2018

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 at 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, SRL staff members have played an essential role in promoting both domestic andinternational joint research. Three end-stations, a time-resolved photoelectron spectroscopy,
a 3D nanoESCA, and a high-resolution soft x-ray emission spectroscopy stations, have theworld’s best time resolution (~50 ps), spatial resolution (~70 nm), and energy resolution(E/E ≈ 10,000), respectively. They have been permanently installed in the beamline toaccept joint-research proposals, while a free-port station equipped with a focusing mirror isavailable to outside users. In 2018, an ambient-pressure operando X-ray photoemission
spectroscopy station was promoted as one of the end-stations accepting joint-research
proposals.
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, we
have promoted soft X-ray science using high-harmonic lasers in the vacuum ultra-violet
and soft X-ray regions at the Kashiwa Campus E-building. In 2014, SRL constructed a new
high-resolution laser spin- and angle-resolved photoelectron spectroscopy (SARPES)
apparatus that is designed to provide high-energy and -angular resolutions and highefficiency spin detection for various types of solids, such as spin-orbit coupled materials
and ferromagnetic materials. Since FY 2015, the SARPES system has been open for users
and accepting joint-research proposals.
Our goal is to provide users with a soft X-ray platform that uses both synchrotron
radiation and high-harmonic lasers by strong collaboration with other LASOR groupmembers.
June 30, 2019
Yoshihisa Harada
Director of SRL-ISSP

Activity Report 2018
TABLE OF CONTENTS
pp2

Preface : Yoshihisa Harada

8

1 Status of Beamline BL07LSU at the SPring-8

122 Status of spin-and angle-resolved photoelectron spectroscopy with laser light at LASOR

14

3 Workshops & Meetings

17

4 Seminar
5 Activities

Synchrotron Radiation Experiments (SPring-8)181) OBSERVATION OF Li DIFFUSION AND ELECTRONIC-STRUCTURE

DISTRIBUTION IN THE ACTIVE MATERIAL OF ALL-SOLID-STATE Li-ION
BATTERY BY OPERANDO PHOTOELECTRON SPECTROMICROSCOPY
Keishi Akada, Takaaki Sudayama, Daisuke Asakura, Hirofumi Matsuda, Naoka Nagamura,
Masaharu Oshima, Eiji Hosono, Yoshihisa Harada

202) X-RAY PHOTOELECTRON SPECTROSCOPY OF LITHIUM STRAGE IN SnO 2

SINGLE-PARTICLE MICROWIRE ANODE MATERIAL
Keishi Akada, Takaaki Sudayama, Daisuke Asakura, Hirofumi Matsuda, Naoka Nagamura,
Masaharu Oshima, Eiji Hosono, Yoshihisa Harada

223) ELECTRONIC STRUCTURE ANALYSIS OF CATHODE MATERIALS FOR LI-ION

BATTERIES BY OPERANDO SOFT X-RAY EMISSION SPECTROSCOPY USINGANAQUEOUS ELECTROLYTE SOLUTION
Daisuke Asakura, Takaaki Sudayama, Jun Miyawaki, Yoshihisa Harada, Eiji Hosono

244) AN OPERANDO SOFT X-RAY SPECTROSCOPY STUDY OF A CATHODE

MATERIAL FOR LI-ION BATTERIES TO INVESTIGATE THE REACTION OF
EACH REDOX PEAK
Daisuke Asakura, Takaaki Sudayama, Hirofumi Matsuda, Jun Miyawaki, Yoshihisa Harada,
Eiji Hosono

265) PROBING OXYGEN-REDOX BATTERY CATHODES
Masashi Okubo

286) REDOX DRIVEN SPIN TRANSITION IN SOLID
Masashi Okubo

307) ENHANCEMENT OF PHOTORESPONSIVITY OF FULLEREN IN

PHTHALOCYANITE/FULLERENE ORGANIC SOLAR CALL
Kenichi Ozawa, Susumu Yamamoto, Marie D’angelo, Yuto Natsui, Naoya Terashima,
Kazuhiko Mase, Iwao Matsuda

328) PHOTOELECTRON NANO-SPECTROSCOPY PF REACTIVE ION ETCHING-

INDUCED DAMAGES TO THE TRENCH SIDEWALLS AND BOTTOMS OF 4h-SIC
TRENCH-MOSFETS
Masaharu Oshima, Daisuke Mori, Aki Takigawa, Akihiko Otsuki, Naoka Nagamura, Shun Konno,
Yoshinori Takahashi, Masato Kotsugi, Hiroshi Nohira, Keishi Akada, Yoshihisa Harada

