electronic structure of Bi/InAs(110)-(2×1) surface.
(a) Spin-integrated and (b) spin-resolved ARPES band mapping along
Bi chain direction. Spin-direction was set to in-plane chain normal(k//[001]) direction. (c) The 2D color code for (b). blue-red color
corresponds to spin polarization and the contrast of the color to theintensity of photoelectrons.
(d) Spin- integrated and (e)spin-resolved EDCs. All data were measured at 45K.

TOPOLOGICAL SURFACE STATE MODIFIED BY ADSORPTION OF
ORGANIC DONOR MOLECULE

Tatsuya Kitazawa1, Koichiro Yaji2, Kosuke Shimozawa1, Hiroshi Kondo1, Takayoshi
Yamanaka1, Hiroshi Yaguchi1, Yukiaki Ishida2, Kenta Kuroda2, Ayumi Harasawa2, Takashi
Iwahashi3, Yukio Ouchi3, Fumio Komori2, Shik Shin2, and Kaname Kanai1
1

Department of Physics, Faculty of Science and Technology, Tokyo University of Science
2
Institute for Solid State Physics, The University of Tokyo
3
Department of Materials Science and Engineering, Tokyo Institute of Technology

Topological insulators (TIs) have attracted much attention in quantum matter physics.
Topological surface states (TSS) have a helical-spin texture, in which the spin-locked
electrons are topologically protected against the backscattering from nonmagnetic impurities
due to time reversal symmetry1. This property of TSS will be important in next-generation
spintronic devices because it will allow lower-energy electronic transport. Thus, controlling
the TSS by creating topological interfaces with other materials is vital for using TIs in
spintronic devices. In 2016, Tu et al. reported a topological p-n junction on the surface of
Bi2-xSbxTe3-ySey, a three-dimensional TI fabricated by using electron-accepting organic
molecule F4-tetracyanoquinodimethane (TCNQ), and they achieved on-off switching of
charge current flow originating from TSS2, suggesting that electronic functional organic
molecules are suitable for TI spintronic devices. Recently, there have been several studies of
the TI interface with organic molecules toward using TIs in spintronic devices. Caputo et al.
examined the cobalt phthalocyanine (CoPc)/Bi2Se3 interface3 and observed a slight shift in the
Dirac point (DP) to higher binding energy by angle-resolved photoemission spectroscopy(ARPES) experiments, indicating charge transfer from CoPc to Bi2Se3. This means that the
CoPc/Bi2Se3 interface is a chemisorption system. However, a negligible shift in the Bi 4f corelevels was observed by X-ray photoemission spectroscopy (XPS), indicating little chargetransfer at the interface. These experimental results contradict each other. The interface
between TCNQ, a typical electron acceptor, and Bi2Se3 was studied by Pia et al.4 Theyobserved a DP shift to higher binding energy by ARPES and a shift of the Bi 4f core leveltoward lower binding energy accompanied by TCNQ deposition by XPS measurements.
These results indicate that there is a chemical interaction between adsorbed TCNQ and Bi2Se3.
Therefore, studies of the interfacial electronic structure of the TCNQ/Bi2Se3 interface should
consider chemical interactions, such as charge transfer.
These earlier experimental studies of the interface between the organic molecules and the TI
surface have led to contradictory interpretations of the interactions at the organic molecule/TI
interfaces. Little attention has been paid to the electronic structure of molecular layers,
although this is essential for properly evaluating the interfacial electronic structure. Also, it
can be pointed out that there are few reports on the electron donating molecules (n-type
dopant) although some researches on the electron-accepting molecules (p-type dopant) like
TCNQ and its derivatives have been reported.
In the present work, we directly observed the electronic structure both of the molecularlayers and the TI surface to fully understand the influences on the TSS upon the adsorption of
the molecules. We focused on the interface between tetrathianaphthacene (TTN) and Bi2Se3.
Bi2Se3 has been extensively investigated as a prototypical TI. A Bi2Se3 single crystal
comprises five-atom layers (Se-Bi-Se-Bi-Se) stacked along the [111] direction, which are
known as quintuple layers (QLs). The QLs are attached to each other by weak van der Waals(vdW) forces and the interlayer gap between QLs is called the vdW gap. We chose Bi2Se3
because it has a single Dirac cone at the Γ point in the (111) surface Brillouin zone and a wideband gap of ~ 0.3 eV. This simple electronic structure allows us to trace the DP shiftaccompanied by the molecular deposition easily. To create the organic molecular layer/TI
interface, we vacuum deposited TTN onto Bi2Se3. TTN is a π-conjugated molecule containingfour electron rich sulfur atoms and has a strong donor nature by a small ionization energy of ~4.4 eV. TTN functions as a strong n-type dopant for Bi2Se3 because the Fermi level of Bi2Se3
is lower than the highest occupied molecular orbital of TTN. The role of donor organic
molecules on the TI surface has not been investigated thoroughly, in contrast to acceptor
organic molecules or n-type dopants, like alkali metals and other inorganic materials. Organic
molecules have the advantages of requiring lower-cost methods for creating the interface with
TI and having a lower risk of intercalation into the bulk. Thus, organic molecules are goodcandidates for manipulating the TSS.
In this work, we observed the entire electronic structure of the TTN/Bi2Se3 interface by
examining the TTN thickness dependence of the interfacial electronic state using UPS, XPS,
and ARPES.
Figure 1 shows the Fermi surfaces and the energy band dispersions measured at the
TTN/Bi2Se3 interface for various TTN thicknesses by ARPES. For the 5Å-thick TTN, theDirac point (DP) shifted toward higher binding energy (E−EF = −0.42 ± 0.02 eV) and the
Fermi surface area of TSS increased. The hexagonal warping of the Fermi surface was also
observed. We also observed the emergence of a new state just below the Fermi level with aparabolic band dispersion and hexagonal Fermi surface upon TTN deposition. This new state
was identified as the two-dimensional electron gas state (2DEG) originating from the bulk
conduction band, which is induced by the band-bending derived from the increase in thenumber of TSS electrons.

Fig. 1: Development of the electronic structure at the TTN/Bi2Se3 interface. a) ARPES
spectra of Bi2Se3 and a TTN molecular layer of various thicknesses, showing the Fermi
surface and the energy band dispersion of photoemission intensity. The dashed lines labeled
DP represent the energy of DPs. These ARPES spectra are acquired along the momentum line
slightly off the ΓK line in the surface Brillouin zone. b) Positions of the DP (EDP) and thebottom of 2DEG band dispersion (E2DEG) and c) the Fermi surface area of TSS (STSS) and
2DEG (S2DEG).
REFERENCES[1] P. Roushan et al., Nature 460, 1106 (2009).
[2] N. H. Tu et al., Nat. Comm. 7, 13763 (2016).
[4] M. Caputo et al., Nano Lett. 16, 3409 (2016).
[5] A. D. Pia et al., Chem. Phys. Chem. 19, 2405 (2018).

INVESTIGATION OF TOPOLOGICAL PROPERTIES IN
SUPERCONDUCTING TASE3
Chun Lin1, Ryo Noguchi1, Kenta Kuroda1, Koichiro Yaji1, Ayumi Harasawa1, Shik Shin1,
Atsushi Nomura2, Masahito Sakoda3, Masakatsu Tsubota4, Satoshi Tanda3,
and Takeshi Kondo1
1

The Institute for Solid State Physics, The University of Tokyo
2
3

Department of Physics, Tokyo University of Science

Department of Applied Physics, Hokkaido University
4

Department. of Physics, Gakushuin University

In recent years, a great deal of research activities are associated with the quantum materials
characterized by either the low dimensionality [1] or the electronic topology [2]. TaSe3 isknown as a quasi-1D semimetal with layered chain-like structure [Fig. 1 (a)] and has been
extensively discussed in the early ages for its superconducting properties and the absence of
charge density wave (CDW) [3]. These characteristics make it distinct from othertransition-metal trichalcogenides, which typically undergo multiple CDW transitions [3]. As
shown in Fig. 1 (b), it is considered that the absence of CDW stems from the relatively poor
nesting conditions in TaSe3 due to its more 3D-like electronic structure and CDW may be
induced by Cu intercalation [4,5] or in mesowires [6], both of which are expected to reducethe dimensionality. A recent first-principle calculation indicate that TaSe3 actually belongs toa strong topological insulator (TI) phase and moreover predict intriguing topological phasetransitions among strong TI, weak TI, Dirac semimetal, and normal insulator phases through aslight change of lattice parameters, as demonstrated in the topological phase diagram of Fig. 1(c) [7]. Therefore, TaSe3 offers a simple system to study the interplay between
superconductivity and topological phases, and the possibility of selecting dirrennt topologicalphase by changing physical parameters (temperature, element substitution etc.).
A direct investigation of the electronic structure of TaSe3 via angle-resolved photoemission
spectroscopy (ARPES) is consequently indispensable to elucidate its novel topologicalproperties. We have conducted laser-based spin-resolved ARPES (SARPES) measurements at
the Institute for Solid State Physics, the University of Tokyo last year and reported the firstunambiguously observation of the electronic structure of TaSe3 and the possible topologicalsurface states (TSSs) [8]. In present study, we show the high-resolution band structure of thepossible TSSs and confirm that these surface states are indeed spin-momentum locked.
Therefore, our results reveal a well-established TI phase in quasi-1D TaSe3.

Fig. 1 (a) Crystal structure and unit cell of TaSe3 [7]. (b) Superconductivity in pure and Cu-doped TaSe3 (left)[4], CDW signature in Cu-doped TaSe3 (middle) [5], and direct observation of CDW in mesowire TaSe3 with

hundreds of nano-meters in width (right) [6]. (c)Topological phase diagram of TaSe3 including
strong topological insulator (TI), weak TI, Diracsemimetal, and normal insulator phases [7].

Figures 2 (a,b) show the 3D Brillouinzone [7] and our recently calculated TSSs
at the X point on (-101) surface. Figure 2(c) is the schematic of the experimental
ARPES setup and the typical sample size.
In Figs. 2 (d,e), we present thehigh-resolution Fermi surface and theband dispersion at the X point. The
observed sharp and high-intense featuresnear EF are nicely consistent with our
calculations in Fig. 2 (b). To further
confirm whether the sharp electronic
states are spin-polarized, Fig. 2 (f) showsone
typicalspin-revolved
energydistribution curves and the corresponding
spin polarization at a momentum positionindicated by the dash line in Fig. 2 (e).
Taking advantage of our high-resolution
SARPES, the spin splitting is clearlyobserved for the spin up and spin downstates, uncovering a spin polarization of ~20%. We confirmed a reversal of the spinpolarization for an opposite momentum
position. Therefore, our results elucidate
the spin-momentum locked TSSs in TaSe3.
To
sum
up,
we
performed
high-resolution SARPES measurements on
TaSe3 focusing on the surface states, theresults confirm the spin-momentum locked
TSSs and hence a TI state.

Fig. 2 (a) 3D Brillouin zone (BZ) of TaSe3 [7]. (b)Calculated topological surface states (TSSs) at the X
points on (-101) surface [7]. (c) ARPES setup. Typicalsamples size is ~ 1×0.05×0.01 mm3. (d) Observed
surface-state Fermi surface on (-101) surface near the
X point at 10 K. (e) The corresponding high-resolution
dispersion at the X point with momentum positionindicated by the red arrow in (d). (f) Spin polarization
at the momentum position indicated by a dash line in(e). Red and blue curves are energy distribution curvesof spin up and spin down states, respectively.

REFERENCES[1] Y. Liu, et al., Nat. Rev. Mater. 1, 1 (2016).
[2] X.-L. Qi, S.-C. Zhang, Rev. Mod. Phys. 83, 1057–1110 (2011).
[3] P. Monceau, Adv. Phys. 61, 325 (2012).
[4] A. Nomura, et al., EPL. 124, 67001 (2019).
[5] A. Nomura et al., EPL. 119, 17005 (2017).
[6] J. Yang et al., ArXiv190609527 (2019).
[7] S. Nie et al., Phys. Rev. B 98, 125143 (2018).
[8] C. Lin et al., ISSP Activity Report of Synchrotron Radiation Laboratory (2018).

ANGLE- AND SPIN-RESOLVED PHOTOEMISSION SPECTROSCOPYRESEARCH ON TYPE-II WEYL SEMIMETAL WTe2
Yuxuan Wan1,2, Lihai Wang3, Keisuke Koshiishi1, Masahiro Suzuki1, Peng Zhang ，Jaewook Kim4, Kenta Kuroda2, Koichiro Yaji2, Ryo Noguchi2, Ayumi Harasawa2, Shik Shin2,
Atsushi Fujimori1, Sang-Wook Cheong3, Takeshi Kondo2
2

1

Department of Physics, The School of Science, The University of Tokyo
2
3

The Institute for Solid State Physics, The University of Tokyo

The Center for Quantum Materials Synthesis, Rutgers University

Introduction
So far, no example of Weyl fermions has been observed in particle physics research. On theother hand, some topological materials are expected to host electrons behaving as Weyl
fermions. These materials, called Weyl semimetals, have spin-polarized bands with cone-likeliner dispersions with zero-mass near Weyl points, which are generated when either timereversal or space-inversion symmetry is broken. The Weyl points are topologically protected,
and intriguingly, topological surface states connect a pair of Weyl points with opposite chirality,
forming arc-like Fermi surface (Fermi arc).
In some Weyl semimetals, the hole- and electron-like pockets touch each other at several
Weyl points. These kinds of Weyl semimetals are called type-II Weyl semimetals, and WTe2 westudy here is one of such candidates. In WTe2, thespace inversion symmetry along c-axis is broken,
thus the top and bottom surfaces should be different[1-3]. Theoretical calculations predict distinct Fermi
arc states for these two surfaces [4], which requiredetailed experimental investigation.
Experiment
The single crystals of WTe2 with large domains
were grown via the iodine vapor transport method[4]. Comparing with previous studies, the single
crystals have a much larger domain size. We
developed a new method to obtain a pair of samples
with opposite polar surfaces as shown in Fig. 1,
namely “sandwich method”. A single crystal ofsample was fixed on a metal substrate with silver
paste as usual ARPES experiment. Then we applied
silver paste on the top surface of the sample andcovered it with another metal substrate. After heating
the sample until the silver paste was hardened, we
separated the two substrates to cleave the sample. The
two pieces of sample would expose different surfaces.
The experiment was carried out with the spinresolved laser ARPES (SARPES) at ISSP, theUniversity of Tokyo. The light source is 7 eV laserfrom KBBF crystal. We measured two kinds of

Fig.1 (a) The method to get a pair ofsamples with polar surfaces. (b) and (c) The
Fermi surfaces on both the polar surfaces
by laser ARPES. (d) and (e) Banddispersions along high symmetry cuts.

surfaces of WTe2: surface A and B. We used the 7 eV laser ARPES to map the energy contours
and measured the band structures. We found two kinds of ARPES results on the polar surfaces.
SARPES measurements on candidate topological surface states were taken after the ARPES
measurements. We distinguished surface states from bulk states.
Results
We measured two kinds ofsurfaces of WTe2: surfaces A and B
with 7 eV laser-ARPES. In Fig. 1(b)-(e), we can see the arc-likesurface states on both surface A and
B. The surface states of the opposite
surfaces are different because of thelack of inversion symmetry as
expected.
Figure 2 shows the SARPES
results of the candidate topologicalsurface states. The spin polarizationof bulk bands inversed on the polar
surfaces, while surface states keep
the same direction. We also found asurface resonance state generated by
the mixture of the surface state andthe hole pocket, suggesting that thehole and electron pocket also
connect below the Fermi level,
which is not predicted by theoreticalstudies. The band structures arelinear near the contact point with a
Dirac-like cone.
We observed two kinds ofsurfaces of type-II Weyl semimetal
WTe2 with laser-ARPES. Clear
surface states which connect the bulk
hole- and electron-like pockets could
be seen. By SARPES measurements,
we found that the surface states are
all spin-polarized, which arecompatible with topological Fermi
arcs.

Fig.2 (a) and (e) The ARPES images of cut 3 on surface A andB, respectively. The red (pink)/blue (light-blue) dashed lines
indicate the band dispersions with sy-up/down polarization.
(b),(c),(f) and (g) Laser-SARPES images for sy -up anddown.(d) and (h) Energy distribution curves (EDCs) for up (red)and down(blue) along dashed line in (b), (c), (f), and (g). The
marks indicated the positions of the peaks: light-blue and pinkrounds for bulk states and blue and red for surfaces, which arealso plotted on (b), (c), (f), and (g) . (i)-(p) Similar set for spinx component along cut 2 on surface A and B.

REFERENCES[1] A.Soluyanov et al. Nature 527.7579 (2015).
[2] Y.Wu et al., Phy. Rev. B 94, 121113(R) (2016).
[3] F.Y.Bruno et al., Phy. Rev. B 94, 121112(R) (2016)[4] W.Zhang et al., Phy. Rev. B 96, 165125 (2017)

DEVELOPMENT OF TIME-, SPIN- AND ANGLE-RESOLVEDPHOTOEMISSION SPECTROSCOPY MACHINE AT ISSP
K. Kawaguchi, K. Kuroda, Z. Zhao, A. Harasawa, K. Yaji, R. Noguchi, S. Tani,
M. Fujisawa, S. Shin, F. Komori, Y. Kobayashi, T. Kondo
The Institute for Solid State Physics (ISSP), The University of Tokyo
Spin- and angle-resolved photoemission spectroscopy (SARPES) is a powerful technique to
directly access spin-polarized electronic structures [1]. In 2014, We have newly built thelaser-SARPES machine at ISSP, which is based on the 6.994-eV laser generated by KBBF
non-linear crystal and the double VLEED-type spin detector [2]. Thanks to the high-flux lightsource as well as the high-efficiency of the detectors, our SARPES machine enables us toreveal the various spin-polarized states and the related phenomena.
Recently, we have started to a project to upgrade our laser-SAPRES machine by combining
femtosecond laser pulses to perform pump-probe time-resolved SARPES (tr-SARPES). In
contrast to the standard SARPES, this technique allows us to investigate the non-equilibrium
spin-polarized electronic states and its spin dynamics disentangled from the charge dynamics.
We have installed a new 10.7 eV-laser pulse developed by Kobayashi group of ISSP [3],which is based on the Yb-doped fiver with the high repetition rate (1 MHz). The 10.7 eV laser
is generated by the third harmonic generation of the 347-nm driver in Xe gas. The
high-energy probe pulse will cover the entire first Brillouin Zone (BZ), which is a great
advantage compared to the conventional 6 eV-lasers based on Ti:sapphire laser.
In this activity report, we report the status of the machine development. At this stage, wehave detected the SARPES signals probed by 10.7 eV laser. We here will show its
demonstrations on polycrystalline-Au, and Bi/Si (111) films.
We characterize energy resolution bymeasuring polycrystalline gold plate(Fig. 1). By fitting analysis of theenergy distribution curves (EDCs) by a
Fermi-Dirac function convoluted by
Gaussian, we estimate the energy
broadening of the laser itself to be 20
meV. This value is reasonable if we
consider the band width of thefundamental laser generated by the
Yb-doped fiber. We also find that there
is no significant space charging effect,
which is the best advantage of thehigh-repetition laser.
Next, we mapped the band dispersionsof Bi/Si (111) film. Fig. 2(a) shows theobserved Fermi-surfaces covering the

Fig. 1: EDCs recorded by 10.7 eV laser on thepolycrystalline gold plate at 17 K. Dashed curve
is a fitting function to deduce the energy
resolution of the laser pulse itself.

BZ. We also measured the clear band dispersions of Bi/Si (111) in wide energy-momentum
windows [Fig. 2(b)]. This guarantees that our laser system is stable for SARPES and ability toinvestigate the spin-polarized band structure for entire first BZ. We are now installing thepump pulses to perform the pump-probe tr-SARPES measurement.

Fig. 2: (a) The Fermi surface of Bi/Si (111) films. 10.7 eV laser is p-polarization set up,
and analyzer slit is along ΓM line. (b) Band dispersion image along Γ-M direction.

References:[1] J. H. Dil, Electron. Struct. 1, 023001 (2019).
[2] K. Yaji et al., Rev. Sci. Instrum., 87, 053111 (2016).
[3] Z. Zhao et al., Opt. Exp. 25, 13517 (2017).

THE WEAK TOPOLOGICAL INSULATOR STATE IN ZRTE5Peng Zhang1, Ryo Noguchi1, Kenta Kuroda1, Chun Lin1, Kaishu Kawaguchi1, Koichiro Yaji2,
Ayumi Harasawa1, Simin Nie3, Hongming Weng4, V. Kandyba5, A. Giampietri5, A. Barinov5,
Qiang Li6, G.D. Gu6, Shik Shin7,1, and Takeshi Kondo1
1

Institute for Solid State Physics, University of Tokyo, Kashiwa, Chiba 277-8581, Japan. 2ResearchCenter for Advanced Measurement and Characterization, National Institute for Materials Science,
Tsukuba, Ibaraki 305-0003, Japan. 3Department of Materials Science and Engineering, Stanford
University, Stanford, California 94305, USA. 4Beijing National Laboratory for Condensed Matter
Physics and Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China. 5Elettra Sincrotrone Trieste, Basovizza, Italy. 6Condensed Matter Physics and Materials Science
Department, Brookhaven National Laboratory, Upton, NY, USA. 7Office of University Professor,
University of Tokyo, Kashiwa, Chiba 277-8581, Japan

Two distinct topological insulators exist in three dimensions: strong topological insulator(STI) and weak topological insulator (WTI). Among them, STI have been widely studied in
the past decades both in theories and experiments. There are 2D spin-momentum locked Diraccones on all the surfaces, in which the perfect backscattering is prohibited, while generalscattering away from 180 degree still exists. On other hand, there are many theory studies on
WTI but experiments are still rare. WTI host surface states only on the side surfaces. They arethought to be weak since two adjacent layers in even-layer WTI may couple with each other,leading to a topologically trivial phase. However, it is found that WTI surface states are
actually robust as a results of delocalization. Furthermore, surface bands of WTI generally
have a quasi-1D dispersion. Similar to the quantum spin Hall edge states, there are lessscattering channels than STI, and smaller scattering at the side surface is expected. However,experiments on surface states of WTI are rare, due to the difficulty to prepare a large anduniform side surface of layered materials. Cleavage is generally useful for top surface, andsuch techniques proved its power on STI. In nature, most materials can only be cleaved fromtop surface, and it is difficult to prepare a side surface. The only example is Bi4I4, which has
two cleaved surface at the same time (specially) [1]. ZrTe5 is another candidate of WTI [2].
Although there are some spectroscopic studies on the top surface, there is no study on the sidesurface. The key evidence for WTI of ZrTe5, the results from side surface, is missing, and theproperties of the surface band is absent.
ZrTe5 has a quasi-1D crystal structure. The layer distance for the top surface is ∼ 7.3 Å, and itis ∼ 6.9 Å for the side surface. As a result of the large layer distances, in principle, it should bepossible to cleave both surfaces. However, Te-Te bonding exists between adjacent layers forthe side surface, making it much more difficulty to cleave than the top surface stacked by van
der Waals forces. We successfully cleaved the side surface of WTe5, and measured its band
structure with laser ARPES. We resolved a Fermi surface only dispersing along the chain
direction (a), as displayed in Fig. 1a. The ARPES intensity plot along ka shows a hole-likeband. By comparing with the calculations [2], we found this band should be the surface band.
The spin polarization is expected for the surface band.
We measured two pairs of EDCs (Fig.1a) by spin-resolved ARPES from three different
samples. The ka locations are also shown in Fig.1b. The spin polarization curves along the b, aand c directions are displayed in Fig.1c - e. In each panel of Fig.1c - e, the five curves withthe same color (blue or red) are taken at two different kb (kb ∼ 0 and 10, indicated in Fig.1a)

-10
-15
-10 -5

L
0

5

-0.1
-0.2
-0.3

10

-10 -5ka (degree)Surface band

H

0
-5

H

0

L

-10 -5 0

5 10

ka (degree)

Polarization

f

0.4
0.2

ka (degree)dSpin Pb

L
5 10

eSpin Pa

Spin Pc

0.0-0.2

Background

Surface band

-0.4
0.4
0.2

g
Spin Pb

EDC 2hSpin Pa

Spin Pc

0.0-0.2

average and remove background

Polarizationc

H

Background

0.0

5

E - EF (eV)kb (degree)

10

EDC 2

0.1

EDC 1

b

15

BackgroundaSurface bandfrom three samples (one sample at only kb ∼ 0, in total 6 - 1 = 5 curves). These five curves
show no obvious difference, confirming the repeatability of the measurements. We notice thatthe spin polarization curves in the background range (< -0.15 eV) are not zero but have offsets.
Such constant polarization in the background may come from the asymmetry of the spin +and spin - channels, or may come from the matrix-element effect in the photoemission process,which is often observed in spin-resolved ARPES. We averaged the five polarization curves
with the same color in Fig.1c - e, and subtracted the resulting curves by a constant to remove
the background polarization. The results are displayed in Fig.1f - h. It is clear that the spin ispolarized along b direction, tangent with the Fermi surface, forming the quasi-1D
spin-momentum locking pattern. Our results prove the existence of topological surface stateson the side surface, and the weak topological insulator nature of ZrTe5.

EDC 1

-0.4
-0.25 -0.20 -0.15 -0.10 -0.05 0.00

-0.20 -0.15 -0.10 -0.05 0.00

-0.20 -0.15 -0.10 -0.05 0.00

E - EF (eV)Figure 1. Spin polarization calibration. (a) Fermi surface of the side surface. The redand blue dots indicate the locations in k-space of the spin-polarized EDCs in (c - h). (b)Intensity plot of the band structure along ka and its MDC curvature plot. The red and bluelines indicate the ka locations of the spin-polarized EDCs. Due to the quasi-1D feature of
the Fermi surface, the band structure at kb ∼ 0 and kb ∼ 10 in (a) are basically the same.
(c - e) Raw spin polarization curves along b, a and c directions measured at the indicatedlocations in (a) and from different samples. The polarization curves at the same ka arealmost the same (independent of kb locations or samples). So the curves are justdistinguished by their ka values with red and blue colors. (f - h) Spin polarization curves
along b, a and c directions, obtained by averaging the same-color curves in (c - e),respectively. Since the background part (< -0.15 eV) should show no spin polarization,
we subtracted a constant for all the curves to remove the background spin polarization.

References:[1] R. Noguchi. et al. Nature 566, 518 (2019).
[2] H. Weng, Dai, X. & Fang, Z. Phys. Rev. X 4, 011002 (2014).

Observation of spin polarized surface states in polar Weyl semimetal
Td-MoTe2
M. Sakano1, Y. Tanaka1, M. S. Bahramy1, 2, H. Takahashi1,3, K. Kuroda4, A. Harasawa4, K.
Yaji4, S. Shin4, S. Ishiwata1,3, K. Ishizaka1, 2
1Quantum-Phase Electronics Center (QPEC) and Department of Applied Physics, The
University of Tokyo
2
RIKEN Center for Emergent Matter Science (CEMS)
3
Graduate School of Engineering Science, Osaka University
4
Institute for Solid State Physics, The University of Tokyo
The electronic structure in the low temperature phase of Td-MoTe2, which is theoretically predicted
as a type-II Weyl semimetal, has been intensively investigated by angle-resolved photoelectron
spectroscopy [1-6]. Until now, some segment-like band features resembling Fermi arcs areexperimentally observed, which are indicative of the topological surface states connecting the bulk
Weyl nodes [5,6]. However, since the complicated electronic structure near the Fermi level makes it
difficult to be observed, the interpretations of ARPES results on the observed Fermi-arc-like bands arestill controversial. Here we investigate the spin polarizations of the Fermi arc features for both polar
terminations by utilizing the high-resolution spin and angle-resolved photoelectron spectroscopy(SARPES) [7].
Td-MoTe2 has the noncentrosymmetric crystal structurewith polarization along the stacking direction (c-axis), so
it has the “top” and “bottom” polar domains. The top
surface is denoted by (001) and the bottom surface isdenoted by (00-1). According to the band calculations,
there are eight Weyl points in the first Brillouin zone of
Td-MoTe2. Their energy levels and coordinates are
predicted to be (E, kx, ky, kz) = (EF + 6.7 meV, ±0.185,±0.013,0) (W1s : near the Γ-X line) and (EF + 59 meV, ±0.181, ±0.053, 0) (W2s : the far from Γ-X line) by thetheoretical research[21]. Fig. 1(a) and (b) show the energy
contours at EF + 6.7 meV for the (001) and (00-1) surfaces
respectively. On the (001) surface, an arc-like surface state
dispersing from the W1 can be seen between the hole
pocket and the electron pocket. In addition, there isanother surface state that crosses the Γ-X line near the
Fig. 1 The calculated energy contour
electron pocket, indicated by “SS”. On the (00-1) surface,at EF + 6.7 meV for (001) (a) andthere is a sharp surface state that connects the pair of W1s(00-1) (b) surfaces.
with different chirality, indicated by the white arrow in
Fig. 1(b).
Fig. 2(a) shows the energy contour at EF on the (001) surface obtained by ARPES at 20 K. The two
types of sharp photoelectron intensities are observed indicated by the pink solid brackets and the green
curve. The pair of segments is the possible Fermi arcs connecting W1 and W2 from the comparison
with the calculation shown in Fig. 1(a). The state indicated by the green curve is also a surface state
but can be distinguished from the Fermi arc.
Next, looking at the energy contours at EF + 20 meV for the (00-1) surface shown in Fig. 2(b), a shortsegment of the photoelectron intensity can be observed at kx ~ 0.20 Å-1 along the Γ-X line, as indicatedby the pink marker. This segment is the possible Fermi arc connecting the pair of W1 across the Γ-Xline, as shown in the calculation for the (00-1) surface (Fig. 1(b)).
To investigate the spin structures of the topological surface states, we perform SARPES along Fermi
arcs and the SS. On the (001) surface, the SARPES measurements were performed along the orange
lines (#0~#7) shown in Fig 2(a). Figs. 2(c)~(e) show the spin-resolved spectrums along #0 ~ #7 for theSx, Sy and Sz respectively. The measurement position (#0 ~ #7) in momentum space is calculated fromthe detector angle shown on the horizontal axis of Figs. 2(c)-(e). The red and blue curves represent thephotoelectron intensities of the up spins and down spins, respectively. From the ARPES results shownin Fig. 2(a), there are the Fermi arc and the SS, on #0, #2~#7. The peak positions shown in the spin
resolved momentum distribution curves (MDCs) are the Fermi arc and SS peak positions obtained byfitting the MDCs by the normal ARPES measurement shown in Fig 2(a). In the spin-resolved
spectrums for Sx shown in Fig. 2(c), the only one peak is observed on the #1, whereas two peakstructures are observed on the #2 reflecting the two different surface states, so we can say that the Arc
and the SS can be measured by SARPES. The spin resolved MDCs for Sy (Fig. 2(d))shows that the SS
has Sy up polarization on the Γ-X line, the up-spin spectrums have the two-peak structure in the regionslightly off the Γ-X line (#2-#5), so we can see that the Sy of the Fermi arc near W1 is up on the (001)surface. For Sz, the Arc and SS seem to have the opposite polarization near the Γ-X line, but they havelarge Sz-up polarization far from the Γ-X line. Next, we move on to the results of the (00-1) surface.
Fig. 2(b) shows the energy contour at EF + 20 meV on the (00-1) surface. SARPES is performed along
the orange line in Fig. 2(b), so the peak in the 3D spin-resolved MDCs shown in Fig. 2(f)-(h)corresponds to the peak of the Arc. There seem to be no difference in the photoelectron intensities of
the up and down of Sx, but the MDC of Sy-down ARPES intensities shown in Fig. 2(g) has a sharp
peak which is consistent with the peak position of the Arc observed by the normal ARPES. Finally,
the MDC of Sz-up ARPES
intensities shown in Fig. 2(h) has apeak structure at the same position
with the result of the normal
ARPES, so we can see the Arc
have
the
small
Sz-downpolarization.
We focus on the spin components
along y of the Arc in the vicinity ofthe Γ-X line. The Arc of the (001)surface has Sy-up polarization (Py ~0.4) just outside of the pair of W1
at ky ~ ±0.01 Å-1. On the other
hand, the Arc on the (00-1) surface
at ky = 0 has Sy-down polarization(Py ~ -0.6). In other words, the signof the spin polarizations of the
Fermi arcs on the top and bottom
surface reverse at the W1s, thejunction point between the Arcs.
This feature seems to reflect the
Berry flux emerging from the
Weyl points, as the source and sinkof Berry flux in momentum space.
REFERENCES[1] A. A. Soluyanov, et al.,Fig.2 ARPES energy contour at EF for (001) (a) and at EF + 20
Nature 527, 495(2015).
meV for (00-1) (b). (c)-(e) 3D SARPES momentum distribution[2] Y. Sun, et al.,Phys. Rev. B
curves along the orange lines in (a). (f)-(h) 3D SARPES
92 161107 (2015).
momentum distribution curves corresponding to the orange[3] Z. Wang, et al.,Phys. Rev.
line in (b).
Lett. 117, 056805(2015).
[4] L. Huang, et al., Nat. Mater.
15, 1155 (2016).
[5] A. Tamai, et al., Phys. Rev. X 6, 031032 (2016).
[6] M. Sakano, et al., Phys. Rev. B 95, 121101 (2017).
[7] K. Yaji et al., Rev. Sci. Instrum. 87, 053111 (2016).

SARPES STUDY OF A PB ATOMIC LAYER ON GE(111)1

Koichiro Yaji1 and Fumio Komori2
Research Center for Advanced Measurement and Characterization,
National Institute for Materials Science

2

Division of Nanoscale Science, Institute for Solid State Physics, The University of Tokyo

Strongly spin-orbit coupled materials, such as topological insulators and Rashba systems, areplaced at a central research field in condensed matter physics not only because of fundamental
interests but also spintronic applications. In the standard model of the strongly spin-orbit
coupled systems, the spin direction is perpendicular to both the direction of surface normal andthe momentum of an electron. Recently, in contrast to the scenario of the spin-momentum
locking, the spin-orbital locking model was reported in several papers [1]. Also, in our previous
studies [2,3], we demonstrated the spin-orbit coupling of surface states on Bi(111) and Bi2Se3
with spin- and angle-resolved photoemission spectroscopy using a vacuum ultraviolet laser(laser-SARPES). Here we show the spin-orbit coupling of surface states of a monolayer Pb film
on Ge(111) (Pb/Ge(111)-b) investigated by laser-SARPES. Moreover, we demonstrate theincident-light-angle dependence of SARPES.
SARPES measurements were performed at E-building in the Institute for Solid State Physics[4]. The Pb/Ge(111) sample was in-situ prepared in a molecular beam epitaxy chamber which
was directly connected to the analysis chamber. We used an n-type Ge(111) substrate. The cleansurface was obtained with the several cycles of Ar+ ion sputtering with the ion energy of 0.5
kV and subsequent annealing at 600 °C. Pb was evaporated from an aluminum crucible. The
thickness of Pb and the orderliness of the monolayer film was judged from low-energy-electrondiffraction pattern. The photoelectrons were excited by 6.994-eV photons and were analyzed
with a ScientaOmicron DA30-L analyzer equipped with very-low-energy-electron-diffraction
type spin detectors. The electron deflector function is useful for studying the light-incidentangle dependence of SARPES. The effective Sherman function was 0.28. The sample
temperature was kept at 40K during themeasurements.
Figure 1 shows an ARPES image recorded
along GK in the surface Brillion zone of
Pb/Ge(111)-b, being parallel to the mirror
plane of the surface. We find several bands
around G, which are ascribed to thesubsurface states of Ge(111) [5]. The bands
crossing the Fermi level (EF) around k ~ 0.400.45 Å-1 are Pb-derived states, where thesplitting is due to the Rashba effect [6]. The
bands lying in the energy range of 0.3-0.8 eV
below EF around k > 0.3 Å-1 are also Pbderived states and show semiconducting Fig. 1 The ARPES image of Pb/Ge(111)-b along
behavior. The overall shapes of the bands GK recorded with the photon energy of 6.994 eV.
observed with hn = 6.994 eV well agree with The color arrangement gives the information of thephotoelectron intensity.
those with hn = 21 eV reported in ourprevious study [7].
The SARPES measurements were performed with p- and s-polarized lights [Fig. 2], which
could be regarded as orbital-selective SARPES. Here, the light incident angles were set to 20°
and 40°. We used the spin detector for resolving the y-spin component, which was perpendicularto the mirror plane of the surface. As shownin Fig. 2, we clearly demonstrate that theobserved spin polarizations are inverted
upon switching the light polarizations. This
indicates that the symmetric and antisymmetric orbitals are independently
coupled to mutually opposite spins as inother spin-orbit coupled systems such as
Bi(111) and Bi2Se3. The total spinpolarization is obtained from the integrationsof the orbital-selective spin polarizations
with p- and s-polarized lights. For the lightincident angle of 20°, the spin-polarized
branches show opposite spin directions eachother, which is in agreement with thescenario of the Rashba effect. On the other
hand, for the 40° incidence, the total spinpolarizations are in the same directions,which is against the Rashba scenario and Fig. 2 Laser-SARPES images for the Pb-derived
violates the time reversal symmetry. surface states along GK recorded with the light
Besides, the theoretical calculation suggests incident angles of 20° (upper panels) and 40° (lower
that these branches exhibit the Rashba-type panels) with the p-polarized light (left panels) and thespin texture [7]. Therefore, we conclude that s-polarized light (center panels). The right panels
the spin polarization observed with SARPES represent the orbital-integrated SARPES image.
White and black colors give the information of thewith the incident angle of 40° is largely negative-positive spin polarizations. The left-upper
influenced by the final-state effect in panel is a schematic drawing of the experimental
photoemission process. It is likely that the geometry.
photoionization cross sections between thep- and s-polarizations are different. We have to mind that the total spin polarization is notalways derived from the integrations of the orbital-resolved ones.
REFERENCES[1] Z.-H. Zhu et al., Phys. Rev. Lett. 110, 216401 (2013), Y. Cao et al., Nat. Phys. 9, 499 (2013), H.
Zhang et al., Phys. Rev. Lett. 111, 066801 (2013), Z. Xie et al., Nat. Commun. 5, 3382 (2014).
[2] K. Kuroda et al., Phys. Rev. B 94, 165162 (2016).
[3] K. Yaji et al., Nat. Commun. 8, 14588 (2017).
[4] K. Yaji et al., Rev. Sci. Instrum. 87, 053111 (2016).
[5] K. Yaji et al., J. Electron Spect. Rel. Phenom. 201, 92 (2015).
[6] K. Yaji et al., Nat. Commun. 1, 17 (2010).
[7] K. Yaji et al., Phys. Rev. B 86, 235317 (2012).

HIGH-RESOLUTION SPIN-RESOLVED ELECTRONIC STRUCTURE
OF TOPOLOGICAL HEAVY-FERMION MATERIALS
Andrés F. Santander-Syro, Franck Fortuna, Maximilian Thees
Institut des Sciences Moléculaires d'Orsay (ISMO), Université Paris-Saclay and CNRS, France

Cris Adriano
Instituto de Física “Gleb Wataghin”, UNICAMP, Campinas-SP, 13083-859, Brazil

Ce-based intermetallic compounds have interesting physical properties that arise from theinterplay between Ruderman-Kittel-Kasuya-Yoshida (RKKY) magnetic interaction,
crystalline electric field and Fermi surface (FS) effects in connection to a strong
hybridization between the Ce3+ 4f electrons and the conduction electrons. Ce-based materials
often present non-trivial ground states, such as heavy fermion, unconventional
superconductivity and non-Fermi-liquid behavior, which frequently appear in the vicinity of amagnetically ordered state. Interestingly, some of these properties, seem to be favored tooccur in low symmetry systems such as tetragonal structure, well-known examples are thefamilies of CeMIn5, Ce2MIn8 (M = Co, Rh, Ir, Pd), CePt2In7 and CeCuSi2.
In this context, recent attention was given to the CeTX2 family (T = transition metal; X =pnictogen), which hosts both ferromagnetic (FM) and antiferromagnetic (AFM) memberswith complex magnetic behavior [1]. In particular, in this project we investigated, using
laser-excited spin- and angle-resolved photoemission spectroscopy (Laser-SARPES), thecompound CeCuBi2 that crystallizes in the tetragonal ZrCuSi2-type structure (P4/nmm [1]).
Previous studies show that CeCuBi2 has an AFM ordering at TN = 16 K with a large magnetic
anisotropy [1].
As shown in Figure 1, in the AFM phase we detected a dispersive hole-like state with afinite spin polarization. This state may arise from unpaired or incompletely paired moments
near the material surface, as also observed in other AFM heavy-fermion systems [2]. Further
data analyses and complementary spin-integrated ARPES measurements are currently being
performed to clarify its origin.

Figure 1. (a) laser-ARPES energy-momentum map showing a hole-like state right below the Fermi level in
CeCuBi2. (b) Bottom panel: spin-resolved energy distribution curves along the momentum marked by the redline in (a); top panel: net spin polarization.

REFERENCES[1] Cris Adriano et al., Phys. Rev. B 90, 235120 (2014).
[2] A. Chikina et al., Nat. Commun. 5, 1-7 (2014).

Study of spin structure in domain-regulated alloy of atomic layer
Ting-Yu Chen1, Li-Cheun Yang2, Santosh Chiniwar1, H. -T. Jeng1, Woei Wu Pai3,
S. -J. Tang1,2,
1. Department of Physics, National Tsing Hua University, Hsinchu 30013, Taiwan
2. National Synchrotron Radiation Research Center (NSRRC), Hsinchu 30076,
Taiwan3. Center for Condensed Matter Sciences, National Taiwan University, Taipei 106,
TaiwanThe enhanced Rashba effect of a 2-dimestional (2D) surface alloy has been a centerof interest in surface-science community. A typical Rashba effect in a 2D system ismainly contributed by the out-of-plane electric field caused by the inversion –symmetry
breaking perpendicular to the surface plane. Consequently, the k momentum ofdegenerate surface-state bands locked with opposite in-plane spin polarizations shift inpositive and negative directions, respectively, forming a concentric 2D constant-energy
contour (CEC). However, previous studies on 2D surface alloys revealed the enhanced
Rashba effect correlated with possible in-plane electric field and thus out-of-plane spinpolarizations. Such effect was attributed to the second-order warping term of spin-orbit
interaction in addition to the first-order term for the typical Rashba effect.
Our group found that the surface-state bands of GeAg2 surface alloy formed on
Ag(111) exhibits anomalous splitting centered at surface zone center Γ as shown in
Fig. 1. The splitting is obvious in one symmetry direction ΓM Ag2Ge but negligible inanother ΓK Ag2Ge . The corresponding 2D CECs centered at Γ is displayed in Fig. 2(a),exhibiting an entangled configuration of inner hexagon and outward snowflakes thatrotate 30° with each other.

Fig. 1: The measured energy band dispersions for Ag2Ge on Ag(111) in two symmetry directions

(a)

(b)

Fig. 2: (a) The measured 2D constant energy contours at −1.0 eVfor surface-state bands of Ag2Ge
surface alloy on Ag(111) centered at Γ and (b) the corresponding fitting result.

We employed the spin-orbit interacting model of the first-order Rashba term and thesecond-order Warping term to simulate the measured CECs in Fig. 2(a); the match isvery agreeable as shown by the orange fitting curve in Fig. 2(b). Moreover, the fitting
result shows that the contribution to the splitting is dominantly from the Warping term,
being in complete contrast to the behaviors in most surface alloys where the first-order
Rashba term dominates. If it is the case, the spin polarizations of the surface-state bandsof Ag2Ge surface alloy centered at Γ would be dominantly out-of-plane. This
motivated us to come to the Synchrotron Radiation Laboratory in the Institute for Solid
State Physics, The University of Tokyo, Japan to use the spin- and angle-resolved
photoelectron spectroscopy (SARPES) to measure the spin texture of surface state
bands of Ag2Ge surface alloy. Figure 3 shows the scanning-tunneling-microscopy(STM) image of Ag2Ge surface alloy coexisting with striped-phase germanene which
form triangle arrays enclosing the Ag2Ge surface alloy. We believe the triangle arrays
break the in-plane symmetry to cause the in-plane electric field.
Figure 4 shows the preliminary results of our spin-resolved photoemission
measurement carried out in SARPES machine time in Nov 2019. The applied magnetic
field was along the out-of-plane direction (z direction). Indeed one can see the energy
distribution curves (EDCs), taken at off-normal 8°, corresponding to two opposite
directions (±z) of magnetic fields reveal difference in intensities for the two peaks ofthe split surface-state bands. However, it is a bit abnormal that the two peaks of red
EDC are both higher than those of blue EDC. I think the surface normal of our sample
was not completely aligned with the detector axis during measurment. In our next
SARPES machine, we will be much more careful in those measurement details.