349) STUDY ON INTERFACIAL HYDROGEN-BOND STATES IN ELECTRON-PROTON-CORRELATED ORGANIC HETEROBILAYERS
Hiroyuki S. Kato, Susumu Yamamoto, Akira Ueda, Shinya Yoshimoto, Hatsumi Mori, Jun Yoshinobu,
Hiroki Wadati, Iwao Matsuda

3610) OPERANDO OBSERVATION OF METHANE ACTIVATION ON HETEROGENEOUS

CATALYSTS BY AMBIENT-PRESSURE XPS
Takanori Koitaya, Susumu Yamamoto, Iwao Matsuda, Jun Yoshinobu, Toshihiko Yokoyama

3811) SYNERGETIC EFFECT AT METAL-OXIDE INTERFACE ON METHANE

ACTIVATION MEASURED BY AMBIENT-PRESSUREXPS
Takanori Koitaya, Susumu Yamamoto, Iwao Matsuda, Jun Yoshinobu, Toshihiko Yokoyama

4012) CHEMICAL STATE ANALYSIS OF INTERMEDIATE WATER BOUNCED BY

BIOCOMPATIBLE POLYMERSMasahiro Kunisu, Masaru Nakada, Koichi Ozaki, Junpei Yahiro, Yitao Cui, Kosuke Yamazoe,
Jun Miyawaki, Yoshihisa Harada

4213) SIZE AND TEMPERATURE DEPENDENCE OF HYDROGEN-STRAGE IN Pd

NANOPARTICLES
Seo Okkyun, Jiayi Tang, Takanori Koitaya, Susumu Yamamoto, Chulho Song, Iwao Mastuda,
Jun Yoshinobu, Osami Sakata

4414) ELUCIDATION OF BAND ALIGNMENT AT SEMICONDUCTOR/ELECTROLYTE

INTERFACE BY X-RAY PHOTOELECTRON SPECTROSCOPY
Yuki Imazeki, Masahiro Sato, Takahito Takeda, Masaki Kobayashi, Susumu Yamamoto, Iwao Matsuda,
Jun Yoshinobu, Yoshiaki Nakano, Masakazu Sugiyama

4615) STRUCTURE ANALYSIS OF BATIO 3/PT (111)/SI QUASCICRYSTAL THIN FILM

BY PHOTOELECTRON HOLOGRAPHY
Shun Fukami, Hiroki Momono, Masafumi Ogi, Peng Dawei, Yusuke Hashimoto, Hiroyuki Matsuda,
Munetaka Taguchi, Stefan Förster, Wolf Widdra, Hiroshi Daimon

4816) PHOTOINDUCED DYNAMICS IN Ba 3CuSb2O9 WITH Cu 3d ORBITAL LIQUIDSMEDIATED BY OXYGEN 2 p HOLES
Kou Takubo, Kohei Yamamoto, Yujun Zhang, Yasuyuki Hirata, Hiroki Wadati, Huiyuan Man,
Satoru Nakatsuji, Takashi Mizokawa

5017) AMBIENT-PRESSURE XPS STUDIES OF CO 2 HYDROGENATION ON PD-BASED

BIMETALLIC ALLOY CATALYSTS
Jiayi Tang, Takanori Koitaya, Susumu Yamamoto, Okkyun Seo, Kozo Mukai, Iwao Mastuda,
Jun Yoshinobu

5218) SOFT X-RAY SPECTROSCOPY OF SOLUTIONS WITH CARBON NANOTUBES

TREATED VIA PLASMA IN LIQUID
Kazuo Terashima, Noritaka Skakibara, Kenichi Inoue, Shion Takahashi, Taku Goto, Tsuyohito Ito,
Keishi Akada, Jun Miyawaki, Yukiya Hakuta, Yoshihisa Harada

5419) Photoelectron spectral imaging of p +-WSe2/n-MoS2 hetero junctions for TFET

device structures
Naoka Nagamura, Keigo Nakamura, Keishi Akada, and Kosuke Nagashio

5620) TIME-RESOLVED PHOTOELECTRON HOLOGRAPHY STUDY OF SILICENE
Kouichi Hayashi, Artoni Kevin R. Ang, Koji Kimura, Yuichiro Fukatsu, Akito Asano, Takahiro Tsukada,
Yohei Okuda, Takahiro Yonezawa, Antoine Fleurence, Yukiko Yamada-Takamura