Fig. 3: The STM image of Ag2Ge surface alloy coexisting with striped-phase germanene in the form of
triangle arrays.

Fig. 4: The preliminary result of spin-resolved photoemission measurement on the split surface-state
bands of Ag2Ge surface alloy. Red and blue EDCs correspond to opposite directions (±z ) of magnetic
fields.

Spin resolved ARPES on interfacial electronic states of Bi/NbSe2
R. Stania1, H. Jung1,2, H.W. Yeom1,2
1Center for Artificial Low Dimensional Electronic Systems, Institute for Basic Science (IBS),
Pohang 37673, Republic of Korea
2Department of Physics, Pohang University of Science and Technology (POSTECH), Pohang
37673, Republic of Korea

Introduction
Bulk, as well as low-dimensional, bismuth (Bi) have provided extremely rich platforms toadvance our knowledge in condensed matter physics, ranging from high thermoelectric
properties[1] to giant Rashba effect with highly spin-polarized electrons [2][3]. Moreover,there have recently been much interest in the topological classification of Bi, and it has beenreported that pure Bi is topologically non-trivial in bulk [4][5], as well as in thin film[6].
More recently, it has also been reported that due to the higher-order topology, Bi also
possesses topologically protected hinge (edge) states even in bulk form [7]. Such new
identifications of Bi are expected to open up further possibilities for Bi-based structures,
particularly when combined with other novel materials.
In order to utilize the rich properties of Bi-based structures, both to explore further
fundamental aspects and for future device applications, it is essential to understand theelectronic interactions of low-dimensional Bi on various substrates, particularly at theirinterfaces (e.g., topological quantum computations when combined with superconductors [7]).
Of special interest are two-dimensional (2D) layered van-der-Waals materials (vdW) which
are known to host exceptional spin-related properties and have strong potential for spintronic
applications [8].
Thus, in our experiments, we aimed to investigate the electronic structures of ultra-thin Bi andits interfacial electronic states on a transition metal dichalcogenide (TMDC). In particular,
2H-NbSe2 not only shows a phase transition from metallic state to charge density wave (CDW)
state [9], but also towards superconducting state at one of the highest transition temperatures
amongst the TMDCs [10]. In addition, NbSe2 was recently predicted to be topologically nontrivial [11]. Thus, Bi/NbSe2 is an ideal playground to explore such interactions and an
important step towards the investigation of topological superconductivity and Majorana
fermions.
Experimental details
The NbSe2 samples were cleaved under UHV conditions and the surface quality wasconfirmed using LEED and He IIα. The Bi layers were grown in a separate chamber andcoverage was determined using He IIα excitation. The high resolution ARPES and SARPES
measurements were performed using a laser light source with a photon energy of 6.994 eV atthe laser-SARPES machine developed at ISSP [12]. All photoemission experiments werecarried out at sample temperatures below 35 K under UHV condition (about 1x10-8 Pa).

Results and discussion
By using high resolution ARPES we were able to distinguish the bands around the Γ point
which not only shows the expected bands for freestanding few-layer Bi(111) but also
additional bands that could not be clearly allocated and may resemble interface states (Figure1a). Spin dependent measurements reveal a strong spin texture (Figure 1b) for the TSS around
EB=0.7 eV and especially for the surface state near the Fermi level, which was expected to beless strong polarized for few BL Bi films. More structural investigation and theoretical
calculation will be required in order to determine the origins of the sub-bands.

Fig. 1 a) Spin integrated measurement of Bi/NbSe2 along the ΓK direction; b) spin dependent EDC along thedashed line in a) with red (blue) corresponding to out of plane spin up (down) measurements and green thepolarization

References[1] B. Poudel, et al., Science, 320, 634 (2008)[2] Y. Koroteev, et al., Phys. Rev. Lett., 93, 046403 (2004)[3] T. Hirahara, et al., Phys. Rev. B, 76, 153305 (2007)[4] S. Ito, et al., Phys. Rev. Lett., 117, 35 (2016)[5] Y. Ohtsubo, et al., New J. Phys., 18, 123015 (2016)[6] I.K. Drozdov, et al,. Nat. Phys., 10, 664 (2014)[7] F. Schindler, et al,. Nat. Phys., 14, 918 (2018)[8] D. Pensin, A.H. MacDonald Nature Mater., 11, 409 (2012)[9] D.E. Moncton, et al., Phys. Rev. Lett., 34, 734 (1975)[10] R. F. Frindt Phys. Rev. Lett., 28, 299 (1972)[11] M. S. Bahramy, et al., Nature Mater., 17, 21 (2017)[12] K. Yaji, et al., Rev. Sci. Instrum., 87, 053111 (2016)

HIGH-RESOLUTION SPIN- AND ANGLE-RESOLVEDPHOTOEMISSION SPECTROSCOPY ON DOPED MAGNETICTOPOLOGICAL INSULATORS
Keisuke Fukutani1,2, Chandan De1, Roland Stania1,2, Koichiro Yaji3, Kenta Kuroda3,
Jaeyoung Kim1,2, and Han-Woong Yeom1,41Center for Artificial Low dimensional Electronic Systems, Institute for Basic Science (IBS), Republicof Korea2
Pohang Accelerator Laboratory, Pohang University of Science and Technology (POSTECH),
Republic of Korea
3
The Institute for Solid State Physics, The University of Tokyo, Japan
4
Depertment of Physics, Pohang University of Science and Technology (POSTECH), Republic of
Korea

The topological classifications in condensed matter physics have led to a paradigm shift in
our understanding of the properties of solids, where the fundamental characteristics of matterscan be determined not only by their symmetries and many-body interactions but also by their
topological orders [1]. Such new classification schemes, have allowed for the predictions anddiscoveries of various new types of materials, leading to the realizations of intriguing
electronic properties [2-7]. Among them, the interplay between the topological order andmagnetism (broken time-reversal symmetry) has garnered much attentions as it is expected tolead to various exotic phenomena, such as quantum anomalous Hall effect [8] and topologicalmagnetoelectric effect [9], and have been extensively studied to date [10].
Recently, a discovery of an intrinsic antiferromagnetic topological insulator MnBi2Te4 has
been reported [11-13], followed by extensive investigations by various experimental andtheoretical means on this material as well as its hetero-structural cousins(MnBi2Te4)n(Bi2Te3)m [14-17]. On the other hand, despite the expected broken time-reversal
symmetry, the electronic band structures of these materials are variously reported with regard
to the presence of the band gap in the Dirac cone [18-23], which is at the heart of topologicalelectronic properties. While there are several proposed hypotheses to explain suchdiscrepancies [20,22,23], the general consensus has not been reached.
The keys to resolve such issue can be provided by high-resolution spin- and angle-resolved
photoemission spectroscopy (SARPES), which allows us to simultaneously probe thetopological surface Dirac band and their momentum-dependent spin textures. Since it is
known for these materials that the microscopic domain sizes and the presence of bulk bandprojections, pronounced above the photon energies of hv ~10 eV [11], can obscure theunambiguous observation of the Dirac bands, utilizations of the micro-spot-sized, laser-based(hv = 7 eV) SARPES available at the Institute for Solid State Physics (ISSP) at the Universityof Tokyo would be an ideal experimental probe to tackle the present issue.
In our experiments, we have performed laser-SARPES measurements on the pristine as well
as Sb-doped (MnBi2Te4)n(Bi2Te3)m at the temperatures above and below the magnetic phasetransition temperatures and have revealed their electronic structures as well as thecorresponding spin textures near the Γ point in the surface Brillouin zone. Fig. 1 shows the
ARPES intensity plots and the corresponding Fermi surface mapping for the pristine
MnBi2Te4 and MnBi4Te7. It can be clearly seen that the both materials possess gapless Diraccones centered at the Γ point with the Dirac point (ED) located at the binding energy of ~2.7
eV with no clearly identifiable band gap. These observations are consistent with some of theearlier ARPES studies [20-23], and likely point to the possibility that near the surface, themagnetic configurations are different from that of the bulk (i.e., possible surface magnetic

reconstructions), so as to provide the effective protection from gap-opening even in theabsence of the time-reversal symmetry [15]. In order to identify the magnetic configurations
near the surface and to identify the possible origin of the gapless Dirac cones, we have further
obtained the spin-resolved ARPES data for the pristine as well as the Sb-doped samples. The
spin-resolved data show unconventional spin textures for topological surface states and are
currently under further investigation in conjunction with theoretical calculations as well astransport measurements.

Figure 1: ARPES intensity maps for (a-c) MnBi2Te4 and (d-f) MnBi4Te7. (a) and (b) schematically show thelayer-stacking sequences for each of the materials, where the topmost layers are determined to be both MnBi2Te4.
(b) and (e) show the Fermi surface mappings and (c) and (f) show the band dispersions around the Γ point. The
Dirac point ED, where the upper and lower Dirac cones meet in the gapless dispersions are indicated by thearrows for both materials. The data were obtained with the s-polarized 7-eV laser at the temperature of T = 10 K.

REFERENCES[1] M. Z. Hasan and C. L. Kane, Rev. Mod. Phys. 82, 3045 (2010).
[2] C.-Z. Chang et al., Science 340, 167 (2013).,[3] C.-K. Chiu et al, Rev. Mod. Phys. 88, 035005 (2016).
[4] P. Zhang et al., Science 360, 182 (2018),[5] C.X. Trang et al., Nat. Commun. 11, 159 (2020).
[6] L. Fu et al., Phys. Rev. Lett. 100, 096407 (2008).
[7] A. Das et al., Nat. Phys. 8, 887 (2012)[8] F. D. M. Haldane, Phys. Rev. Lett. 61, 2015 (1988).
[9] X.-L. Qi et al., Phys. Rev. B 78, 195424 (2008).
[10] Y. Tokura et al., Nat. Rev. Phys. 1, 126 (2019).
[11] M. M. Otrokov et al., Nature 576, 416 (2019).
[12] E. D. L. Rienks et al., Nature 576, 423 (2019).
[13] Y. Gong et al., Chinese Phys. Lett. 36, 076801 (2019).
[14] R. C. Vidal et al., Phys. Rev. X 9, 041065 (2019).
[15] Y.-J. Hao et al., Phys. Rev. X 9, 041038 (2019).
[16] L. Ding, et al., Phys. Rev. B 101, 020412(R) (2020).
[17] R. C. Vidal et al., Phys. Rev. B 100, 121104 (2019).
[18] J. Wu et al. Science Advances 5, eaax9989 (2019).
[19] K. N. Gordon et al., arXiv:1910.13943 (2019).
[20] P. Swatek et al., Phys. Rev. B 101, 161109(R) (2020).
[21] B. Chen et al., Nat. Commun. 10, 4469 (2019).
[22] Y.J. Chen et al., Phys. Rev. X 9, 041040 (2019).
[23] Y. Fu et al., Phys. Rev. B 101, 161113(R) (2020).

PHYSICAL REVIEW B 99, 134414 (2019)

Half-metallicity of the ferrimagnet Mn2 VAl revealed by resonant inelastic
soft x-ray scattering in a magnetic field
R.Y. Umetsu,1,* H. Fujiwara,2 K. Nagai,2 Y. Nakatani,2 M. Kawada,2 A. Sekiyama,2,3 F. Kuroda,4,5 H. Fujii,4,5T. Oguchi,3,4,5 Y. Harada,6 J. Miyawaki,6 and S. Suga4,71

Institute for Materials Research and Center for Spintronics Research Network, Tohoku University,
2-1-1 Katahira, Sendai 980-8577, Japan
2
Division of Materials Physics, Graduate School of Engineering Science, Osaka University,
1-3 Machikaneyama, Toyonaka, Osaka 560-8531, Japan
3
Center for Spintronics Research Network, Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka 560-8531, Japan
4
Institute of Scientific and Industrial Research, Osaka University, 8-1 Mihogaoka, Ibaraki 567-0047, Japan
5
CMI2 -MaDIS, National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan
6
Institute for Solid State Physics, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8581, Japanand Synchrotron Radiation Research Organization, The University of Tokyo, 1-1-1 Koto, Sayo-cho, Sayo, Hyogo 679-5148, Japan
7
Forschungszentrum Jülich, PGI-6, 52425 Jülich, Germany(Received 22 October 2018; published 9 April 2019)
Detailed information on the electronic states of both V and Mn 3d electrons in the ferrimagnet Mn2 VAl is
obtained by bulk sensitive resonant inelastic soft x-ray scattering (SX-RIXS) excited with circularly polarized
light under an external magnetic field. The results under the V L-edge excitation have revealed the negligible
partial density of states (PDOS) of the V 3d states around the Fermi energy as well as their rather localized
character. Under the Mn L-edge excitation, on the other hand, the spectra are dominated by fluorescence withclear magnetic circular dichroism with noticeable excitation photon energy dependence. Compared with thetheoretical prediction of the RIXS spectra based on the density-functional-theory band structure calculation, an
itinerant, spin-dependent character of the Mn 3d states and decays of the Mn 2p core states are confirmed inconsistence with the half-metallicity of the Mn 3d states.
DOI: 10.1103/PhysRevB.99.134414
I. INTRODUCTION

Since the half-metallic electronic structure was predictedin the half-Heusler alloys of NiMnSb and PtMnSb [1] and in
full-Heusler alloys such as Co-based Heusler alloys [2,3], alarge number of investigations have been carried out from theinterest in the field of spintronics. When electrons around the
Fermi energy (EF ) are completely spin polarized, a system
must be very useful as a ferromagnetic electrode for the spin
injection and tunnel magnetoresistance as well as various spin
utilizable devices [4]. For example, magnetoresistance of themagnetic tunneling junction with using complete half-metal
ferromagnets would ideally be infinite [5]. Very recently, othertypes of Heusler alloys such as Mn-based Heusler alloys
and their quasiternary alloys have also been pointed out toshow the half-metallic electronic states [6–12]. Among them
Mn2 VAl is one of the most attractive materials for device
application. Its magnetic properties and theoretically predicted electronic structures were reported in the early 1980’s[13–15].
If the Mn2 VAl orders completely, atoms of Mn, V, and Al
occupy the Wyckoff positions 8c, 4b, and 4a, respectively,
with the space group F m3̄m. (See the Supplemental Material[16] for more details.) The spontaneous magnetization per*

rieume@imr.tohoku.ac.jp2469-9950/2019/99(13)/134414(10)formula unit of 1.9 μB /f.u. at 4.2 K is close to 2 μB /f.u. [13]predicted by the generalized Slater-Pauling rule [17] and ismuch smaller than those in Co-based Heuser alloys. The magnetic critical Curie temperature TC of this ferrimagnet Mn2 VAl
is quite high, about 760 K, with antiferromagnetically coupled
V and Mn spins below TC [13,14]. This material is thought tobe very promising for spintronic devices at room temperature
because the expected current to switch its spin would berather low. To investigate the magnetic properties of Mn2 VAl,
extensive studies have recently been carried out by using x-ray
absorption magnetic circular dichroism (XAS-MCD) for bulk
[18] and film [19–21] specimens. The electronic structures
have also been discussed by analyzing the XAS-MCD spectra
for bulk single crystals, showing the itinerant character of theMn 3d states, whereas the V 3d states are essentially localized [18]. On the other hand, the spin-polarized unoccupied
electronic structures have been predicted by the analysis of
XAS-MCD spectra for the film samples [19].
For the fundamental investigations of half-metallic materials, researchers should pay attention to how to provide
convincing evidence of the half-metallic electronic states. To
date fundamental magnetic properties reflecting the specificcharacter of the half-metallic electronic states have been investigated. For example, rather small high-field magnetic susceptibility [22] and the negligibly small pressure dependenceof the magnetization in some Co-based Heusler alloys havebeen reported. These results are understood as reflecting the134414-1©2019 American Physical Society

R.Y. UMETSU et al.

PHYSICAL REVIEW B 99, 134414 (2019)

fact that the electronic state near EF is insensitive to appliedexternal fields in the case of half-metallic ferromagnets [23].
Recently, anisotropic magnetoresistance is also predicted toserve as one of the practical screening tests of the half-metallic
ferromagnets from both theoretical and experimental aspects[24,25].
We report here a method to prove the spin-polarized
electronic structures of the half-metallic ferrimagnet Mn2 VAl
Heusler alloy with highly ordered L21 -type structure bymeans of resonant inelastic soft x-ray scattering (SX-RIXS)
measurements of the V and Mn 2p core excitation with useof right- and left-helicity circularly polarized light under anexternal magnetic field. RIXS is a bulk sensitive photon-in andphoton-out spectroscopy, and very powerful for investigating,for example, the d-d excitations for open shell 3d orbitals
and magnetic excitations for spin systems [26–29] as wellas 2p-3d transitions in element- and symmetry-specific ways.
These excitations and decays are sensitive to spin, electron
correlation, crystalline symmetry, in addition to the strengthof hybridization with the ligand band. Furthermore, RIXS
is insensitive to the surface conditions because of its longprobing depth (>100 nm) in contrast to any kind of photoelectron spectroscopy (PES). Since the emitted light is probed,
RIXS is not affected by any external perturbation such as
magnetic field in contrast to PES for electrons. In the present
study, the photon energy (hvin ) dependence of the magnetic
circular dichroism (MCD) of RIXS was measured in detailin order to obtain the spin-dependent information. The results
obtained in the present experiments and theoretical analyses
confirmed the half-metallicity of Mn2 VAl, demonstrating that
the RIXS and RIXS-MCD are very powerful for the study ofthe electronic structures of the half-metallic ferromagnetic or
ferrimagnetic materials.

II. EXPERIMENT

expected value of the magnetic moment for Mn2 VAl by the
Slater-Pauling rule, which is predicted by Galanakis et al.
[17], is 2 μB /f.u. A slight deviation of the saturation magnetic
moment 1.82 μB /f.u. of our sample from 2 μB /f.u. might becaused by a small amount of the off-stoichiometric composition. Although the atomic content of Mn is well controlled
in the present specimen, the atomic content of V is slightly
increased compared to that of Al. The half-metallic electronic
structures are, however, robust for the slight off-stoichiometriceffect in the present case, which is simulated by the density
functional theory (DFT) [18].
B. Measurements

Resonant inelastic soft x-ray scattering (SX-RIXS) for V
and Mn 2p core excitation was measured at room temperature
at the HORNET end station installed at the end of the longundulator beam line BL07LSU of SPring-8, Japan [30,31].
Measurement was performed with an external magnetic fieldof 0.25 T, which was applied by a permanent magnet with twopoles for passing the excitation light [32,33]. The directionof the magnetic field was repeatedly reversed by the rotatable
feed through supporting the magnet. (See the Supplemental
Material [16] for more details on the experimental geometry.)
Right- and left-helicity circularly polarized lights (RCP and
LCP) parallel to the magnetic field were incident at 45◦ onto
the (100) plane for excitation. Light emitted at 45◦ fromthis surface was dispersed by a grating and detected by a
two-dimensional detector. The 100 axis was contained in
the scattering plane. The direction of the magnetic field wasreversed to confirm the genuine magnetic circular dichroismin this RIXS experiment. The total energy resolution was
set to ∼140 (170) meV at the V (Mn) 2p3/2 edge. RIXS
measurements were performed on a surface of the sample
obtained by fracturing in an Ar globe box in advance andtransferred to a RIXS chamber with a vacuum of 1 × 10−5 Pawithout any exposure to atmosphere.

A. Sample preparation

A mother ingot of polycrystalline Mn2 VAl was fabricatedby induction melting in an argon atmosphere. Since the vapor
pressure of Mn is high during the melting, excess Mn elements
are contained in the mother ingot. A single crystal was grownby the Bridgman method with a size of 12 mm in diameterand about 30 mm in length. The obtained ingot was annealed
at 1473 K to grow the crystal grains. Furthermore, a two-stepannealing process at 1123 and then 873 K was employed
in order to control the microstructures and to heighten the
degree of order. These sample preparation processes resulted
in a rather high degree of order as S = 0.84 in our sample
compared with S = 0.5 by Kubota et al. [20] and S = 0.4
by Meinert et al. [19] in their samples. Crystal orientation
was checked by the back Laue method and the specimen
was cut in a strip form in the direction parallel to 100.
The sample composition was confirmed to be Mn: 50.5, V:
26.9, Al: 22.6 (at. %) with an electron probe microanalyzer.
Sample magnetization was measured with a superconducting
quantum interference device (SQUID) magnetometer. The
magnetization (M-H) curve measured at 5 K for Mn2 VAlshowed that more than 90% of saturation magnetization isrealized already at 0.2 T of the external magnetic field. The

III. RESULTS AND DISCUSSION
A. RIXS and RIXS-MCD experimental spectra

Figures 1(a) and 1(b) show the x-ray absorption spectra(XAS) of V and Mn L3 edges, respectively, by means of thetotal electron yield recorded at 20 K [18]. The numbers above
the vertical bars on the XAS indicate the incidence photon
energy hvin for RIXS spectra. Figures 1(c) and 1(d) show
the RIXS spectra for the V and the Mn L3 edges at room
temperature, respectively, measured by parallel (μ+ : blue) andantiparallel (μ− : red) configurations between the light helicity
and the direction of the magnetic field as a function of theenergy loss given by the horizontal axis. In the figures, strong
intensity peaks without any energy loss are always observed
and they are called the elastic component. In the larger energy
loss region, the so-called fluorescence peaks are observed with
their energy loss increasing linearly with hvin . The relative
height of the fluorescence peak is noticeably smaller thanthat of the elastic peak in the case of V L3 -edge excitationand the difference of fluorescence between μ+ and μ− is notprominent, whereas its difference is clearly observed in the
Mn L3 -edge excitation.

134414-2

HALF-METALLICITY OF THE FERRIMAGNET Mn …

(a)Intensity (arb. units)

V L3 XAS
T = 20 K

1

2

510

511

V L-edge
RIXS
T = 300 K
0.25 T

PHYSICAL REVIEW B 99, 134414 (2019)

10
9 11
8 12
7
6
5
3 4

16
15 1718
14
19
20
13

512
513
514
Photon Energy (eV)

µ+
µ-

11
10

12
13

9

2

1

515

(b)

Mn L3 XAS
T = 20 K

516

636

637

(c)

Mn L-edge
RIXS
T = 300 K
0.25 T

3

45

6

7

8

638
639
640
Photon Energy (eV)

641

642

(d)

µ+
µ-

Intensity (arb. units)

20
19
18

13

17
12

16

11

15
14

10

13
12

9

11

8

10

7

9

6

8
7

5

6
5

4

4

3

3

2

2

1

1

8

6

4
2
Energy Loss (eV)

0

8

6

4
2
Energy Loss (eV)0FIG. 1. XAS and RIXS spectra for V and Mn L3 edges. (a), (b) are XAS for the V and the Mn L3 edges, respectively, at 20 K and 2 T
magnetic field [18]. (c), (d) are the RIXS spectra for the V and the Mn L3 edges, respectively, obtained at room temperature. In (c), (d), the
RIXS spectra were measured by parallel (μ+ ) and antiparallel (μ− ) configurations between the light helicity and the direction of the magnetic
field. The numbers above the vertical bars on the XAS indicate the excitation photon energy hvin for the RIXS spectra.

Figures 2(a) and 2(b) show the detailed intensity mapsof the RIXS spectra for the V L3 edge obtained by thecircularly polarized light excitation. The horizontal axis showsthe energy loss from the incident photon energy hvin . The
spectral differences of the RIXS between the parallel and antiparallel configurations, RIXS-MCD, are shown in Fig. 2(c).
The corresponding results for the Mn L3 edge are shown in
Figs. 2(d)–2(f), respectively. From the comparison of these
spectra between the V and the Mn L3 edges, several significant
features are recognized as have been only partly discussed
by the polarization averaged RIXS spectra [18]. The mostcharacteristic feature is that the fluorescence peak associatedwith the V L3 edge does not branch off from the elastic peak.
On the other hand, there is almost no gap in the energy loss
between the elastic peak and the appearance of the fluorescence peak for the Mn L3 edge. In addition, weak structure isobserved around the constant energy loss ∼2 eV for a wide
excitation region above hvin ∼ 515 eV for the V L3 edge. This
inelastic energy loss feature is considered to be associatedwith the d-d excitation because the existence of any feature
at a constant energy loss cannot be due to any fluorescencefeature. When we compare the fluorescence MCD for the V

134414-3

R.Y. UMETSU et al.

PHYSICAL REVIEW B 99, 134414 (2019)

Mn2VAl V L3-edge RIXS
(a)

(b)

(c)

518

518

518

516

516

516

514

514

514

Incoming Photon Energy (eV)highhigh (+)

512

512
low

510
8

512

µ+
510
6

4

0

µ2

0

low (-)

510
8

6

4

2

0

8

6

µ+ - µ4

2

0

2

0

Energy Loss (eV)Mn L3-edge RIXS

(d)

(e)

644

644

644

642

642

642

640

640

640

(f)

high

high (+)

638

638

638

+

µ

low

8

6

4

0

-

µ
2

0

8

6

low (-)

4

2

0

8

6

µ+ - µ4

Energy Loss (eV)FIG. 2. Intensity maps of hvin -dependent RIXS and MCD at V and Mn L3 edges in Mn2 VAl. (a), (b) are the intensity maps of the RIXS
of V as a function of hvin obtained at room temperature in a magnetic field of 0.25 T for μ+ and μ− configurations. RIXS-MCD is given by
μ+ − μ− in (c). (d)–(f) show the corresponding results for the Mn L3 edge.

and Mn L3 edges in Figs. 2(c) and 2(f), it is clearly recognized
as the negative Mn fluorescence MCD is spread in the photon
energy range in hvin = 638−639 eV, while very weak positive
V fluorescence MCD is observed around hvin = 513 eV.
Typical RIXS spectra and their MCD at hvin = 512.5 eV
for the V L3 edge and hvin = 638.6 eV for the Mn L3 edge
are reproduced in Figs. 3(a) and 3(b), respectively, by solidlines. The RIXS-MCD features are observed in both cases of
V and Mn L3 -edge excitations, and the sign of the MCD of the
major fluorescence feature is found to be opposite between
the cases of V and Mn in agreement with the ferrimagnetic
character of this material, Mn2 VAl. Here, one notices that a
broad peak is observed in the V L3 -edge excitation, while
double-peak features split by 1.0–1.2 eV are observed inFig. 3(b) in the case of the Mn L3 -edge excitation beside theelastic peak. Detailed RIXS-MCD for the Mn L3 thresholdexcitation with changing hvin is shown later in Figs. 4(a1)–4(a6). These characteristic features must be closely correlatedwith the electronic structures of V and Mn.
B. Theoretical basis

For interpreting the observed RIXS and MCD spectra,
theoretical calculations were performed by means of DFT[18]. Based on the DFT, spin-resolved partial density of states(PDOS) of the V, Mn, and Al are calculated as shown in
Fig. 3(c) together with the total density of states (DOS). The eg
and t2g derived components are separately shown in Figs. 3(d)and 3(e) for V and Mn, respectively. First of all, the PDOS
of Al is almost negligible near EF . In the case of the V 3dstates, the PDOS are found to be very small in the region of
EF ± 0.6 eV. High PDOS of V of the t2g occupied states arearound −1.5 eV and those of the unoccupied t2g and eg states
are located at around +1.6 eV.
In the case of the Mn 3d states, however, rather complex
electronic structures strongly dependent on the spin are predicted. Although the occupied valence bands below −0.4 eV
are composed of both t2g and eg states with both spin-up
and -down states, clear differences are recognized above thisenergy. Namely, spin-down t2g states are crossing EF withhigh PDOS but the spin-up t2g states have negligible PDOS
between −0.5 eV and +0.7 eV. In the case of the eg statesof Mn, up-spin states have negligible PDOS between −0.4
and +0.4 eV, though down-spin states have a certain PDOS
between −0.4 and +1 eV before showing high PDOS around+1.4 eV. Thus half-metallic PDOS behavior so far predicted
is reconfirmed in Fig. 3(e).
As already pointed out in Figs. 2(a) and 2(b), the fluorescence component of RIXS in the V L3 edge does not branch
off from the elastic peak. The absence of any additional

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HALF-METALLICITY OF THE FERRIMAGNET Mn …

PHYSICAL REVIEW B 99, 134414 (2019)

V L3-RIXS (hν in = 512.5 eV)
300 K
0.25 T
Experiment Simulation

10

(a)

Mn2VAlTotal DOS(c)5

up spin0
down spin-10

Mn L3-RIXS (hν in = 638.6 eV)
300 K
0.25 T
Experiment Simulation(b)

DOS (states /eV spin cell)

Intensity (arb. units)

-5

-4
3

PDOS
Mn (×2)
V
Al

-2

0

2

4

(d)

2

up spin

1
0
-1
-2
-3
-4
3

down spin

V d PDOS
eg
t2g

-2

0

2

4

(e)

2

up spin

1
0
-1
-2
-3
-4

down spin

Mn d PDOS
eg
t2g

-2

0

2

4

Energy Relative to EF (eV)Energy Loss (eV)FIG. 3. Representative RIXS spectra for V and Mn L3 edges, and theoretically predicted PDOSs of Mn, V, and Al in Mn2 VAl. RIXS spectra
recorded for μ+ and μ− configurations at incoming photon energy hvin of 512.5 eV (a) and 638.6 eV (b) at the V and Mn L3 edges, respectively,
together with the MCD. (a), (b) include simulated spectra based on the DFT for V and Mn L3 edges, respectively. (c) shows the theoreticalprediction of the spin-dependent total DOS as well as PDOSs of Mn, V, and Al [18]. (d), (e) correspond to the eg and t2g components of thepartial DOSs of V and Mn, respectively [18].

fluorescence peak between the elastic peak and the peakaround ∼2 eV for the L3 threshold excitation is consistent withthe weakness or negligible PDOSs of the V 3d states aroundEF for both spin-up and spin-down V 3d states. The energy
splitting between the V unoccupied eg + t2g states PDOS andthe occupied t2g PDOS ranges from 2 to 4 eV [Fig. 3(d)].
Although this predicted d-d splitting energy is slightly larger
than the experimental d-d splitting energy roughly estimatedas 2–3 eV in the RIXS energy loss peak in Figs. 2(a) and 2(b),this constant loss energy feature in Figs. 2(a) and 2(b) can be
unambiguously ascribable to the genuine RIXS feature due tothe d-d excitation. On the other hand, the RIXS feature for
the Mn L3 -edge excitation branches off really from the elastic
peak and moves linearly with hvin revealing its fluorescenceorigin, suggesting the finite PDOS at EF of the Mn 3d states.
In the PDOS for the Mn shown in Fig. 3(e), finite PDOS
exists around the EF in the down-spin state, supporting theexperimental results.
The RIXS-fluorescence spectra were simulated based on
the Kramers-Heisenberg formula as shown in Eq. (A8) in the

Appendix, where details of the calculations are explained.
Since the correlation between the fluorescence RIXS-MCD
and the band structures must be discussed, more detailed
calculation with taking the magnetization into account mustbe performed. Under the external magnetic field of 0.25 T,
magnetization is almost saturated with the magnetic moment along the magnetic field [13,18]. Simulated RIXSfluorescence spectra and the MCD based on the DFT for
V and Mn L3 edges are shown by dotted lines in Figs. 3(a)and 3(b), respectively, in addition to the experimental spectra. It is seen that the simulated RIXS-MCD qualitatively
reproduces the experimental feature, for example, such as
the double-peak feature of the fluorescence in the Mn L3edge. If we compare the spectra of theoretically predicted
fluorescence and its MCD with corresponding experimental
results, one notices that the predicted spectra are more widely
energy spread than the experimental results. This may be
due to the larger electron correlation energy between the
Mn 3d electrons beyond that employed in the ordinary DFT
calculation.

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R.Y. UMETSU et al.

PHYSICAL REVIEW B 99, 134414 (2019)
Mn L3 RIXS-MCD
Experimenthν = 638.4 eV

Intensity (arb. units)Experimenthν = 638.2 eV

6

5

4

3

2

1

0

6

5

4

3

2

1

0

6

5

4

3

2

1

0

6

5

4

3

2

1

0

6

5

4

5

4

3

Experimenthν = 638.8 eV

2

1

0

6

Simulationhν = 0.4 eV

3

2

1

0

6

Simulationhν = 0.2 eV

Simulationhν = 0.0 eVm

64Simulationhν = 0.2 eV

Simulationhν = 0.0 eV

65Experimenthν = 638.6 eV

5

4

2

1

0

6

5

4

4

3

2

1

0

6

3

2

1

0

6

5

4

3

2

1

0

6

Simulationhν = 0.6 eV

3

2

1

0

6

5

4

5

4

Experimenthν = 639.2 eV

3

2

1

0

6

Simulationhν = 0.8 eV

Simulationhν = 0.6 eV

Simulationhν = 0.4 eV

3

5Experimenthν = 639 eV

5

4

3

2

1

0

6

Simulationhν = 0.8 eV

3

2

1

0

6

5

4

5

4

3

2

1

0

3

2

1

0

3

2

1

0

Simulationhν = 1.0 eV

5

4Simulationhν = 1.0 eV

3

2

1

0

6

5

4

Energy Loss (eV)FIG. 4. RIXS and RIXS-MCD spectra for Mn L3 edges. (a1)–(a6) show the experimental RIXS results for the μ+ and μ− configurations
and their MCD spectra for the Mn L3 edge as a function of the energy loss. Theoretically calculated RIXS and RIXS-MCD spectra based on
DFT calculation are in (b1)–(b6). The definition of hν1 is given in the text. (c1)–(c6) show m j resolved RIXS-MCD spectra for the Mn L3 edge,
and the summation of the whole components of these spectra corresponds to the RIXS-MCD spectra in (b1)–(b6).
C. Detection of the spin-polarized Mn 3d electronic states

In order to further discuss the Mn 3d states around EF ,RIXS and the MCD were systematically investigated in the
Mn L3 -edge threshold excitation region. Figures 4(a1)–4(a6)show the experimental RIXS spectra for μ+ and μ− configurations and their MCD with changing the hvin (638.2 hvin  639.2 eV). Before performing detailed calculations tosimulate the experimental spectra, PDOS as well as excitation and deexcitation processes must be considered. Figure 5
shows schematic views of the Mn L3 RIXS-MCD processes
by considering the PDOS for the half-metallic magnetic system. The difference of the photon energy hvin from the energy
E2p0 between the EF and the m j = −3/2 Mn 2p core stateis defined hereafter by hv1 . PDOS of the up- (down-) spinstate is schematically shown on the left- (right-) hand sideof the energy axis (vertical axis) in each figure. The Zeeman
splitting of the Mn 2p core level states of around 0.5 eV due
to the effective magnetic field induced by the spin-polarized
3d states is taken into account in the present simulation. The
excitation to the empty conduction band states is shown in
Fig. 5 by the upward arrows with the filled circles. The gap of
0.3 eV from the EF to the bottom of the DOS in the majority
up-spin state of the Mn 3d states is taken into account.
Five energy ranges of hν1 = hνin −E2p0 can be considered
for the excitation as (1) below 0.16 eV, (2) 0.16–0.46 eV, (3)0.46–0.63 eV, (4) 0.63–0.8 eV, and (5) above 0.8 eV. Since
the experimental results were obtained at 300 K, one shouldnote that the energy broadening of around 0.1 eV cannot be
neglected in the comparison between the experimental results
and theoretical prediction. In both the 2p core excitation andthe 3d-2p fluorescence decay processes, the spin is conservedin the dipole transition. However, the hole spin can be relaxed
before the fluorescence decays into the core hole with m j =−1/2 and +1/2 states which are composed of both spin-up
and -down states due to the spin-orbit coupling, so evenwhen the m j = −1/2 state with spin-down state is excited
to the empty conduction band, the core hole spin can bepartially relaxed to the spin-up state before the fluorescencetakes place. This means that one should take into accountthe fluorescence in both spin-down and -up channels with thefixed relative weight given in the initial core hole states when
the fluorescence decays to the core holes with m j = −1/2 or+1/2 states are calculated. In Fig. 5, the width of the arrows
corresponds to the transition probability.
The fluorescence spectra for the Mn L3 -edge excitation arecalculated as shown in Figs. 4(b1)–4(b6). In the experimental
MCD spectra in Figs. 4(a1)–4(a6), the intensity increases
with increasing hvin showing the double-peak future, whichbecomes less clear above hvin = 638.8 eV. Such a tendency
is qualitatively reproduced by the calculated spectra. Figures 4(c1)–4(c6) show the predicted MCD spectra of fluorescence to the Mn 2p m j = −3/2, −1/2, +1/2, and +3/2
states. That is, the summation of the total four components
in these spectra [(c1)–(c6)] corresponds to the MCD spectra
shown by the black lines in Figs. 4(b1)–4(b6). It is clear thatthe fluorescence decay to the m j = −3/2 state is dominating at the low hv threshold. Since the down-spin t2g states
have substantial PDOS of the unoccupied states from EF
to +0.8 eV, and the down-spin eg states have high PDOS
around +1.4 eV, the pure down-spin m j = −3/2 states can becontinuously excited at least up to hv1 ∼ 0.8 eV in addition to
the region in +1.2−1.6 eV inducing the remarkable negative

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HALF-METALLICITY OF THE FERRIMAGNET Mn …

PHYSICAL REVIEW B 99, 134414 (2019)

FIG. 5. Schematic views of the RIXS-MCD processes in the case of half-metallic density of states. Excitation from the Zeeman split 2p m jcore states is considered in the present simulation as indicated in the table and the Zeeman splitting is around 0.5 eV due to the effective
magnetic field. The difference of the photon energy (hvin ) from the energy E2p0 between the Fermi energy (EF ) and the Mn 2p m j = −3/2
state is given here by hv1 . PDOS of the up- (down-) spin states is schematically shown on the left- (right-) hand side of the energy axis (verticalaxis) in each panel. A gap of 0.3 eV from the EF to the bottom of the DOS in the majority up-spin state is also employed. The spin is thought tobe fully down and up in the m j = −3/2 and +3/2 states, respectively. On the other hand, the spin at m j = −1/2 and +1/2 states is composedof both spin-up and -down states due to the spin-orbit coupling. The width of the arrows corresponds to the transition probability.

fluorescence MCD in Figs. 4(c1)–4(c6). Since the PDOS of
the occupied Mn 3d states in the region from EF down to−0.4 eV for the down-spin state is rather high, noticeable
fluorescence and its clear MCD is predicted just from theenergy loss of 0 eV as clearly seen in Figs. 4(b1) and 4(c1),and 4(b2) and 4(c2). The fluorescence feature with an energy
loss peak near 0.3 eV in Figs. 4(b1) and 4(b2) is thought toreflect the high PDOS of the down-spin occupied t2g states in
the above-mentioned region.
With increasing hv1 up to ∼0.46 eV, excitation from the
m j = −1/2 state becomes gradually feasible in addition to
the excitation from the m j = −3/2 state, providing negative
but small MCD of the fluorescence to the m j = −1/2 state
as shown in Figs. 4(c2) and 4(c3). The origin of the doublet
feature of the fluorescence and its MCD separated by ∼1 eV[Figs. 4(a2)–4(a4)] is relatively well predicted by the calculation as shown in Figs. 4(b1)–4(b4) and 4(c1)–4(c4). Both ofthe doublet features must be due to the fluorescence transition
into the down-spin m j = −3/2 core hole state.
For 0.46 eV < hv1 < 0.63 eV, both down- and up-spinstates are excited from the m j = −1/2 core states. For
0.63 eV < hv1 < 0.8 eV, a similar situation takes place forthe m j = +1/2 core states. In each case, both spin-down and-up states are excited and fluorescence takes place after therelaxation of the core hole spin. With the increase in the
hv1 , the relative weight of the transition to the m j = −1/2,+1/2, and +3/2 states increases gradually with the increaseof the magnitude of the individual fluorescence MCD. As aresult, the total magnitude of the predicted fluorescence MCD
decreases relatively as seen in the series of Figs. 4(b1)–4(b6)in consistence with the experimental results in Fig. 4(a3)–
4(a6).
In the present study, detailed hvin dependence of RIXSMCD was clearly observed, providing the spin-dependent
information on the bulk electronic structures with the single
magnetic domain. This technique has a great advantage toinvestigate the half-metallic bulk electronic structures compared to various other investigations, which have been performed up to now. Ultraviolet-photoemission spectroscopy(UPS) [34] and hard x-ray angle-resolved photoelectron spectroscopy (HAXARPES) [35,36] are such examples. In thecase of UPS, it is known that the surface electronic structure accessible by UPS is noticeably different from thatin the bulk. The HAXARPES is much more bulk sensitive. However, its orders-of-magnitude reduced photoionization cross sections for the valence band electronic states,
compared with the UPS, strongly hinder the usefulness of
HAXARPES studies. Moreover the possible recoil shift effects for the valence electron states spoil reliable discussions
on the most important electronic structures near the EF in
Mn2 VAl [37,38]. Although spin-polarized and angle-resolved
photoelectron spectroscopy (SP-ARPES) will be desired todetect the half-metallicity, its low detection efficiency (ordersof 10−4 or less) makes it almost impossible to make a reliable

134414-7

R.Y. UMETSU et al.

PHYSICAL REVIEW B 99, 134414 (2019)

experiment. Therefore, the electronic structure, especiallythe half-metallicity of the bulk Heusler alloys, has not yet
been fully clarified. The qualitative agreement obtained in
the present experiments and theoretical analyses confirmed
the half-metallicity of Mn2 VAl, demonstrating that the RIXS
and RIXS-MCD are powerful for the study of the electronic
structures of the half-metallic ferromagnetic or ferrimagnetic
materials.
IV. CONCLUSIONRIXS and its MCD measurements in magnetic field were
performed on the single-crystal half-metallic Heusler alloy,
Mn2 VAl, in order to obtain reliable information on the bulk
electronic state of the 3d electrons. The d-d excitation due
to the t2g -eg splitting is clearly observed for V in Mn2 VAlunder the L3 -edge excitation. The loss energy of the V d-d
RIXS maximum is found to be about 2 eV, being comparable
to the splitting energy between the theoretically predicted eg
and t2g states. The delayed branching off in the V 3d-2p
fluorescence peak from the elastic peak demonstrates thenearly absent V 3d PDOS around EF . The clear appearanceof the t2g -eg RIXS of V reflect the rather localized characterof the V 3d states. The RIXS-MCD of the fluorescence peaksof the Mn 3d-2p transition under the L3 -edge excitation showsa negative sign with clear hvin dependence. The sign andthe shape of the RIXS-MCD are qualitatively reproducedin consistence with the DFT calculations and confirmed theabsence of the up-spin Mn 3d PDOS at the EF , demonstrating the half-metallicity of the Mn2 VAl Heusler alloy.
Thus the bulk sensitive RIXS studies under external magnetic field are confirmed to be essential to study the detailed
electronic structures of various Heusler alloys and familymaterials.
ACKNOWLEDGMENTS

We sincerely thank K. Yubuta, T. Sugawara, and Y.
Murakami for their help in making single crystals and EPMA
experiments, and Professor T. Kanomata for valuable discussions. This research was supported by Precursory Research for Embryonic Science and Technology (PRESTO),
Japan Science and Technology Agency (JST), and Grantin-Aid for Scientific Research Grants No. JP18K03512, No.
JP18H01690, No. JP16H03851, No. JP16H02267, and No.
JP17H06154 from the Ministry of Education, Culture, Sports,
Science and Technology Japan. This work was carried out
under the approval of BL07LSU at SPring-8 Synchrotron Radiation Research Organization and the Institute for Solid State
Physics, The University of Tokyo (Proposals No. 2013A7447,
No. 2013B7459, No. 2014B7478, No. 2016B7512, and No.
2017B7552). F.K., H.F., and T.O. acknowledge a support by
the “Materials research by Information Integration” Initiative
project from JST.

all-electron full-potential augmented plane-wave (FLAPW)
method [39]. The generalized gradient approximation (GGA)
using the Perdew-Burke-Ernzerhof scheme has been used forthe exchange-correlation potential [40,41]. The relativistic
effects are considered for the 2p core states including the spinorbit coupling. On the other hand, the spin-orbit coupling forthe 3d states is negligible compared with the 2p states, since
the orbital magnetic moments of Mn and V were estimated as
0.026 μB /Mn and 0.037 μB /V by XAS-MCD measurements,
respectively [18]. Plane-wave expansion cutoffs were set to
20 Ry for the wave functions and 80 Ry for the charge
density and potential functions. The muffin-tin sphere radius
was chosen as 1.1 Å for all elements. For the Brillouin-zone
integration, a 16 × 16 × 16 uniform mesh was used with thetetrahedron integration technique. The atoms were placed on
the general form X2Y Z of the L21 structure with X at the 8csite in the Wyckoff position, Y at the 4b site, and Z at the 4asite. The lattice constant was set to 5.875 Å [7].
The RIXS spectra were simulated by using the KramersHeisenberg formula [29,42], described as2  b|F̂ (μ2 ) |ii|F̂ (μ1 ) |a 
2
1
σ (vin , vout ) ∝Ei − Ea − hvin − ii fi

× δ[(hvin − hvout ) − (Eb − Ea )],(A1)

where a, b, and i denote the initial, final, and intermediate
states having the energy of Ea , Eb , and Ei , respectively. The
incoming and outgoing photon energies are described as hvinand hvout , respectively. Lifetime broadening of a core holeis given by i of 0.36 (0.28) eV for Mn (V) [43], and thedipole transition operator is described as F̂ (μ) for the photon
helicity (μ) of the circularly polarized light. The electron
configurations of the initial, intermediate, and final states with|a = |2p6 v, |i = |2p5 ve, and |b = |2p6 v −1 e  are taken
into account with the relative energy of Ea = 0, Ei = εe −E2p ,and Eb = εe − εv−1 . Here v and e represent the valence electrons and the electron in the empty conduction band, and v −1and εv−1 stand for the valence electron state with one holeinduced by the decay into the 2p core hole state and the energyof valence electrons with one hole. Then the denominator of
Eq. (A1) can be expressed by considering these energies andthe content of the δ function as
Ei − Ea − hvin − ii = εv−1 + εe − εe − E2p − hvout − ii .
If we assume εe = εe , where no change is considered forthe electron excited into the empty conduction band on the
fluorescence decay, the denominator of Eq. (A1) is further
approximated as
Ei − Ea − hvin − ii = εv−1 − E2p − hvout − ii .