5821) SOFT X-RAY INELASTIC SCATTERING AND DIFRACTION TECHNIQUESAPPLIED FOR THE STUDY ON THE ROKE OF WATER AND HYDRATION IN
BIOPOLYMERS, POLYSACCHARIDES AND ELECTROLYTES
Jun Miyawaki, Kosuke Yamazoe, Yitao Cui, Yoshihisa Harada

6023) OPERANDO OBSERVATION OF BURIED INTERFACES OF DEVICES BY USING

SOFT X-RAY PHOTOELECTRON MICROSCOPY IN COMBINATION WITH
CHARGE NEUTRAL CONDITION
Hirokazu Fukidome, Naoka Nagamura, Masaharu Oshima

6224) REVEALING THE MECHANISM OF THE METAL-INSULATOR TRANSITION IN

VO2 PROBED BY MOMENTUM RESOLVED RIXS
H. Fujiwara, T. Kashiuchi, K. Nishimoto, N. Nishikawa, A. Irizawa, J. Miyawaki, Y. Harada

6425) ELECTRONIC-STRUCTURE MAPPING OF ELECTRODE MATERIALS BY

OPERANDO PHOTOELECTRON SPECTROMICROSCOPY USING AN ALL-SOLIDSTATE LI-ION-BATTERY
Eiji Hosono, Takaaki Sudayama, Daisuke Asakura, Keishi Akada, Naoka Nagamura, Koji Horiba,
Masaharu Oshima, Yoshihisa Harada

6626) VALENCE-CHANGE MAPPING OF TRANSITION METALS BY OPERANDO

PHOTOELECTRON SPECTROMICROSCOPY USING AN ALL-SOLID-STATE
LI-ION-BATTERY
Eiji Hosono, Takaaki Sudayama, Daisuke Asakura, Keishi Akada, Naoka Nagamura, Koji Horiba,
Masaharu Oshima, Yoshihisa Harada

6827) THE ELECTRONIC STRUCTURE OF HYDRATED WATER IN BIOCOMPATIBLEPOLYMERS
Daiki Murakami, Kosuke Yamazoe, Tomoya Ueda, Toshiki Sonoda, Jun Miyawaki, Yoshihisa Harada,
Masaru Tanaka

7028) SOFT X-RAY ABSORPTION AND EMISSION SPECTROSCOPY OF THE

INTERMEDIATE WATER IN BIOCOMPATIBLE POLYMERSDaiki Murakami, Kosuke Yamazoe, Tomoya Ueda, Jun Miyawaki, Yoshihisa Harada, Masaru Tanaka

7229) Dispersionless Charge Excitations for La 2-xSrxNiO4+δ probed by Oxygen K -edge

Resonant Inelastic X-ray Scattering
Kohei Yamagami, Kenji Ishii, Yasuyuki Hirata Keisuke Ikeda, Jun Miyawaki, Yoshihisa Harada,
Shun Asano, Masaki Fujita, Hiroki Wadati

7430) Charge Excitations for single-and bi-layered Manganate probed by Resonant

Inelastic X-ray Scattering at Oxygen K-edge
Kohei Yamagami, Kenji Ishii, Jun Miyawaki, Yoshihisa Harada, Hideki Kuwahara, Hiroki Wadati

7631) Development of Coherent Soft X-ray Diffraction Imaging Technique for

Observation of Ultra-fastMagnetic Dynamics
Yuichi Yamasaki, Kohei Yamamoto, Yuichi Yokoyama, Yasuyuki Hirata, Hiroki Wadati,
Taka-hisa Arima

7832) Development of time-resolved ambient pressure X-ray photoelectron spectroscopy

system At SPring-8 BL07LSU
Susumu Yamamoto, Iwao Matsuda

8033) Sabatier reaction on a Ni/SiO 2 catalyst studied by operando soft X-ray

spectroscopies
Susumu Yamamoto, Jun Miyawaki, Jiayi Tang, Takanori Koitaya, Takuma Tokunaga, Yu Shibata,
Yuka Kosegawa, Kosuke Yamazoe, Kozo Mukai, Shinya Yoshimoto, Yoshihisa Harada, Jun Yoshinobu,
Iwao Matsuda