(A2)On the same assumption the transition matrix elements aretransformed as
i|F̂1(μ1 ) |a = e|F̂1(μ1 ) |2p,(A3)

APPENDIX: SIMULATION BASED ON THE DENSITYFUNCTIONAL THEORYb|F2(μ2 ) |i = v −1 |F̂2(μ2 ) |2p−1 δ(e − e).

(A4)

The electronic structure calculation based on DFT has been
performed using the HILAPW code, which is based on the

The summation for the intermediate states |i can be expressed by the two integrals as follows, and the RIXS intensity

134414-8

HALF-METALLICITY OF THE FERRIMAGNET Mn …

can be described as
σ (vin , vout ) ∝




dεv−1

dεe

f

PHYSICAL REVIEW B 99, 134414 (2019) −1 (μ2 ) −1 2 
v |F̂ |2p  · e|F̂ (μ1 ) |2p221(εv−1 − E2p − hvout )2 + i2

δ[(hvin − hvout ) − (εe − εv−1 )].

(A5)

If we assume that the square of the matrix element in the numerator is proportional to the partial density of states of theoccupied (unoccupied) states denoted as Docc (Dunocc ) at the energy of εv−1 (εe ) of occupied (unoccupied) valence states multipliedby the transition probability w (μ)
jm j ms between the 2p and 3d states depending on the helicity (μ) [44–46], we obtain (μ )

e|F̂ (μ1 ) |2p2 ∝w jm1j ms Dunocc(A6)(m+μ1 )ms (εe ),1
jm j ms (μ )
 −1 (μ2 ) −1 2
v |F̂ |2p  ∝w j  m2 j m s Docc(m +μ2 )m s (εv −1 ).
2(A7)

j  m j m sHere j, m j , ms , and those with a prime ( ) stand for the total angular momentum of the 2p core states, their z component andits spin, while m and ms , as well as those with a prime denote the magnetic quantum number and the spin of the 3d states underconsideration. Here m = m j −ms is assumed for the 2p states. In the present calculation, the relativistic effects are considered
for the 2p j=3/2 core states including the spin-orbit interaction for the states with m j = ±3/2 and ±1/2, while the 3d spin is welldefined by ms . Furthermore, we take into account the Zeeman splitting of the 2p states due to the effective magnetic field of the
3d states. The weight coefficient w (μ)
jm j ms is given by multiplication of the Clebsch-Gordan coeffcient and the Gaunt coefficient.
Finally we obtained the RIXS intensity as
(μ1 )
2)
occ
unocc

w (μ
jm j m s w jm j ms D(m +μ2 )m s (εv −1 )D(m+μ1 )ms (εe )μ1 ,μ2(vin , vout ) ∝
σ
dεv−1

2
εv−1 − E2p jm j − hvout + i2
jm j ms m s=

jm j ms m s

(μ1 )
2)
occ
unocc
w (μ
jm j m s w jm j ms D(m +μ2 )m s (εv −1 )D(m+μ1 )ms [εv −1 + (hvin − hvout )]
.
dεv−1

2
εv−1 − E2p jm − hvout + i2

(A8)

j

In the simulation, we practically give the energy of unoccupied valence states εe using the energy offset (hv1 ) from Fermi
energy (EF ) as εe = EF + hv1 . Then, we further transform Eq. (A8) as
(μ1 )
2)
occ
unocc

w (μ
jm j m s w jm j ms D(m +μ2 )m s (εv −1 )D(m+μ1 )ms (hv1 + EF )μ1 ,μ2 (v1 ,vout )
σ
∝
.
(A9)
dεv−1
(εv−1 − E2p jm j − hvout )2 + i2
jm j m mss

In this model, the magnitude of the RIXS-MCD is proportional to the Dunocc or the quantity of the core holes and the line
shape reflects the energy distribution of the Docc . Moreover, the spin polarization of the 2p core hole plays an essential role inthe spin selective transition with dipole selection rule. Thus, the RIXS-MCD is clearly observed at the pre-edge of the L3 XAS
since the excitation from the spin-polarized m j = −3/2 states is dominant. In the case of the m j = ±1/2, however, the spins of
the core hole are mixed, and the core hole spins can be relaxed between spin up and down before the fluorescence takes place.
This allows the optical path in which the spins in the absorption process and emission process are different. The fluorescence
spectra thus obtained are shown in Figs. 4(b) and 4(c). Note that the transition probability for the circularly polarized photon
was taken into account in the absorption process only, but it was averaged in the emission process since the polarization of theoutgoing photon was not measured in the experiment.

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[16] See Supplemental Material at http://link.aps.org/supplemental/
10.1103/PhysRevB.99.134414 for details on the crystal
structure of Mn2 VAl Heusler alloy with L21 type (seeRefs. [13,14,47]), and for details on the scattering geometryin SX-RIXS measurements. The single crystal of the Mn2 VAlspecimen was tilted 45◦ from the direction of the incident x-ray.
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is used for RIXS excitation.
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Carbon 152 (2019) 680e687

Contents lists available at ScienceDirect

Carbonjournal homepage: www.elsevier.com/locate/carbon

Inﬂuence of interface dipole layers on the performance of grapheneﬁeld effect transistors
Naoka Nagamura a, b, Hirokazu Fukidome c, *, Kosuke Nagashio d, Koji Horiba e,
Takayuki Ide c, Kazutoshi Funakubo c, Keiichiro Tashima c, Akira Toriumi d,
Maki Suemitsu c, Karsten Horn f, Masaharu Oshima gaNational Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki, 305-0047, Japan
PRESTO, Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi, Saitama, 332-0012, Japan
Research Institute of Electrical Communication, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai, 980-8577, Japand
Department of Materials Engineering, Graduate School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-8656, Japane
Photon Factory, Institute of Materials Structure Science, High Energy Accelerator Research Organization, 1-1 Oho, Tsukuba, Ibaraki, 305-0801, Japanf
Department of Physical Chemistry, Fritz Haber Institute of the Max Plank Society, 14195, Berlin, GermanygSynchrotron Radiation Research Organization, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-8656, Japanbca r t i c l e i n f o

a b s t r a c t

Article history:
Received 29 March 2019
Received in revised form
25 May 2019
Accepted 9 June 2019
Available online 11 June 2019

The linear band dispersion of graphene's bands near the Fermi level gives rise to its unique electronic
properties, such as a giant carrier mobility, and this has triggered extensive research in applications, suchas graphene ﬁeld-effect transistors (GFETs). However, GFETs generally exhibit a device performancemuch inferior compared to the expected one. This has been attributed to a strong dependence of theelectronic properties of graphene on the surrounding interfaces. Here we study the interface between agraphene channel and SiO2, and by means of photoelectron spectromicroscopy achieve a detailed
determination of the course of band alignment at the interface. Our results show that the electronic
properties of graphene are modulated by a hydrophilic SiO2 surface, but not by a hydrophobic one. By
combining photoelectron spectromicroscopy with GFET transport property characterization, we
demonstrate that the presence of electrical dipoles in the interface, which reﬂects the SiO2 surface
electrochemistry, determines the GFET device performance. A hysteresis in the resistance vs. gate voltage
as a function of polarity is ascribed to a reversal of the dipole layer by the gate voltage. These data pave
the way for GFET device optimization.
© 2019 Published by Elsevier Ltd.

1. Introduction
Interfaces of a graphene channel, such as those with gate oxides
or contact metals, demand precise and accurate control of electronic level alignment. In graphene, the linear band dispersion near
the Fermi level in principle provides excellent intrinsic electronic
properties, e.g. an extremely high mobility of carriers, derived fromtheir zero-effective mass. These intrinsic properties make graphenea promising material for next-generation electronics applications,such as graphene-based ﬁeld-effect transistors (GFETs) and highelectron mobility transistors (HEMTs). At present, however, actual
graphene channels used in GFETs actually exhibit transport* Corresponding author.
E-mail address: fukidome@riec.tohoku.ac.jp (H. Fukidome).
https://doi.org/10.1016/j.carbon.2019.06.038
0008-6223/© 2019 Published by Elsevier Ltd.

properties inferior to those anticipated from the intrinsic electronic
properties [1e4]. This inferiority is ascribed to the high susceptibility of the electronic properties of graphene to the surrounding
interfaces as well as to technological immaturity in graphene device fabrication. This high susceptibility is also in part due to thelinear band dispersion. In conventional semiconductors, the2=daverage kinetic energy per electron is EK  Z2 nd =2m* where m* isthe effective mass and nd is the average electron density in d spatial
dimensions. The Coulomb energy per electron is of the order
1=dEC  e2 nd =ε0 where e is the electron charge. Therefore, theeffective coupling constant aeff, which indicates the ratio of
Coulomb over kinetic energy and is related to the strength of theelectron-electron interactions, is given by aeff ¼ EC =EK ¼
1=d2m* e2 nd =Z2 ε0 . This depends on carrier density. On the other
hand, the average kinetic energy per electron in graphene is of theorder EK  ZvF n1=2 where vF is the Fermi-Dirac velocity and n is the

N. Nagamura et al. / Carbon 152 (2019) 680e687

2D electronic density, owing to its linear dispersion. So, the aeff in
2D graphene is described as aeff ¼ EC =EK ¼ ðe2 =ε0 Þ=ZvF . This isindependent of the electronic density, but affected by the dielectric
constant of the surrounding environment [5e7].
Of the various interfaces, the interface with gate oxides is ofparticular concern [8,9] because oxide ﬁlms are the most popularmaterials for insulating layers in semiconductor devices. Theinterface between a graphene channel and a gate oxide not only
acts as scattering centers for carriers but also causes drastic changes
in electronic characteristics of graphene such as e-e interactions
described above. Changes in the dielectric constants of gate oxides,
such as SiO2, thus inﬂuence the transport properties or in otherwords, the device performance [10]; for example, depositing ice on
a graphene channel enhances carrier mobility in the channel [11].
This is explained by a reduction in aeff by the high dielectric constant of ice. Hence the interface chemistry also inﬂuences thetransport properties. This is shown by reports that a graphene
channel interfaced with hydrophilic SiO2 exhibits degraded transport properties, such as a reduced carrier mobility and hysteresis in
the resistance-gate bias curve in the gate-bias sweep direction,
when compared to a channel interfaced with hydrophobic SiO2
[12,13]. The difference between hydrophobic and hydrophilic SiO2
lies in the presence of adsorbed water molecules on the latter,
which are sandwiched between graphene and hydrophilic SiO2
[14].
The impact of interface physics and chemistry on graphene
channels should thus be fully understood for further developmentin the GFET technology. Imaging techniques are most useful to
extract interface characteristics and microstructures [15e18]. To
this end, we have developed a core-level photoelectron spectromicroscopy technology, called “3D nano-ESCA,” where we canscan the sample with a high lateral spatial resolution (70 nm) [19]to record photoelectron spectra (the “third dimension”) to quantitatively analyze electronic level information, such as the Fermi
level in graphene, and also chemical states, for example theoxidation valency of SiO2 at the desired points, from core level line
positions [20]. We have demonstrated in our previous reports that
3D nano-ESCA is useful for the microscopic investigation of GFETs[21,22]. In fact, microscopic spatial variations of the potential
landscape in a GFET were elucidated by measuring the Fermi level
using 3D nano-ESCA. Thus, 3D nano-ESCA is a most suitable tool toanalyze electronic states, chemical states and, indirectly, transport
properties.
This study describes how SiO2 surface chemistry, i.e., hydrophilicity vs. hydrophobicity, modulates the electronic states of thegraphene channel from a microscopic viewpoint and then compares the inﬂuence of states with the transport properties obtained
from the macroscopic electrical characteristics of GFETs.
2. Methods
2.1. Sample preparation
Exfoliated graphene was transferred onto SiO2 thin ﬁlms
(90 nm) on pþ-Si(100) substrates. The color contrasts in opticalimages depending on the layer number were emphasized at thegraphene sheets on 90 nm SiO2/Si substrates, so the presence of amono-layer of graphene was conﬁrmed by the optical contrasts andRaman spectroscopy [23]. To prepare a hydrophobic SiO2 thin ﬁlm,
we performed the so-called reoxidation process of the SiO2 thinﬁlm by annealing it at 1273 K for 5 min in a 100% oxygen gas ﬂow[13]. This process induces the desorption of H2O molecules from thesurface and produces surface siloxane groups. On the other hand, toprepare hydrophilic SiO2 thin ﬁlms, an O2 plasma treatment withan O2/Ar mixture (1:9) ﬂow rate of 50 cm3/min was carried out [13].

681After the exfoliation of monolayer graphene on the prepared SiO2
thin ﬁlms, Ni contact electrodes were prepared by vacuum evaporation, and structured by electron-beam lithography. The post
annealing procedure was not adopted, and the measurements forsample characterization were performed on as-fabricated devices.
2.2. Sample characterization
Spatially resolved C 1s, Si 2p, and O 1s core-level photoelectron
spectra measurement was carried out using the 3D nano-ESCA
instrument installed at the University of Tokyo outstation beamline, BL07LSU at SPring-8 [19,24]. In this beamline, the synchrotron
radiation (SR) beam has a high energy-resolving power (E/DE > 104). The photon energy of the SR beam used for measurements was 1000 eV. The lateral spatial resolution, i.e., the spot sizeof the X-rays focused using a Fresnel zone plate, was 70 nm. Theenergy resolution of the spectrometer was set to 300 meV and theaccuracy of the angle resolution was 0.9 . The binding energy scale
was calibrated using the photoelectron peaks of a gold mesh foil
(Au 4f 7/2, binding energy: 84.0 eV) at the same potential as thesource electrode, and the Fermi levels detected in valence spectra
on Ni electrodes. Details of the experimental setup can be foundelsewhere [19,22]. The resistance-gate voltage characteristics were
evaluated in ambient air conditions using a semiconductor
parameter analyzer (B1500A, Keysight Technologies Inc.).
3. Results and discussion
To quantitatively analyze the impact of SiO2 surface chemistryon band level alignment, we ﬁrst demonstrate the applicability of
3D nano-ESCA; we then discuss the inﬂuence of interface chemistry
between graphene and SiO2 on the channel performance. In section
3.3, we compare the electronic states with the device performance(e.g., hysteresis), which varies with SiO2 surface chemistry, andﬁnally in section 3.4, we show that SiO2 surface chemistry affects
the electronic states of graphene near metal contacts as well.
3.1. 3D nano-ESCA imaging of GFET
3D nano-ESCA, as illustrated in Fig. 1(a), is used to analyze theelectronic structure of a GFET and to quantitatively clarify the effectof surface chemistry of SiO2 thin ﬁlms on the graphene channel. A
GFET structure on a 1 cm  1 cm substrate is mounted on a sample
holder as shown in Fig. 1(b). Electrodes, including source, drain, andgate, are connected to the chamber ground. The optical micrographof the GFET device structure consisting of a graphene ﬂake channel
region and contact metal electrodes is shown in the upper picturein Fig. 1(c). The faint shape of the graphene ﬂake is barely visible.
On the other hand, highly spatially resolved elemental mapping of
the GFET device, where the intensities of the C 1s, Si 2p, and Ni 3p
core-level spectra are red, green, and pink, respectively, are shownin the lower picture of Fig. 1(c), which is the same region as theupper one. A sharp image is obtained by using the nano-focused Xray beam (70 nm) with a Fresnel zone plate [19]. 3D nano-ESCA
thus has a high enough lateral spatial resolution to reﬂect the
GFET architecture. As discussed above, hydrophobic SiO2 thin ﬁlms
deposited on Si(100) substrates were subjected to the so-calledreoxidation process, which leads to the surface being covered bysiloxane groups (Fig. 1(d)), while the hydrophilic one is covered
with silanol groups (Fig. 1(e)) [13,22]. The cross sections of bothdevices are schematically shown in Fig. 1(d) and (e), respectively.
3.2. Interface chemistry of graphene with SiO2
The electronic and chemical states at the interfaces of graphene

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N. Nagamura et al. / Carbon 152 (2019) 680e687

Fig. 1. Schematic of the measurements and graphene-oxide interface in GFET. (a) Schematics of the 3D nano-ESCA imaging system. (b) Photo of a sample holder upon which a
GFET device structure on a substrate is mounted. (c) (upper) Optical micrograph of a GFET, barely showing the graphene ﬂake. (lower) Elemental mapping of a GFET using 3D nanoESCA, where green is the silicon substrate, pink are the nickel contacts, and red the graphene ﬂake. (d, e) Schematic cross-sections of Ni electrodes and graphene on (d) hydrophobicand (e) hydrophilic SiO2 thin ﬁlms, respectively, indicative of the chemical composition. (A colour version of this ﬁgure can be viewed online.)channels in GFETs with hydrophobic or hydrophilic SiO2 ﬁlms were
examined by performing a point-for-point spectroscopic analysis of
the C 1s, Si 2p, and O 1s core levels at the center of the graphene
channels with 3D nano-ESCA, as shown in Figs. 2 and 3. Along thegraphene channels, we used the core-level binding energies toinvestigate the potential level alignment of graphene on the hydrophobic (blue curves in Figs. 2 and 3) and hydrophilic (red curves
in Figs. 2 and 3) SiO2 thin ﬁlms on the Si(100) substrates. The C 1s
spectrum of graphene on the hydrophobic SiO2 thin ﬁlm on Si(100)has a higher binding energy, compared to that on the hydrophilic
SiO2 thin ﬁlm on Si(100) (Fig. 2(a)). The C 1s spectra can be

decomposed into two components by precisely examining thebinding energy, which directly reﬂects the Fermi level position
relative to the Dirac point, of the graphene channels (Fig. 2(b)). The
lower binding energy component is attributed to graphene, while
the higher binding energy component, which is somewhat broader,
is attributed to contaminations probably arising from lithographic
processing, according to our previous angle-resolved analysis of the
C 1s spectra of GFETs [22]. Although these contaminants could havean inﬂuence as p-type dopants and scattering centers in the graphene channels [25], the amount of residual carbon contaminants
is almost the same between graphene channels on hydrophilic SiO2
and hydrophobic SiO2 according to the intensity of peak components assigned to contaminants in Fig. 2(b), so we assume that theeffect of contaminants is the same on the hydrophilic and hydrophobic SiO2. It is obvious that the peak of graphene on a hydrophilic
SiO2 thin ﬁlm has a lower binding energy than that on a hydrophobic SiO2 thin ﬁlm. This result can be explained by the fact that
doping induces a shift in the Fermi level, resulting in a shift in the C
1s binding energy [26], as schematically shown in Fig. 2(c). The
binding energy of graphene on a hydrophobic SiO2 thin ﬁlm is284.45 eV, which is very close to that of neutral graphene [27,28].
Graphene on a hydrophobic SiO2 thin ﬁlm on Si(100) therefore
exhibits negligible doping; on the other hand, it can be inferredfrom the lower binding energy (284.23 eV) of the C 1s peak thatgraphene on a hydrophilic SiO2 thin ﬁlm is hole-doped. The difference between the spectra is about 0.22 eV. Assuming a linear
band dispersion of graphene with respect to the wave vector, we
use the following equation to estimate the concentration of doped
holes (Nh) as follows [29,30],

pﬃﬃﬃpﬃﬃﬃﬃﬃﬃ
EDP  EF ¼ ZvF p Nh

Fig. 2. Electronic states of graphene channels in contact with SiO2 gate oxides. (a)Pinpoint C 1s core-level spectra recorded at the center of the graphene channels onhydrophobic (blue) and hydrophilic (red) SiO2 thin ﬁlms on Si(100) substrates. (b)Decomposition of the spectra displayed in (a). (c) Schematic diagram to explain thepeak shift in graphene at different SiO2 surface conditions. For comparison, the data onhydrophilic SiO2 thin ﬁlms was sourced from Ref. [22]. (A colour version of this ﬁgure
can be viewed online.)(1)Here, EF, EDP, -, and nF are the Fermi level, Dirac point energy,
reduced Planck's constant, and the Fermi velocity of electrons ingraphene (~1.1  106 m/s), respectively. Inserting our experimental
value of 0.22 eV as the value of (EDPeEF), the concentration of the
doped holes (Nh) in graphene on hydrophilic SiO2 thin ﬁlms is
estimated to be 2.4  1012 cm2. The SiO2 surface chemistry thus
has a strong inﬂuence on the Fermi level position relative to the

N. Nagamura et al. / Carbon 152 (2019) 680e687

683

Fig. 3. Electronic and chemical states of the SiO2-graphene channel interface. (a, b) Si 2p and O 1s core level spectra at the center of the graphene channels on hydrophobic (blueplus sign) and hydrophilic (red circle) SiO2 thin ﬁlms on Si(100) substrates, respectively. For comparison, the data of a hydrophilic SiO2 thin ﬁlms is sourced from ref. 22. (c, d)
Decomposed Si 2p and O 1s spectra, respectively. (A colour version of this ﬁgure can be viewed online.)

Dirac point, i.e., the doping strength, in the graphene channel.
In order to investigate the inﬂuence of SiO2 surface chemistry on
the electronic states of graphene, pinpoint measurements of the Si
2p and O 1s core lines of the SiO2 thin ﬁlms underneath graphene
were conducted at the center of the graphene channels, as shown in
Fig. 3(a) and (b), respectively, with the decomposition of these corelines shown in Fig. 3(c) and (d). The Si 2p core lines of hydrophobic
SiO2 contain a small shoulder around 103 eV, which is not observedin hydrophilic SiO2. This shoulder, which occurs at lower binding
energies, is ascribed to the surface siloxane, as schematically shownin Fig. 1, which has a lower valency than stoichiometric SiO2 [31].
This is corroborated by the angle dependence of the peak intensity
ratio of the siloxane peak over the bulk SiO2 peak (Fig. 4(a) and (b));
the results indicate that siloxane is present on the surface. Both the
O 1s and Si 2p core lines of the hydrophilic SiO2 thin ﬁlm are shifted
towards lower binding energies by a considerable amount (~1.2 eV)
when compared to the shifts in their counterparts corresponding to
the hydrophobic SiO2 thin ﬁlm. This means that there is an interfacial layer that affects the binding energies.
We interpret the shift towards lower binding energies as being
not of chemical origin, they would be too large anyway, but due tothe potential alignment in the GFET device. The difference inbinding energies is very similar (the width of the O 1s peak isslightly different and there is a chemical shift which reveals itself by
the presence of a second component in both interfaces). The
considerable shift in binding energies can be analyzed using theschematic band diagram of graphene/SiO2/Si interfaces in GFETs(Fig. 5(a)). In our pinpoint analysis, the graphene channel, metal
contacts, and back gate (Si substrate) are grounded, i.e., the applied
gate bias (VG) ¼ 0 V, as shown in Fig. 1(b). The Fermi level thenextends through all three materials as a straight line. Band alignment at the Si/SiO2 interface can be derived from the Fermi level at
the Si surface [20]. SiO2, which has a large bandgap (~9 eV), causes alarge potential drop in the Si substrate at the interface, as shown in
Fig. 5(a).
A comparison of graphene/hydrophobic SiO2 and graphene/hydrophilic SiO2 interfaces, in terms of the binding energies of thecore levels and valence band, is shown in Fig. 5(b). Here, the Fermi
level of SiO2 is again aligned with that of graphene. We could detect
the graphene, the topmost layer of the SiO2 thin ﬁlm under graphene, and the graphene/SiO2 interface because the probing depth
was a few nanometers, considering the escape depth of photoelectrons with a kinetic energy of about 500 (O 1s) and 900 (Si 2p)eV [32] at the incident photon energy 1000 eV. The binding

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N. Nagamura et al. / Carbon 152 (2019) 680e687

Fig. 4. Emission angle dependence of surface siloxane and bulk SiO2. (a) Si 2p core level spectra measured in various emission angles. Photoelectron signal from grazing angle issurface sensitive, and that from near normal angle contains information of buried bulk regions. (b) Emission angle dependence of the intensity ratio between the shoulder peak at
103 eV derived from the surface siloxane and the main peak component at 105 eV derived from the bulk SiO2 substrate. (A colour version of this ﬁgure can be viewed online.)

Fig. 5. Schematic band diagrams of interfaces in GFET structures. (a) Schematicband diagram across the GFET. (b) Schematic band diagram of graphene and SiO2 thinﬁlms to demonstrate the inﬂuence of SiO2 surface chemistry. (A colour version of thisﬁgure can be viewed online.)

energies of the Si 2p and O 1s core levels thus reﬂect the changes invalence band alignment with respect to the Fermi level. The point
to be noted is the presence or absence of a dipole layer at thegraphene/SiO2 interface, which depends on the SiO2 surface
chemistry. With respect to the graphene/hydrophilic SiO2 interface,graphene and SiO2 are charged positively and negatively, respectively, as can be inferred from the shifts in the C 1s (Fig. 2(a)), Si 2p(Fig. 3(a)), and O 1s core levels (Fig. 3(b)). We attribute the existenceof silanol groups on the surface of the hydrophilic SiO2 thin ﬁlm tothe negative charges on the surface. According to previous theoretical predictions, neither silanol nor siloxane groups cause dopingin graphene [33]. While this prediction awaits experimental
conﬁrmation, the potential shift due to silanol groups, when incontact with water molecules, may be causing the doping in graphene. In fact, this suggestion is supported by the low value of the
acid-dissociation constant (pKa) of the SiO2 surface (~4.5) [34],which indicates a negative charge by the process of giving up a
proton in water, which has a higher value pKa (pH) of 7. This has

been veriﬁed by in-situ electrochemical Fourier transform infrared
spectroscopy (FTIR) in combination with quantum chemical calculations, which indicate that negatively charged silanol groups areformed when a SiO2 surface is in contact with water [35].
Furthermore, our suggestion is supported by previous theoreticalstudies pointing out the role of water in the doping of graphene on
substrates such as SiO2 [36,37]. The dipole layer thus consists ofpositively-charged graphene and a negatively-charged hydrophilic
SiO2 thin ﬁlm, resulting in a potential drop in the layer, as shown in
Fig. 5(b). The potential drop shifts the energy position of the Si 2p
and O 1s core lines upwards relative to the Fermi level. In the case of
the hydrophobic SiO2 thin ﬁlm, no dipole layer is present at theinterface, hence graphene is not doped, (see Fig. 2), when the surface of the hydrophobic SiO2 thin ﬁlm is covered with uncharged
siloxane groups. A negligible dipole layer is then formed at theinterface. This results in a negligible potential drop, as expressed by
the straight line across the interface (Fig. 5(b)). Thus, pinpoint corelevel spectroscopy demonstrates that the SiO2 surface chemistry
has a great impact on the interfacial electronic level alignment
between graphene and SiO2.
3.3. Inﬂuence of SiO2 surface chemistry on GFET electricalcharacteristics
The above difference in level alignment in graphene on a hydrophilic or hydrophobic SiO2 substrate is expected to have a strong
inﬂuence on GFET electrical characteristics as well [12,13], because
the electronic states of the graphene channel determine the GFET
electrical characteristics. Therefore, we compared the resistance(R)-gate voltage (VG) curves of GFETs using hydrophobic and hydrophilic SiO2 thin ﬁlms as the gate oxides, as shown in Fig. 6(a). In
the GFETs, Ni thin ﬁlms and pþ-Si(100) substrates are used as thesource/drain electrodes and back gate, respectively. The moststriking feature in these curves is the large hysteresis found in the
R-VG curve of the GFET using a hydrophilic SiO2 thin ﬁlm as the gate
oxide, but not in the GFET using hydrophobic SiO2 [12,13]. The
curves for forward and backward sweep on hydrophobic SiO2 are
identical and are thus not resolved in Fig. 6(a). Such hysteresis,
which is reproduced over many consecutive sweeps [13], indicatesN. Nagamura et al. / Carbon 152 (2019) 680e687

685

channels in terms of their interface chemistry with SiO2 gate oxides, which was described in the previous subsection as demonstrated through 3D nano-ESCA.
3.4. Inﬂuence of SiO2 surface chemistry near the metal contact

Fig. 6. Inﬂuence of SiO2 surface chemistry on GFET electrical characteristics. (a) RVG curves of the GFETs using hydrophobic (blue) and hydrophilic (red) SiO2 thin ﬁlms
on Si(100) substrates. (It should be explained in the Figure which of the red curves isforward and backward) No hysteresis occurs in graphene on hydrophobic SiO2, hence
the forward and backward sweep coincide. (b) Schematic diagram explaining themechanism behind the hysteresis of the R-VG curve; the interface dipole layer plays acrucial role. (A colour version of this ﬁgure can be viewed online.)that the doping type changes with a change in the direction of thegate voltage sweep. Because the dipole layer induces a difference inthe level alignment between the two GFETs (Fig. 5(b)), it is obviousthat the dipole layer formed between graphene and the hydrophilic
SiO2 thin ﬁlm affects the doping level of the graphene channel, as
schematically shown in Fig. 6(b). What we observe here is that thereversal of gate voltage inverts the polarity of the dipole layer,
which arises from the polarity inversion of the charging states ofgraphene and the hydrophilic SiO2 surface. As an aside we note thatthe consumption of gate voltage by the dipole layer (Fig. 6(b)) cancause a broadening in the width of the R-VG curve of the GFET withhydrophilic SiO2 rather than hydrophobic SiO2 [38]. This is because
the effective gate capacitance C increases by the dipole layer in theformula [39] which represents the graphene resistance R;

R¼

L

w

1
qﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ þ RC2
em n0 þ ðCjVG  VD jÞ2(2)

where L/w is the aspect ratio of the transistor, m is the mobility, n0 is
the residual charge, and RC is the constant background resistance.
The next thing to be discussed is the comparison of Dirac voltages of the GFETs, where the resistivity (R) is the highest and the
Fermi level is considered to coincide with the Dirac point, by
relating the results obtained by 3D nano-ESCA, as described in theprevious subsection. To relate with the 3D nano-ESCA measurements, we used the R-VG curve in the forward sweep for the GFET
using a hydrophilic SiO2 thin ﬁlm. The reason for this choice is asfollows. The R-VG curves were measured by sweeping VG
from 30 V to þ30 V (forward sweep) and later from þ30 V
to 30 V (backward sweep); it was stopped at 30 V, after which
3D nano-ESCA measurements at VG ¼ 0 V were carried out. Therefore, these measurements can be regarded to occur during a forward sweep. The Dirac voltage of the GFET using a hydrophilic SiO2
thin ﬁlm in the forward sweep is more positive than that recorded
using a hydrophobic SiO2 thin ﬁlm. This indicates the graphene
channel in the GFET using a hydrophilic SiO2 thin ﬁlm in the forward sweep is more hole-doped than that using a hydrophobic SiO2
thin ﬁlm. This result is consistent with the pinpoint C 1s core level
spectra of the graphene channel, which indicate the binding energy
shift toward lower energy on a hydrophobic SiO2 thin ﬁlm due tohole doping as shown in Fig. 2, although we must consider adsorbed molecules other than water, such as O2, during the R-VG measurements [12,14]. These changes in the R-VG curves are thus
explained by the modulation in the electronic states of graphene

Surprisingly, SiO2 surface chemistry also exerts an inﬂuence on
the electronic states near the interface with the metal contact,which is also a key component in GFET. One of the consequences ofsuch metal-contact inﬂuence is the formation of a charge transfer
region (CTR) [13,40], which can extend up to a width of 1 mm in the
GFET using hydrophilic SiO2 as the gate oxide [22]. The CTR is
supposed to be formed due to the disappearance of the density ofstates (DOS) near the Dirac point in graphene. Unfortunately,
however, the inﬂuence of SiO2 surface chemistry on the CTR is still
unclear.
To clarify the inﬂuence of SiO2 surface chemistry on the electronic states of the graphene channel near the interface between
graphene and contact metal, we performed spatially resolved C 1s
core level spectroscopy near the metal contact using 3D nano-ESCA.
The spatial variation in the binding energy of graphene, which reﬂects the change in doping (work function), on hydrophilic andhydrophobic SiO2 thin ﬁlms is shown in Fig. 7. It can be inferred thatacross the entire measured range, the binding energy of graphene
on a hydrophilic SiO2 thin ﬁlm is smaller than that on a hydrophobic SiO2 thin ﬁlm. This means that graphene on a hydrophilic
SiO2 thin ﬁlm is more positively charged, compared to that on ahydrophobic SiO2 thin ﬁlm. The value of binding energy(~284.45 eV), which is very close to that of neutral graphene [27],indicates that the graphene channel is negligibly doped when ahydrophobic SiO2 thin ﬁlm was used, in agreement with the data in
Fig. 2.
In sharp contrast to the hydrophobic SiO2 thin ﬁlm, the binding
energy of graphene on hydrophilic SiO2 becomes smaller near thecontact metal as shown in Fig. 7. The binding energy shift originatesfrom local charge density as shown in Fig. 2(c), so the results whichdisplay the spatial distribution of the binding energy shift in Fig. 7
can be interpreted as a direct measurement of the screening

Fig. 7. Inﬂuence of SiO2 surface chemistry on the potential variation across thegraphene-contact metal interface. Change in the C1s core level of graphene peakacross the graphene/Ni interface in GFETs using hydrophilic (red) and hydrophobic(blue) SiO2 thin ﬁlms as the gate oxides. For comparison, data on hydrophilic SiO2 thinﬁlms is sourced from ref. 22. (A colour version of this ﬁgure can be viewed online.)

686

N. Nagamura et al. / Carbon 152 (2019) 680e687

potential in graphene. Sub-micron CTR formation is detected in thegraphene channel on the hydrophilic substrate, as reported inprevious studies [22]. The determining factor in CTR formation is, inprinciple, supposed to be the charge transfer between materials ofdifferent work functions [13], 4.5 eV for graphene and 5.4 eV for Ni[41], which thermodynamically equilibrate the graphene/Ni system[22].
To explain the difference in the screening potential of the hydrophilic and hydrophilic substrates, we performed theoreticalestimations of the screening potential according to the ThomasFermi approach proposed by Khomyakov et al. [42]. For a single
layer of graphene, they described the screening potential in termsof the charge density in an ungated condition as followsVðxÞ ¼ mF þ signðsÞ

sﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃVB jVB j



m
m
j
j
F F 
 x=l

(3)

s

where x is the distance from the metal/graphene contact edge, ls ¼
Zv=pajVB j is a scaling length, VB ¼ VB1 þ VB2 , Zv ¼ 6:05 eV$Å, anda ¼ e2 =4pε0 kZv ¼ 2:38=k is the ﬁne-structure constant in graphene.
k is the effective dielectric constant. VB1 and VB2 are boundary potential constants, which can be written as

VB1 ¼

1
p
ðW  WG Þ; VB2 ¼ ðWM  WG Þ
4
4

(4)

where WG is the work function of free-standing graphene (4.5 eV),
WM is the work function of the contact metal layer (5.4 eV; Ni in thiscase), and W is the work function of the graphene-covered metal.
The parameter b depends on the contact geometry and b ¼ p whena distance x is large enough compared to d, a thickness of thecontact metal (x [ d), where d ~ 25 nm in this case. W was evaluated using density functional theory (DFT) calculations [43]. In thecase of graphene on Ni(111), the conical dispersion in the graphene
band is destroyed by strong graphene-metal bonding interactions[44]. However, in our process, the resistant residue prevents
chemisorption between graphene and the Ni contact. Later, we canrefer the value of an Au contact, which shows physisorption withgraphene, and has a work function (~5.4 eV) similar to that of a Ni
contact. For large graphene-metal separations due to resistantresidues, WeWG ~0.4 [43]. Subsequently, we obtained k ~1.8 ± 0.9
for graphene on a hydrophobic substrate and k ~77 ± 4 for graphene
on a hydrophilic substrate1 by curve ﬁtting to the measured points
in Fig. 7 using eq. (3) with VB ~ 0.325. If we can neglect polarization
effects at the graphene channel, the effective dielectric constant k isgiven by the average of the dielectric constant of SiO2 (~3.9 eV) andthat of the vacuum due to the image effect [5], i.e., k ~ 2.5. This value
is close to the experimentally obtained value on the hydrophobic
substrate. The large value of k on the hydrophilic substrate is due tothe polarization of the water layer, which has a large dielectric
constant at the graphene/substrate interface. Lacking spatial resolution, the interface dipole layer, of the order of several nanometers,
cannot be detected in our system. However, the screening potentialchanges moderately at large values of k and we can detect spatial
shifts in the screening potential by 3D nano-ESCA with a spatial
resolution of ~100 nm. Therefore, the difference in the potential
variation between the hydrophilic and hydrophobic substrates is

1
The estimated value of the effective dielectric constant, k, is different from ourprevious study in ref. 22 because we adopted the undoped limit of Eq. (3) as a roughapproximation. However, our argument that k shows larger value on a hydrophilic
substrate than typical value of graphene’ s k (~2.5) is consistent. The chemical
potential mF, in other words, the doping level of the graphene on the hydrophilic
substrate is evaluated compared to the hydrophobic case in this study, so now we
can use Eq. (3), which is more general ﬁtting function than a previous study.

caused by the difference in the effective dielectric constants, rather
than the presence/absence of CTR. Although further theoreticalinvestigation with quantum chemistry is required, we believe thatthe positive charging of graphene due to interactions with a hydrophilic SiO2 thin ﬁlm may assist graphene-Ni interactions, which
in turn increases the amount of hole-doping in graphene near Ni,
assuming that charge transfer occurs through bonding between
graphene and Ni in the wide region which is larger than an interfacial dipole layer region.
4. Conclusions
In summary, a combination of 3D nano-ESCA and device characteristics enabled us to quantitatively elucidate that SiO2 surface
chemistry as well as the metal contacts determine the electronic
states of graphene channels and consequently, the GFET device
performance. By using samples in a device geometry and layer
arrangement, we observe a gate voltage induced reversal of theinterface dipole orientation. The results obtained will serve as thebasis for a quantitative understanding of the GFET operation
mechanism, which will help in the realization of high-performancegraphene-based devices.
Acknowledgments
This work was supported by Japan ministry of internal affairs
and communications for SCOPE (1816-2004), the Japan Society for
the Promotion of Science through its “Funding Program for WorldLeading Innovative R&D on Science and Technology (FIRST Program)” and “Grant-in-Aid for Scientiﬁc Research B (Grant Number:
15H03560 and 15K17463),” ,“Grant-in-Aid for Scientiﬁc Research S(Grant Number: 16H06361), ”the Ministry of Education, Culture,
Sports, Science and Technology through its “Grant-in-Aid for Scientiﬁc Research on Innovative Areas (Grant Number: 26107503),”
Japan Science and Technology Agency through its “Core Research
for Evolutional Science and Technology,” and “Materials Research
by Information Integration” Initiative project and PRESTO (Grant
Number: JPMJPR17NB), and the Research Program for CORE lab of“Dynamic Alliance for Open Innovation Bridging Human, Environment and Materials” in “Net-work Joint Research Center for Materials and Devices.” This work was performed at the Synchrotron
Radiation Research Organization, University of Tokyo (Proposal
Nos. 7402 for 2009e2011, 7418 for 2011, 7435 for 2012, and 7419 for2012e2014).
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Science and Technology of Advanced Materials

ISSN: 1468-6996 (Print) 1878-5514 (Online) Journal homepage: https://www.tandfonline.com/loi/tsta20

Spectrum adapted expectation-maximization
algorithm for high-throughput peak shift analysis
Tarojiro Matsumura, Naoka Nagamura, Shotaro Akaho, Kenji Nagata &Yasunobu Ando
To cite this article: Tarojiro Matsumura, Naoka Nagamura, Shotaro Akaho, Kenji Nagata &Yasunobu Ando (2019) Spectrum adapted expectation-maximization algorithm for high-throughput
peak shift analysis, Science and Technology of Advanced Materials, 20:1, 733-745, DOI:
10.1080/14686996.2019.1620123
To link to this article: https://doi.org/10.1080/14686996.2019.1620123© 2019 The Author(s). Published by National
Institute for Materials Science in partnershipwith Taylor & Francis Group.
Accepted author version posted online: 20
May 2019.
Published online: 27 Jun 2019.
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SCIENCE AND TECHNOLOGY OF ADVANCED MATERIALS
2019, VOL. 20, NO. 1, 733–745
https://doi.org/10.1080/14686996.2019.1620123

Spectrum adapted expectation-maximization algorithm for high-throughput
peak shift analysis
Tarojiro Matsumuraa, Naoka Nagamurab,c, Shotaro Akahod, Kenji Nagatac,d and Yasunobu Andoaa

Research Center for Computational Design of Advanced Functional Materials (CD-FMat), National Institute of Advanced Industrial
Science and Technology, Tsukuba, Japan;bResearch Center for Advanced Measurement and Characterization, National Institute for Materials Science (NIMS), Tsukuba, Japan;cJapan Science and Technology Agency, PRESTO, Saitama, Japan;dArtiﬁcial Intelligence Research Center (AIRC), National Institute of Advanced Industrial Science and Technology, Tokyo, Japan
ABSTRACTARTICLE HISTORY

We introduce a spectrum-adapted expectation-maximization (EM) algorithm for highthroughput analysis of a large number of spectral datasets by considering the weight of
the intensity corresponding to the measurement energy steps. Proposed method was applied
to synthetic data in order to evaluate the performance of the analysis accuracy and calculation time. Moreover, the proposed method was performed to the spectral data collected fromgraphene and MoS2 ﬁeld-eﬀect transistors devices. The calculation completed in less than
13.4 s per set and successfully detected systematic peak shifts of the C 1s in graphene andS 2p in MoS2 peaks. This result suggests that the proposed method can support theinvestigation of peak shift with two advantages: (1) a large amount of data can be processedat high speed; and (2) stable and automatic calculation can be easily performed.

Received 19 November 2018
Revised 14 May 2019
Accepted 14 May 2019

1. Introduction
Interpretation of spectral data is essential in spectroscopy measurements for investigating electronic
properties of new materials and devices [e.g. 1–4].
In the case of X-ray photoelectron spectroscopy(XPS), as researchers generally adopt suitable parameters of ﬁtting curves according to previous reports
and their experiences, peak assignment of core-level
spectra in compounds strongly resorts to the manual
trial and error. This procedure surely aﬀects the eﬃciency of the spectral data analysis.