8234) PHOTOEMISSION MICROSCOPY OF MOTT INSULATOR Ca 2RuO4 UNDERELECTRIC FIELD
Y. Takasuka, A. Hishikawa, D. Ohtsuki, C. Sow, S. Yonezawa, Y. Maeno, F. Nakamura, N. Nagamura,T. Yoshida8435) OBSERVATION OF ELECTRONIC STRUCTURE FOR Ca 2RuO4 UNDERELECTRIC FIELD BY SOFT X-RAY ABSORPTION SPECTROSCOPY
Teppei Yoshida, Yukie Takasuka, Daisuke Shibata, Daiki Ootsuki, Jun Miyawaki, Yoshihisa Harada,
Chanchal Sow, Shingo Yonezawa, Yoshiteru Maeno, Fumihiko Nakamura

8636) HYDROGENATION OF FORMATE SPECIES ON A SINGLE ATOM ALLOY MODELCATALYST Pd-Cu (111) STUDIED BY AMBIENT PRESSURE XPS
Takuma Tokunaga, Susumu Yamamoto, Shion Nakano, Tomoki Matsuda, Kozo Mukai, Iwao Matsuda,
Jun Yoshinobu

8837) INVESTIGATING ANTIFERROMAGNETIC-FERROMAGNETIC PHASE

TRANSITION IN GdBaCo 2O5.5 THIN FILM BY RESONANT SOFT X-RAY
MANGETIC DIFFRACTION
Yujun Zhang, Yasuyuki Hirata, Kou Takubo, Kohei Yamagami, Kohei Yamamoto, Keisuke Ikeda,
Tsukasa Takayama, Akira Chikamatsu, Tetsuya Hasegawa, Hiroki Wadati

Experiments at E-labo
901) Spin- and angle-resolved photoemission study of MAX phase compound V 2AlC
Takahiro Ito, Masashi Ikemoto, Damir Pinek, Koichiro Yaji, Masashi Nakatake, Shik Shin,Thierry Ouisse

922) SPIN-ORBITAL TEXTURE IN QUASI-ONE-DIMENSIONAL GIANT RASHBASYSTEM
ON BI/INAS(110)-(2×1)
TakutoNakamura,
Yoshiyuki Ohtsubo, Ayumi Harasawa, Koichiro Yaji, Shik Shin, Fumio Komori,Shin-ichi Kimura

943) SPIN-POLARIZED QUASI-ONE-DIMENSIONAL METALLIC SURFACEELECTRONIC STATE OF BI/GASB(110)-(2×1)
Takuto Nakamura, Yoshiyuki Ohtsubo, Kenta Kuroda, Ayumi Harasawa, Koichiro Yaji, Shik Shin,
Fumio Komori, Shin-ichi Kimura

964) MODIFICATION OF SURFACE OF TOPOLOGICAL INSULATOR Bi 2Se3 BY

ADSORPTION OF ORGANIC DONOR MOKECULE TTN
T. Kitazawa, Shimozawa, and Shin groups

985) INVESTIGATION OF TOPOLOGICAL PROPERTIES IN SUPERCONDUCTINGTASE3Chun Lin, Ryo Noguchi1, Kenta Kuroda, Peng Zhang, Shunsuke Sakuragi, Jiadi Xu, Koichiro Yaji,
Ayumi Harasawa, Shik Shin, Atsushi Nomura, Masahito Sakoda, Masakatsu Tsubota, Satoshi Tanda,
Takeshi Kondo

1006) Observation of the interface state of Bi layer deposited on a MnGa thin film
M. Kobayashi, T. Takeda, N. H. D. Khang, M. Suzuki, A. Fujimori, K. Kuroda, K. Yaji, S. Shin,
M. Tanaka, P. N. Hai

1027) SWITCHABLE WEAK TOPOLOGICAL INSULATOR STATE IN QUASI-ONE-DIMENSIONAL BISMUTH IODIDE
R. Noguchi, T. Takahashi, K. Kuroda, K. Yaji, A. Harasawa, S. Shin, T. Sasagawa, T. Kondo

1048) Electronic and spin structure of heavy element thallium single crystal thin films
Yoshitaka Nakata, Takahiro Kobayashi, Koichiro Yaji, Kazuyuki Sakamoto

1069) SPIN-POLARIZED BANDS OF AN IN BYLAYER FILM FORMED ON Si(111)Takahiro Kobayashi, Yoshitaka Nakata, Koichiro Yaji, Shunsuke Yoshizawa, Fumio Komori,Shik Shin, Takashi Uchihashi, Kazuyuki Sakamoto