KEYWORDS

EM algorithm; peakseparation; spectral data;
XPS analysis; machinelearning
CLASSIFICATION

60 New topics / Others; 502
Electron spectroscopy

The method of spectral data analysis using
machine learning technique has been studied to
improve the resorting to the manual trial and error[5–11]. For example, Nagata et al. [7] proposeda Bayesian peak separation with the exchange
Monte Carlo method [12] and estimated an appropriate number of peaks while avoiding parameter
solutions trapped into local minima. The eﬀectivenessof this method was practically demonstrated in theanalysis of synthetic data and reﬂectance spectral datafor olivine. Murata et al. [9] extended this method [7]

CONTACT Yasunobu Andoyasunobu.ando@aist.go.jp
Research Center for Computational Design of Advanced Functional Materials (CD-FMat),
National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki 305-8568, Japan
This article has been republished with minor changes. These changes do not impact the academic content of the article.
© 2019 The Author(s). Published by National Institute for Materials Science in partnership with Taylor & Francis Group.
This is an Open Access article distributed under the terms of the Creative Commons Attribution-NonCommercial License (http://creativecommons.org/licenses/by-nc/4.0/),
which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Sci. Technol. Adv. Mater. 20 (2019) 734

to time-series spectral dataset, and a highly accurate
analysis was demonstrated to extract latent dynamics
in the dataset. Moreover, Shiga et al. [10] proposeda new non-negative matrix factorization (NMF) technique to analyze spectral imaging data, namely electron energy-loss/energy-dispersive X-ray spectral
datasets from a speciﬁed region of interest at anarbitrary step width. This technique has helped toresolve problems associated with previous NMF techniques, such as the calculation not always converging
and the number of separated peaks being speciﬁed by
the manual trial and error.
Little attention has been paid to the computational
cost because the number of datasets is not so large inconventional spectroscopy measurements. Recently,
as extremely high brilliant quantum beams such assynchrotron radiation (SR) X-rays, X-ray free electron lasers, and neutron beams are available forprobes of spectroscopy, researchers can perform various kinds of high-resolution analysis (e.g. pumpprobe method with sub-10 fs time resolution andimaging microscopy with spatial resolution of nm
order) [see 1–4]. Such advanced spectroscopy measurements potentially produce huge number of datasets, and then the computational cost has becomea serious problem in the spectral data analysis.
Developing an eﬃcient method for the spectral
data analysis is an urgent issue in the multidimensional measurements. For example, operando
SR X-ray scanning photoelectron microscopy system,
called ‘3D nano-ESCA’ (three-dimensional nanoscale
electron spectroscopy for chemical analysis) [13],provides spatial, time and electric ﬁeld dependenceof photoemission spectra. Incident SR X-rays arefocused by a Fresnel zone plate, and the photoemission spectra are obtained at the beam spot (~70 nm)
on a sample. As a sample is scanned on a piezodriven stage, high spatial resolution XPS analysis
can be conducted during device operation by a bias
voltage applying circuit induced in sample stage (i.e.
operando analysis [14,15]). XPS analysis of core-level
spectra typically means peak ﬁtting and assignment of
decomposed peak components determined by thechemical shifts that takes a particular value depending on a local chemical environment of a speciﬁc
element. In contrast, the 3D nano-ESCA system also
carries out the potential mapping of microstructuresin operating devices by observing the spatial distribution of the core level peak shift. In other words, ‘theelectric potential shift’ reﬂects the change of thevacuum level, the Fermi energy, and the local carrier
density, and the value of the electric potential shift
dynamically ﬂuctuates, unlike the chemical shift.
However, the 3D nano-ESCA has been performed
only for the pinpoint or line-scan analysis that deals
with few tens of spectral datasets by the ineﬃciencyof peak ﬁtting procedure, although spatial and time-

T. MATSUMURA et al.

resolved measurement potentially provides overthousands of the datasets.
In this paper, we adapted an expectationmaximization (EM) algorithm for the spectral data
to investigate the peak shift by the peak ﬁtting andassignment of decomposed peak components. We
derived the spectrum-adapted EM algorithm anddemonstrated this method to the synthetic andexperimental data. In the synthetic data analysis, weevaluated the performances of the analysis accuracy
and calculation cost of the proposed method depending on the initialization procedure and compared theperformances of the proposed method with those of
the exchange Monte Carlo method, and Newton’s
method. In the experimental data analysis, the proposed method was applied to the datasets that werecollected previously from graphene [14,16] and MoS2
[17] ﬁeld-eﬀect transistors (FETs) by 3D nano-ESCA.

2. EM algorithm adapted for thehigh-throughput peak separation
The EM algorithm is one of the machine learningtechniques for estimating the parameters of the mixture model (i.e., Gaussian mixture model, GMM),
including latent parameters, based on maximum likelihood estimation with iterative calculation between
the expectation (E) step and the maximization (M)step [18–20]. This algorithm has been widely studied
and applied in image processing [21–25].
When the conventional EM algorithm is applied
to the peak separation using a linear superpositionof distributions such as Gaussian distributions, theanalyzed data are required to be one-dimensional(a1, a2, a3, . . .). However, the spectral data consistof N measurement steps of energy (x = {x1, . . ., xn,. . ., xN}) corresponding to the intensity (w = {w1, . . .,wn, . . ., wN}). Hence, the spectral data (Dat) arerepresented in two dimensions:
x . . . xN
Dat ¼ 1:(1)
w1 . . . wN
When using the conventional EM algorithm, as
Dat (Equation 1) is converted to be onedimensional (x* = {x1;1 , . . ., x1;w1 , . . ., xn;1 , . . .,xn;wn . . ., xN;1 , . . ., xN;wN }), the size of x* becomesNP
wn . This size is
the sum of the intensity
n¼1

signiﬁcantly larger (103 times or more in general)than the total number of measurement steps (N),and the calculation cost increases greatly high.
Therefore, the conventional EM algorithm is
unsuitable for the high-throughput peak separation in terms of the calculation cost.
We solved this disadvantage by using the intensity(w) as a weight for each measurement step (x). Here,Sci. Technol. Adv. Mater. 20 (2019) 735

T. MATSUMURA et al.

we explain this procedure for the peak separation byusing a GMM. The GMM can be written as a linear
superposition of Gaussians (Nðxjμk ;πk Þ) as follows:K X
GMM xjπk ;μk ;σ k ¼
πk Nðxjμk ;σ k Þ;

n¼1

(2)

k¼1

where K is the number of mixture Gaussian distributions corresponding to the number of separated
peaks, and πk , μk and σ k are, respectively, the mixingcoeﬃcient of k-th Gaussian distribution (0  πk  1
KPandπk ¼ 1), mean and standard deviation
k¼1(σ k > 0). For a given GMM, the aim of the EM algorithm is to maximize the log-likelihood function withrespect to the parameters (πk , μk and σ k ) by iterative
calculation between the E-step and the M-step.
The E-step calculates responsibilities γðznk Þ, whichcorrespond to posterior probabilities when the measurement steps (x) are observed [26]; they are calculated using the current parameters (πk , μk and σ k ) as
follows:
πk Nðxn jμk ;σ k Þ;γðznk Þ¼ PK
j¼1 π j Nðxn jμj ;σ j Þ(3)where znk is a latent variable associated with xn . In
GMM, xn is assumed to be generated from one of theGaussian components. znk represents the component
that generated xn ; i.e. znk is equal to 1 when xn is
generated from k-th component, otherwise znk isequal to 0. Theoretical details of znk are describedby McLachlan and Krishnan, and Bishop [20,26].
Then, in the M-step, the parameters are updated
by using the current responsibilities and intensities(w = {w1, . . ., wn, . . ., wN}) that correspond to themeasurement steps of energy (x = {x1, . . ., xn, . . ., xN})
as follows:Nk
πnew
k ¼ PNn¼1 wn;N
1 X¼
wn γðznk ÞxnμnewkN k n¼1

k¼1

The log-likelihood value monotonically increases in
iterative calculation between E-step and M-step, andthe parameters are converged to a local optimal solution [27].
Convergence criterion of the iterative calculation isdeﬁned as the distance of the log-likelihood values
between Equation (8) and that at the step immediately before the update. When this distance is morethan 1 × 10−8 after the M-step, the calculation isreturned to the E-step. In contrast, when the distanceis below 1 × 10−8, the calculation is determined tonewnewhave converged and the parameters (πnewk , μk , σ k )at that time are adopted as the solution.
Calculation was conducted by using our own sourcecode developed in R (http://cran.r-project.org/). R is
an open-source programming language and softwareenvironment for statistical analysis and graphics. The
reason for using our own code is that major
R packages for the calculation of the EM algorithm[e.g. 28] cannot deal with the weight at each data
point. The computer carrying out the calculations
had an Intel(R) Core(TM) i7 CPU with four cores at
2.9 GHz with 16 GB memory.

3. Application to the synthetic data
We applied the proposed method to synthetic data 1
and 2 in order to evaluate its spectral analysis capability. The synthetic data 1 were used for the examinations of initialization procedures in the proposed
method. The synthetic data 2 were used for the comparison of the analysis accuracy and calculation timewith the proposed method, exchange Monte Carlo
method and, Newton’s method (see also Appendix).
3.1. Synthetic data 1(5)Synthetic data 1 consist of step (x) and intensity (y),and true step-intensity function gðxÞ was the sum of
two Gaussian functions
g ðxÞ¼N
1 X
new T
wn γðznk Þðxn μnewk Þðxn μk Þ ; (6)N k n¼1

where
Nk¼

N
X

wn γðznk Þ:

(8)

(4)

and
σ new
k ¼

ln pðxn jπnew ; μnew ; σ new Þ
(
)
N
K
X
X

new new 
new
¼
wn ln
πk N xn jμk ; σ k
:

(7)

n¼1
new
new
Using these parameters (πnewk , μk , σ k ), the loglikelihood value is updated as follows:

2
X
k¼1

πk Nðxjμk ; σ k Þ:(9)The parameters were given as {π1 , π2 } = {0.3, 0.7},{μ1 , μ2 } = {1.0, 1.8} and {σ 1 , σ 2 } = {0.3, 0.3}. Here, μ2
was increased by 0.02 from 1st to 100th of the spectral
datasets. Hence, 100 sets of synthetic data 1 weregenerated and {μ1 , μ2 } varies from {1.0, 1.8} to{1.0, 2.0}.
The step (x) was collected from the range [0:3] insteps of 0.02 so that the total number of steps was
151. The intensity (y) was given by 106 × gðxÞ, andSci. Technol. Adv. Mater. 20 (2019) 736

T. MATSUMURA et al.

each data point is added the noise following Gaussianthat is one of the most common noise models [10].
The intensity including the noise of each data point(y ) was calculated as follows:y ¼ Nðyjy;σ e Þ;

(10)where σ e is the magnitude of noise (σ e = {103, 104,
5 ×104}). These procedures generated the synthetic
datasets practically simulating spectral datasets.
At the calculation, the number of peaks (K) wereK = 2, and we demonstrated three initialization procedures; (1) manual, (2) random and (3) heuristic. In
the manual initialization, the initial values of eachparameter (πk , μk and σ k ) were {0.5, 0.5}, {1, 1.9}and {1, 1}, respectively. In the random initialization,
the initial values of each parameter were randomly
collected from the range [0:1], [0:3] and [0.1:3.0],
respectively. In the heuristic initialization, random
initialization was repeated 5 times, and the resultwith the maximum value of Equation (8) was
selected.
3.2. Synthetic data 2Synthetic data 2 consist of step (x) and intensity (y),and true step-intensity function hðxÞ was the sum of
three Gaussian functions
hðxÞ¼

3
X
k¼1

πk Nðxjμk ; σ k Þ:(11)

The parameters were {π1 , π2 , π3 } = {0.2, 0.5, 0.3}, {σ 1 ,σ 2 , σ 3 } ={0.15, 0.15, 0.15} and {μ1 , μ2 , μ3 } = {1.1, 1.5,
1.9}. The step (x) was collected from the range [0:3]in steps of 0.02, so that the total number of steps was
151. The intensity (y) was given by 106 × hðxÞ, andthe intensity including the noise (y ) was calculated
from Equation (10).
We generated three synthetic datasets with diﬀerent magnitude of noise (σ e = {103, 104, 5 ×104}) andrepeated the calculation 100 times. At the calculation,
we set K = 3, and the parameters (πk , μk and σ k ) werecollected by the random initialization from the range[0:1], [0:3] and [0.1:3], respectively.
3.3. Result in the analysis of synthetic data 1
Figure 1 shows the example of the peak separation forsynthetic data 1. As ﬁtting curves showed good ﬁttingin each data (Figure 1), the proposed method could
perform reasonable analysis. The relationshipbetween estimated and true peak positions (μ1 = 1.0
and μ2 = 1.8 to 2.0) is shown in Figure (2). Analyzing
low and medium noise data (σe = 103 and 104),
estimated peak positions were close to the true; shifting peak (μ2 = 1.8 to 2.0) and ﬁxed peak (μ1 = 1.0)
were clearly observed. In contrast, when σe = 5 ×104,

estimated peaks were occasionally deviated from thetrue position (Figure 2).
Table 1 respectively shows the root-mean-squareerror (RMSE) between the estimated and true peak
position and the calculation time (s) in each initialization. There was almost no diﬀerence in the RMSE
between these initializations when σe = 103. In contrast, the random initialization showed larger RMSE
than the others when σe = 104, and the heuristic
initialization showed smaller RMSE than the others
when σe = 5 ×104. The calculation times were
11.4–11.8 s, 11.4–12.4 s and 54.1–59.8 s in manual,
random and heuristic initialization for analyzing 100
sets of synthetic data 1 with noise (σe = 103, 104 and 5×104), respectively. There is little diﬀerence in calculation time due to the magnitude of noise, although
the heuristic initialization requires relatively large
calculation time.
3.4. Result in the analysis of synthetic data 2
Figure 3 shows an example of the peak separation
using the proposed method, exchange Monte Carlo
method and Newton’s method. The proposed method
showed good ﬁtting curve for synthetic data 2 with
low noise, whereas unclear peak (= 1.1) could not be
detected for the data with medium and high noise. In
contrast, the exchange Monte Carlo method could
detect an accurate peak position regardless of themagnitude of noise. Newton’s method occasionally
failed to detect the peaks because the parameter solution did not converge. For example, appropriate
peaks were not detected for a set of data with medium and high noise (Figure 3).
Table 2 shows the median of RMSE and averagecalculation time (s) by using each method, respectively. The median of RMSE shows that the exchange
Monte Carlo method could perform a more accurate
analysis than the other methods. In contrast, theproposed method and Newton’s method completed
the calculation over 1000 times faster than theexchange Monte Carlo method. However, Newton’s
method showed signiﬁcantly large RMSE. Thus, theproposed method could analyze more accurately than
Newton’s method at the same order of calculation
time. This result strongly suggests that the proposed
method, and the exchange Monte Carlo method havea trade-oﬀ relationship between the analysis accuracy
and calculation time. In contrast, as Newton’s
method showed insuﬃcient accuracy, it is diﬃcult
to use it for the high-throughput analysis.
We also examined the eﬃciency of the proposed
method relative to the conventional EM algorithm byusing synthetic data 2 in σ e = 103. The proposed
method and conventional EM algorithm with thesame initial values required 1775 and 1166 iterationsof the E- and M-step loop, respectively. However, theSci. Technol. Adv. Mater. 20 (2019) 737

T. MATSUMURA et al.

Figure 1. Example of the ﬁtting curve for the sets of synthetic data 1 at each noise (σe = 103 (Low noise), 104 (Medium noise),
5 × 104 (High noise)) by using the random initialization. Circles show the generated intensity (y*) at each step (x). Blue solid line
is ﬁtting curve, and the dotted blue line is each Gaussian distribution.

total calculation times to reach the convergence were
1.1 s and 21611.4 s, respectively. Therefore, the proposed method signiﬁcantly improves the eﬃciency of
the E- and M-step loops. Moreover, the accuracy ofthe proposed method is better than the conventional
method; RMSE obtained by the proposed method is8607.1, whereas that by the conventional method is41371.9. These results suggest that the proposed
method successfully adapted the conventional
method to the spectrum ﬁtting.

4. Application to the experimental data
4.1. Experimental datasets
The experimental datasets were systematically collectedfrom the FETs [14,16,17] by the 3D nano-ESCA system

in order to investigate the local electronic states in the
structures of devices [13,29,30]. Fukidome et al. [14]collected spectra for the graphene FET on the graphene channel region applying gate biases (Vg). Sutoet al. [17] collected spectra by line scanning on theinterface between a Ni electrode and a four-layer
MoS2 sheet. Nagamura et al. [16] collected spectraby line scanning on the interface between a metalelectrode and a monolayer graphene sheet. The previous works [14,16,17] have reported systematic corelevel peak shifts (i.e. the electric potential shift) for
the C 1s in graphene [14,16] and S 2p in MoS2 [17]peaks. These datasets from Fukidome et al. [14],Suto et al. [17] and Nagamura et al. [16] are labeledhere as Graphene FET-1, MoS2 FET, and Graphene
FET-2, respectively. The background was processed
as a linear background.

Sci. Technol. Adv. Mater. 20 (2019) 738

T. MATSUMURA et al.

Figure 2. Relationship between the number of datasets and estimated two peak positions at each noise (σe = 103 (Low noise),104 (Medium noise), 5 × 104 (High noise)). Initialization (1), (2) and (3) are manual, random and heuristic initialization,
respectively. Green and yellow dots show each estimated peak position. Red dashed line represents the true peak position.

Table 1. Analysis accuracy and computational cost of each
initialization. Initialization (1); manual, initialization (2);random, and initialization (3); heuristic. RMSE values
were calculated from the diﬀerence between true peak
position and estimated peak position. Time (s) represents
the calculation time to complete the analysis for 100 setsof data.
RMSE in Peak 1
Initialization (1)Initialization (2)Initialization (3)RMSE in Peak 2
Initialization (1)Initialization (2)Initialization (3)Time (s)Initialization (1)Initialization (2)Initialization (3)Low noise

Medium noiseHigh noise

0.002
0.002
0.001

0.009
0.038
0.009

0.203
0.212
0.151

0.001
0.001
0.001

0.004
0.010
0.004

0.036
0.041
0.027

11.8
12.2
59.8

11.6
12.4
57.5

11.4
11.4
54.1

4.2. Initial condition for the analysis
Decomposed number of peaks (K) were K = 2, 2 and 3
for the Graphene FET-1, MoS2 FET and Graphene
FET-2, respectively. The initial values of the parameters(πk , μk and σ k ) were, respectively, collected by theheuristic initialization from the range [0:1], [711:713
eV] and [1:3] for Graphene FET-1; [0:1], [832:834 eV]
and [1:3] for MoS2 FET; and [0:1], [709:713 eV] and[1:3] for Graphene FET-2.

4.3. Result in the analysis of experimental data
4.3.1. Graphene FET-1
Analysis using the spectrum-adapted EM algorithm
for the Graphene FET-1 determined the ﬁtting

Sci. Technol. Adv. Mater. 20 (2019) 739

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Figure 3. Example of ﬁtting curve for synthetic data 2 at each noise (σe = 103 (Low noise), 104 (Medium noise), 5 × 104 (Highnoise)). Circles show the generated intensity (y*) at each step (x). Blue solid and dotted line are ﬁtting curve and each gaussian
distribution estimated by the proposed method. Red solid and dotted line are ﬁtting curve and each gaussian function
estimated by the exchange Monte Carlo method. Green solid and dotted line are ﬁtting curve and each gaussian function
estimated by Newton’s method.
Table 2. Analysis accuracy and computational cost of the
proposed method, exchange Monte Carlo method andNewton’s method. Median of RMSE was obtained from the
100 calculations of the peak separation. Each RMSE value wascalculated from the diﬀerence between estimated and true
ﬁtting curve. Time represents the average calculation time (s)of the 100 calculations of the peak separation.
Low noise
Proposed method
Median of RMSE
8607
Time (s)0.77
Exchange Monte Carlo method
Median of RMSE
138
Time (s)610.82
Newton’s method
Median of RMSE
537,532
Time (s)0.17Medium noiseHigh noise

65,0890.32

64,5750.43

1375
472.956849
404.95533,8650.18

537,728
0.48

curves of the GMM that ﬁt the spectral data well.
The calculation for 13 sets of spectral data (eachwith 211 measurement steps) was completed in
14.5 s (1.1 s per set) to separate each spectrum into
two Gaussian distributions. Figure 4(a) shows theexample of the ﬁtting curve and the two decomposed Gaussian distributions. In the previous studyof Fukidome et al. [14], as peak ﬁtting was performed
with two components, we adopt K = 2. The component
at higher kinetic energy is interpreted as C 1s core level
spectrum derived from graphene sp2 bonds, whereasthat at lower kinetic energy is ascribable to carbon
oxide contaminants. The ﬁtting curves showed thatthe graphene peak position is systematically shifted by~130 meV depending on the gate bias (Vg) in the range−40 to −5 V (Figure 4(b)). This peak shift is consistent
with the peak shift of about 200 meV corresponding to
the gate bias (in the range −40 to 0 V) in Fukidome
et al. [14]. Here, the binding energy of graphene isexpressed in terms of the gate bias (Vg) [14,31–33]:
pﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ
EBE ðGÞ ¼ EBE ðDPÞ  6:0  102 V CNP V g ; (12)where the EBE(G) and VCNP are the binding energy ofthe graphene and the charge neutrality point (VCNP =28 [14]), respectively. Also, EBE(DP) is the binding

Sci. Technol. Adv. Mater. 20 (2019) 740

T. MATSUMURA et al.

Figure 4. (a) Example of GMM ﬁtting curve for the spectra of Graphene FET-1 at gate biases of −5 and −40 V. The horizontal axis
is the kinetic energy (eV), the vertical axis is the intensity (arbitrary units), and the open black circles show the observed spectral
data. Blue solid and dotted line are ﬁtting curve and each Gaussian distribution. Red dot indicates the peak position of thegraphene (712.85 and 712.98 eV at gate biases (Vg) of −5 and −40 V, respectively). (b) The graphene peak shift of the binding
energy (EBE(G)) against the gate bias (Vg). Red dotted curve is the ﬁtting curve given by Equation (12) using EBE(DP) = 283.95 eV
and VCNP = 28 V [14]. The binding energy (eV) is obtained by converting the kinetic energy; i.e., binding energy = 996.45 −kinetic energy.

energy of graphene when the Fermi level coincides
with the Dirac point, i.e., the energy diﬀerence
between the Dirac point energy and the C 1s corelevel of graphene [14]. The theoretical curve(Equation 12) ﬁtted to the graphene peak position
at Vg = −40 – −5 V shows EBE(DP) as 283.95 eV and
VCNP as 28 V [14] (Figure 4(b)). The value of EBE(DP) (283.95 eV) is close to the binding energy ofneutral graphene (284.4 eV) [34]. The slight
diﬀerence between the binding energy of EBE(DP)(283.95 eV) and neutral graphene (284.4 eV) may
be ascribable to minute uncertainties in the incident
photon energy or Fermi-edge measurements used todetermine the binding energies [14]. However, thebinding energy of graphene peak at Vg = 0 was
underestimated about 300 meV relative to the theoretical curve (Equation 12) in the previous study [14].
This is because that the contaminant component
derived from the 0 th order of diﬀracted beam of
the Fresnel zone plate would be large in the case of
Vg = 0. Estimation of the appropriate peak position
using GMM may not be successful when the contaminant component is large or the asymmetry of thepeak shape cannot be negligible.
4.3.2. MoS2 FET
The proposed method showed GMM ﬁtting curves andS 2p3/2 and S 2p1/2 peak positions from the MoS2 FET

Sci. Technol. Adv. Mater. 20 (2019) 741

T. MATSUMURA et al.

Figure 5. (a) Example of GMM ﬁtting curve for the spectra of MoS2 FET at the Ni/MoS2 interface. The horizontal axis is the kinetic
energy (eV), the vertical axis is the intensity (arbitrary units), and the open black circles indicate the observed spectral data. Bluesolid and dotted line are ﬁtting curve and each Gaussian distribution. Red dots indicate the positions of the S 2p1/2 and S 2p3/2
peaks of the MoS2 sheet at 6000 nm (832.18 and 833.35 eV) and 6800 nm (832.08 and 833.29 eV) of the relative position (Figure5(b)), respectively. (b) Plot of the S 2p3/2 (green point) and S 2p1/2 (yellow point) peak position of the binding energy against therelative position. The S 2p3/2 and S 2p1/2 peak position focusing on the spectral data at the vicinity of the contact of a Nielectrode with 4-layer MoS2. The binding energy (eV) is obtained by converting the kinetic energy; i.e. binding energy = 995.690− kinetic energy.

spectral data (Figure 5). In Suto et al. [17], peak ﬁtting
was performed with two components of S 2p3/2 andS 2p1/2, so that we also adopt K = 2. The calculation to
separate each of the 347 individual spectral data (eachwith 76 measurement steps) into two Gaussian distributions was completed in 30.6 s (0.09 s per set). This
calculation completed in a signiﬁcantly short time
because the spectra of MoS2 FET consists of relatively
small measurement steps. Figure 5(a) shows the example of GMM ﬁtting curves and the two decomposed
Gaussian distributions. The ﬁtting curves show that
S 2p3/2 and S 2p1/2 peak positions systematically shiftedby ~150 and ~100 meV from 5800 nm to 7000 nm,
respectively (Figure 5(b)). This peak shift is observablenear the interface between the Ni electrode and MoS2
sheet (approximately 5750~ nm). Suto et al. [17]reported that such systematic peak shift overlaps witha charge transfer region; they detected this region at the
MoS2/metal–electrode interface expanding over ~500
nm, with the electrostatic potential variation of binding
energy (~70 meV) mainly causing the transfer ofcharges by contacting MoS2 with the Ni metal electrode. This peak shift has been considered due to band
bending in the MoS2 electronic structure with a Fermi
level shift [17,35,36].
4.3.3. Graphene FET-2
Analysis using the proposed method for theGraphene FET-2 spectral data showed ﬁtting
curves, three decomposed Gaussian distributions(Figure 6(a)) and the proﬁle of the graphene peak
positions (Figure 6(b)). The calculation to separate
each of the 44 sets (each with 211 measurement
steps) into three Gaussian distributions was completed in 589.7 s (13.4 s per set). Nagamura et al.
[16] performed peak ﬁtting to the Graphene FET-2
spectral data with two components: the higherkinetic-energy component is interpreted as theC 1s core level spectrum derived from graphene
sp2 bonds, and the lower-kinetic-energy component
is interpreted as that from surface contaminants.
However, we adopt K = 3 in these data because it isbetter to consider multiple components of the surface contaminants from polymer residue in thedevice fabrication process and naturally involved
amorphous carbon [37–39]. The graphene peak
position is systematically shifted by ~140 meV to~700 nm from the vicinity of the interface between
the metal electrode and the monolayer graphenesheet (Figure 6(b)). Such a peak shift overlaps
with a charge transfer region at the graphene/
metal-electrode boundary [40,41]. Nagamura et al.
[16] reported a ~60 meV peak shift at ~500 nm of
the charge transfer region in graphene at a metalboundary. Assuming the measurement energy step
containing ~±50 meV error due to the resolution ofequipment, the proposed method could detect
acceptable peak position and the same order ofenergy shift relative to the result in the previous
research [16].

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T. MATSUMURA et al.

Figure 6. (a) Example of GMM ﬁtting curve for the spectra of Graphene FET-2 at the interface between the metal electrode andthe monolayer graphene sheet. The horizontal axis is the kinetic energy (eV), the vertical axis is the intensity (arbitrary units),
and open black circles indicate the observed spectral data. Blue solid and dotted line are ﬁtting curve and each Gaussian
distribution. Red dot indicates the position of the C 1s peak of the graphene (711.88 and 711.76 eV at the 0 nm and 450 nm of
the relative position (Figure 6(b)), respectively). (b) Proﬁle for the binding energy of the graphene peak position (black dots)
against the relative position of the spectra. The binding energy (eV) is obtained by converting the kinetic energy; i.e. binding
energy = 995.996 − kinetic energy.

5. Discussion and implication
The spectrum-adapted EM algorithm was proposedand successfully applied to synthetic and experimental datasets. The advantage of the proposed method isthe fast and stable calculation. The peak separation
for the synthetic and experimental datasets was completed less than 1.0 and 13.4 s per set of the data,
respectively. As the parameters are converged toa local optimal solution by monotonically increasing
log-likelihood value in the iterative calculation, theproposed method can stably conduct the peak separation. Thus, it is unnecessary to conduct the manual
trial and error in order to converge the parameter

solution in using ordinary gradient methods such as
Newton’s method. In contrast, the exchange Monte
Carlo method can perform a more accurate analysis
than the proposed method (Figure 3). Moreover, theappropriate number of peaks can be determined by
calculating the model selection criteria such as themarginal likelihood [7]. However, the exchange
Monte Carlo method requires high computationalcost, and it is not easy to set a suitable prior distribution and inverse temperature for non-expert. As sucha setting is unnecessary to use the proposed method,
the peak shift analysis can be performed easily.
The random and heuristic initialization are suitable initialization procedures for the high throughput

Sci. Technol. Adv. Mater. 20 (2019) 743

analysis because these procedures can automaticallyprocess the spectral data. Especially, heuristic initialization helped to ﬁnd a reasonable solution in theanalysis of noisy data (Figure 2) and showed acceptable performance in the peak shift analysis of theexperimental data (Figures 4–6). These applications
suggest that the proposed method with the heuristic
initialization may be applicable to other spectral
deconvolution problems for investigating electronic
properties of materials and devices at an adequately
small computational cost. However, it is necessary tosophisticate the initialization procedure because
heuristic initialization cannot systematically selectan appropriate number of peaks, and the computational cost remains relatively higher than the others(Table 1). Moreover, further studies are needed to beable to use other common ﬁtting functions such as
Lorentzian and Voigt function, and asymmetric lineshape functions such as the Doniach–Sunjic function[42] according to Section 4.3.1. To overcome thesedisadvantages is important for further improvementof the high throughout the method.

6. Conclusions
We proposed the spectrum-adapted EM algorithm asa high-throughput method to investigate the peak shift
from a large number of spectral datasets. Application to
the synthetic datasets suggested that heuristic initialization can perform more accurate analysis than otherinitializations with relatively large calculation time,
and the proposed method, and the exchange Monte
Carlo method have a trade-oﬀ relationship between theanalysis accuracy and calculation time. Moreover, theproposed method was applied to experimental datasetscollected from two graphene [14,16] and one MoS2
[17] FETs and detected the systematic peak shifts
close to the results in the previous works [14,16,17] inless than 13.4 s per set. These applications suggest thatthe proposed method has acceptable accuracy to investigate the peak shift at high speed. Even a non-expert
analyst can easily and automatically use this method forthe spectral data analysis.

Acknowledgments
This paper is based on results from a project (P16010)
commissioned by the New Energy and Industrial
Technology Development Organization (NEDO), JST
CREST (JPMJCR1761), the ‘Materials Research by
Information Integration’ Initiative project and
PRESTO (Grant number: JPMJPR17NB), commissionedby the Japan Science and Technology Agency (JST), andthe Research Program for CORE lab (2016002) of‘Dynamic Alliance for Open Innovation Bridging
Human, Environment and Materials’ of the ‘Network
Joint Research Center for Materials and Devices’. We
thank Prof. Kosuke Nagashio of the University of

T. MATSUMURA et al.

Tokyo and Prof. Hirokazu Fukidome of Tohoku
University for measuring the presented data. The spectral datasets were obtained with the support of the
University of Tokyo outstation beamline at SPring-8(Proposal
Numbers:
2012B7402,
2013A7443,
2013B7451, 2014B7472, and 2015A7482).

Disclosure statement
No potential conﬂict of interest was reported by theauthors.

Funding
This work was supported by the Precursory Research for
Embryonic Science and Technology [JPMJPR17NB]; New
Energy and Industrial Technology Development
Organization [P16010]; Research Program for CORE lab[2016002]; Core Research for Evolutional Science andTechnology [JPMJCR1761].

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Appendix. Procedure of the peak separation
using the Newton’s method and the exchange
Monte Carlo method
Here, we describe the procedure of the peak separation
using the Newton’s and exchange Monte Carlo method tothe synthetic data 2. The peak separation model is used thesum of three Gaussian functions s(x):sðxÞ ¼bk
ak exp  ðx  μk Þ2
2
k¼1

3
X

(A1)

where ak , bk and μk are the strength, bandwidth, and centerof k-th Gaussian function. The set of parameters θ ¼
fak ; bk ; μk gKk¼1 are optimized by minimizing the meansquared error function EðθÞ between the synthetic data
fxn ; yn gNn¼1 and the function (Equation A1):

Sci. Technol. Adv. Mater. 20 (2019) 745

N
1 XEðθÞ ¼ðy  sðxn ; θÞÞ2
2N n¼1 n

T. MATSUMURA et al.

(A2)φðbk Þ ¼ Gamma ðbk ; ηb ;λb Þ(A4)φðμk Þ ¼ N ðμk ; η0 ; λ0 Þ(A5)We applied Newton’s method to the optimization of EðθÞ.
The initial values of parameters fak ; bk ; μk gare randomly
chosen from the ranges [500,000–150,000], [1–100] and[0–3].

The hyperparameters fηa ; λa g, fηb ; λb g and fη0 ; λ0 g were
{10, 1e-5}, {10, 1/5} and {1.5, 1/5}, respectively. The determination of the prior densities and hyperparameters is heuristics inthis study.

Using EðθÞ (Equation A2), we also performed Bayesianpeak separation by the exchange Monte Carlo method(theoretical details are shown in Nagata et al. [7]).
Application of the exchange Monte Carlo method
requires to set (1) Prior densities, (2) Inverse temperature, and (3) Initial condition. These settings are shown
below.

(2) Inverse temperature
According to Nagata et al. [7] and Nagata and Watanabe[43,44], the number of inverse temperatures L was 24, andthe inverse temperature was given by(1) Prior densities
The prior densities φðak Þ, φðbk Þ and φðμk Þ of theparameters were, respectively, deﬁned in terms of thefollowing Gamma, Gamma and Gauss distribution:βl is each inverse temperature ð0 ¼ β1 > β2 > . . . >βL1 > βL ¼ 1Þ.

φðak Þ ¼ Gamma ðak ; ηa ;λa Þ

(A3)


βl ¼

0
1:5lLðif l ¼ 1ÞðotherwiseÞ

(A6)(3) Initial condition
The initial values of parameters fak ; bk ; μk g are randomly
chosen from the range [500,000–150,000], [1–100] and [0–3],
respectively. The iteration was set as 10,000 steps for the burnin period, and 2000 steps for the expectation value calculation.

６．Staff
Director:

HARADA Yoshihisa, Professor
KOMORI Fumio, Professor
MATSUDA Iwao, Associate Professor
KONDO Takeshi, Associate Professor
NIIBE Masahito, Visiting Associate Professor
YAMAMOTO Susumu, Research Associate (~2019.10)
YAJI Koichiro, Research Associate
MIYAWAKI Jun, Research Associate
HIRATA Yasuyuki, Research Associate (~2019.9)
HARASAWA Ayumi, Technical Associate
SHIBUYA Takashi, Technical Associate
KUDO Hirofumi, Technical Associate
FUKUSHIMA Akiko, Technical Associate

Technical Assistant:

FUJISAWA Masami
KOSEGAWA Yuka
ARAKI Mihoko

Secretary:

AIHARA Yumiko
HARADA Misa
YOSHIZAWA Motoko
TSUTSUMI Yumiko
IKEDA Kuniko

Graduate Student:

IKEDA Keisuke
WATANABE Ryusuke
UGALINO Ralph John
SATO Yusuke
ZHAO Yuhao
YANG Tianqi

Contract Researcher:

OSHIMA Masaharu
AKADA Keishi
YAMAZOE Kosuke
YAMAGAMI Kohei (~2020.2)
OHDAIRA Takeshi
EL Moussaoui Souliman
ZHANG Wenxiong
KURAHASHI Naoya (2019.11~)

Harima Paper 2019
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1

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3

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6

7

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Daisuke Asakura, Yusuke Nanba, Eiji Hosono,
Mn 2p resonant X-ray emission clarifies the redox
Phys. Chem.
Masashi Okubo, Hideharu Niwa, Hisao Kikuchi, Junreaction and charge-transfer effects in LiMn2O4
Chem. Phys.
Miyawaki, Yoshihisa Harada
Modulation of Electronic States near Electrodes in
Hirokazu Fukidome, Kazutoshi Funakubo, Naoka
Sensors andGraphene Transistors Observed by Operando
Nagamura, Koji Horiba, Yasunori Tateno, Masaharu
Materials
Photoelectron Nanospectroscopy
Oshima, Maki Suemitsu
Masaharu Ito, Hidefumi Takahashi, Hideaki Sakai,High pressure synthesis of a quasi-one-dimensional
Hajime Sagayama, Yuichi Yamasaki, Yuichi
Chem. Commun.
GdFeO3-type perovskite PrCuO3 with nearly divalent Cu Yokoyama, Hiroyuki Setoyama, Hiroki Wadati,(Communication
KanakoTakahashi,
Yoshihiro
Kusanoand
Shintaro)
ions
Ishiwata
Y. Kubota, M. Suzuki, T. Katayama, K. Yamamoto,Polarization control with an X-ray phase retarder for highK. Tono, Y. Inubushi, T. Seki, K. Takanashi, H.
J.Synch. Rad.
time-resolution pump–probe experiments at SACLA
Wadati and M. Yabashi

29 偏光変調型軟X線を用いた複素誘電率の直接測定
30

K. Ozawa, S. Yamamoto, K. Mase, I. Matsuda
久保田雄也、平田靖透、赤井久純、松田巌

Spectrum adapted the expectation-maximization algorithm Tarojiro Matsumura, Naoka Nagamura, Shotaro
for high-throughput peak shift analysis
Akaho, Kenji Nagata & Yasunobu Ando

Sci.Technol.
Adv. Mater.

Magnetic and electronic properties of B -site-ordered37
double-perovskite oxide La2CrMnO6 thin films

K.Yoshimatsu, J.Ishimaru, K.Yamamoto, Y.Hirata,
H.Wadati, Y.Takeda, K.Horiba, H.Kumigashira,O.Sakata, A.Ohtomo

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まぐね

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140-145

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J. Chem. Phys.

150204201(1-7)

2019

B. Feng, H. Zhou, Y. Feng, H. Liu, S. He, I. Matsuda,
41 Superstructure-induced splitting of Dirac cones in silicene L. Chen, E. F. Schwier, K. Shimada, S. Meng, and K. Phys. Rev. Lett.
Wu

122

196801(1-6)

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Segmented Undulator for Extensive Polarization Controls I. Matsuda, S. Yamamoto, J. Miyawaki, T. Abukawa, e-J. Surf. Sci.
in ≦1 nm-rad Emittance Rings
and T. Tanaka
Nanotechnol.

17

41-48

2019

Direct Evidence of Interfacial Hydrogen Bonding in
43 Proton-Electron Concerted 2D Organic Bilayer on Au
Substrate

17

49-55

2019

44

3

44004
(1-5)

2019

9

4539-4550

2019

99

134414(1-10)

2019

233

64-68

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3

024004
(1-8)

2019

480

419-4262019

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4388-4395

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463

1161-1167

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51

115-119

2019

Naoka Nagamura, Hirokazu Fukidome, Kosuke
Influence of interface dipole layers on the performance of Nagashio, Koji Horiba, Takayuki Ide, Kazutoshi
38graphene field effect transistors
Funakubo, Keiichiro Tashima, Akira Toriumi, Maki
Suemitsu, Karstern Horn, Masaharu Oshima
39 時間分解X線磁気円二色性で見るスピンダイナミクス
山本航平、和達大樹

40

42

45

46

47

48

49

50

51

Measurements of ultrafast dissociation in resonant
inelastic x-ray scattering of water

Kosuke Yamazoe, Jun Miyawaki, Hideharu Niwa,
Anders Nilsson, Yoshihisa Harada

Susumu Yamamoto, Hiroyuki S Kato, Akira Ueda,
Shinya Yoshimoto, Yasuyuki Hirata, Jun Miyawaki, e-J. Surf. Sci.
Kohei Yamamoto, Yoshihisa Harada, Hiroki Wadati, Nanotechnol.
Hatsumi Mori, Jun Yoshinobu, Iwao Matsuda
M. Cameau, R. Yukawa, S. Ito, R. Ishibiki, K.
Electronic structure of a monatomic Cu2Si layer on a Si
Phys. Rev.
Horiba, Y. Obata, T. Kondo, H. Kumigashira, M.
Materials
substrate
D'angelo, and I. Matsuda
CO2 Activation and Reaction on Zn-Deposited Cu
T.Koitaya, S.Yamamoto, Y.Shiozawa, Y.Toshikura,
M.Hasegawa, J.Tang, K.Takeuchi, K.Mukai,
ACS Catalysis
Surfaces Studied by Ambient-Pressure X-ray
S.Yoshimoto, I.Matsuda, J.Yoshinobu
Photoelectron Spectroscopy
R.Y. Umetsu, H. Fujiwara, K. Nagai, Y. Nakatani,
Half-metallicity of Mn2VAl ferrimagnet revealed by
M. Kawada, A. Sekiyama, F. Kuroda, H. Fujii, T.
Phys. Rev. B
resonant inelastic soft x-ray scattering in a magnetic field
Oguchi, Y. Harada, J. Miyawaki, and S. Suga
Keishi Akada, Takaaki Sudayama, Daisuke Asakura,
Operando measurement of single crystalline Li4Ti5O12
J.Electron
Hirokazu Kitaura, Naoka Nagamura, Koji Horiba,
Spectrosc.Relat.
with octahedral-like morphology by microscopic X-ray
Masaharu Oshima, Eiji Hosono, and Yoshihisa
Phenom.
photoelectron spectroscopy
HaradaI. Tateishi, N. T. Cuong, C. A. S. Moura, M. Cameau,
Semimetallicity of free-standing hydrogenated monolayer R. Ishibiki, A. Fujino, S. Okada, A. Yamamoto, M. Phys. Rev.
boron from MgB2
Araki, S. Ito, S. Yamamoto, M. Niibe, T. Tokushima, Materials
D. E. Weibel, T. Kondo, M. Ogata, and I. Matsuda
Mass transport in the PdCu phase structures during
Jiayi Tang, Susumu Yamamoto, Takanori Koitaya,hydrogen adsorption andabsorption studied by XPS under Akitaka Yoshigoe, Takuma Tokunaga,Kozo Mukai, Appl. Surf. Sci.
hydrogen atmosphere
Iwao Matsuda, Jun Yoshinobu
Enhanced Photoresponsivity of Fullerene in the Presence Kenichi Ozawa, Susumu Yamamoto, Marie D’of Phthalocyanine: A Time-Resolved X-ray Photoelectron angelo, Yuto Natsui, Naoya Terashima, Kazuhiko
J. Phys. Chem. C
Spectroscopy Study of Phthalocyanine/C60/TiO2(110)
Ma, and Iwao Matsuda
Hydrogen adsorption and absorption on a Pd-Ag alloy
J. Tang, S. Yamamoto, T. Koitaya, Y. Yoshikura, K.
surface studied using in-situ X-ray photoelectronAppl. Surf. Sci.
Mukai, S. Yoshimoto, I. Matsuda, J. Yoshinobu
spectroscopy under ultrahigh vacuum and ambientpressure

Yusuke Hashimoto Munetaka Taguchi Shun Fukami Surface andSite‐sensitive X‐ray photoelectron spectroscopy of Fe3O4
52
Hiroki Momono Tomohiro Matsushita Hiroyuki
Interfaceby photoelectron diffraction
Matsuda Fumihiko Matsui Hiroshi Daimon
AnalysisMonatomic Two-Dimensional Layers: Modern
53 Experimental Approaches for Structure, Properties, andIndustrial Use
E-labo. Paper 2019
No. Title

Iwao Matsuda ed.

Elsevier

Authors
JournalPeng Zhang, Zhijun Wang, Xianxin Wu, Koichiro
Yaji, Yukiaki Ishida, Yoshimitsu Kohama,
Guangyang Dai, Yue Sun, Cedric Bareille, Kenta
Kuroda, Takeshi Kondo, Kozo Okazaki, Koichi
1 Multiple topological states in iron-based superconductors Kindo, Xiancheng Wang, Changqing Jin, Jiangping Nature Physics
Hu, Ronny Thomale, Kazuki Sumida, Shilong Wu,
Koji Miyamoto, Taichi Okuda, Hong Ding, G. D. Gu,
Tsuyoshi Tamegai, Takuto Kawakami, Masatoshi
Sato & Shik Shin(232 pages) 2019

Vol.

Page

Year

15

41

2019

2

A weak topological insulator state in quasi-onedimensional bismuth iodide

3

Coexistence of Two Types of Spin Splitting Originatingfrom Different Symmetries

4

Giant Rashba system on a semiconductor substrate withtunable Fermi level: Bi/GaSb(110)–(2×1)

5Unified description of the electronic structure of M2AC
nanolamellar carbides

Ryo Noguchi, T. Takahashi, K. Kuroda, M. Ochi, T.
Shirasawa, M. Sakano, C. Bareille, M. Nakayama,
M. D. Watson, K. Yaji, A. Harasawa, H. Iwasawa, P.
NatureDudin, T. K. Kim, M. Hoesch, V. Kandyba, A.
Giampietri, A. Barinov, S. Shin, R. Arita, T.
Sasagawa & Takeshi Kondo
Koichiro Yaji, Anton Visikovskiy, Takushi Iimori,
Kenta Kuroda, Singo Hayashi, Takashi Kajiwara,
Phys. Rev.Lett.
Satoru Tanaka, Fumio Komori, and Shik Shin
Takuto Nakamura, Yoshiyuki Ohtsubo, Naoki
Tokumasu, Patrick Le Fèvre, François Bertran, ShinPhys. Rev.
ichiro Ideta, Kiyohisa Tanaka, Kenta Kuroda,
Materials
Koichiro Yaji, Ayumi Harasawa, Shik Shin, Fumio
Komori, and Shin-ichi Kimura
Damir Pinek, Takahiro Ito, Masashi Ikemoto,
Koichiro Yaji, Masashi Nakatake, Shik Shin, andPhys. Rev. B
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