10810) MULTIPLE TOPOLOGICAL STATES IN IRON-BASED SUPERCONDUCTORS
Peng Zhang, Zhijun Wang, Xianxin Wu, Koichiro Yaji, Yukiaki Ishida, Yoshimitsu Kohama,
Guangyang Dai, Yue Sun, Cedric Bareille, Kenta Kuroda, Takeshi Kondo, Kozo Okazaki, Koichi Kindo,
Xiancheng Wang, Changqing Jin, Jiangping Hu, Ronny Thomale, Kazuki Sumida, Shilong Wu,
Koji Miyamoto, Taichi Okuda,Hong Ding, G. D. Gu, Tsuyoshi Tamegai, Takuto Kawakami,
Masatoshi Sato, Shik Shin

11011) ELECTRONIC BAND STRUCTURE OF TRANSFER GRAPHENE ON SI(111) SUBSTRATE
Koichiro Yaji, Fumio Komori and Shik Shin

11112) STUDY ON ELECTRONIC STRUCTURES IN HALF-METALLIC FERROMAGNET

CoS2 BY HIGH-RESOLUTION SPIN- AND ANGLE-RESOLVED PHOTOEMISSION
SPECTROSCOPY
Hirokazu Fujiwara, K. Terashima, T. Wakita, Y. Yano, W. Hosoda, N. Kataoka, A. Teruya, M. Kakihana,
M. Hedo, T. Nakama, Y. Ōnuki, K. Yaji, A. Harasawa, K. Kuroda, S. Shin, Y. Muraoka, T. Yokoya

11313) ANGLE-RESOLVED PHOTOEMISSION SPECROSCOPY STUDY OF TYPE-II

WEYL SEMIMETAL WTe2
Yuxuan Wan, Lihai Wang, Keisuke Koshiishi, Masahiro Suzuki, Jaewook Kim, Kenta Kuroda,
Koichiro Yaji, Ryo Noguchi, Ayumi Harasawa, Shik Shin, Sang-Wook Cheong, Takeshi Kondo,
Atsushi Fujimori

115

6 Staff

116

7 Publications List

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. A scientific aim of the beamline is to promote advanced spectroscopy
for solid state and soft (including bio) materials. There are four regular endstations: timeresolved soft X-ray spectroscopy (TR-SX spectroscopy), soft X-ray diffraction, 3Dscanning photoelectron microscope (3D nano-ESCA) and ultrahigh resolution soft X-ray
emission spectroscopy (HORNET) that are open for users. There is also a free port station
for users who bring their own experimental apparatus. In 2018, we have completedcommissioning of the ambient pressure X-ray photoelectron spectroscopy station at thefree port station and opened the system for general users.
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. In
2018, we have introduced a soft X-ray chopper that can transmit only a selected pulse of
the incident beam in order to improve signal to noise ratio of specific experiments liketime-resolved spectroscopy of slow dynamics, operando spectroscopy under applied bias
or to reduce radiation induced damages.
At the beamline endstations, various scientific researches were carried out by both thelaboratory staffs and general users (G-type application). Below are brief introduction ofrecent activities at each station.
(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 addition, an optical parametricamplifier (OPA) has been installed for both laser systems to cover a wider spectrum range
of optical laser.
In 2018, time-resolved photoelectron diffraction experiments were performed on amonatomic two-dimensional layer of Si, Silicene. Two-dimensional detection ofphotoelectron diffraction patterns and a chemical shift in high energy resolution
measurements by ARTOF allow us to trace temporal evolution of Si atom positions in asite-specific manner.
Photo-excited carrier dynamics in a model organic phovoltaic system was studied using
time-resolved soft X-ray photoelectron spectroscopy. Monolayer-thick p-type metal
phthalocyanine (CuPc) and n-type fullerene (C60) layers were deposited on a TiO2(110)
surface. Electrons excited in the unoccupied states of CuPc and C60 are swiftly transferredto the conduction band of TiO2 substrate, leaving the molecules in transient cationic states.
The cationic states have long lifetime of the order of microseconds, suggesting that anelectron-hole recombination is efficiently suppressed at the organic/TiO2 interface.
(2) 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 2018, we have devised new technique enabling to probe the electronics states ofburied interfaces of GaN-HEMT in academia-industry collaboration with Sumitomo
Electric Industries, by combing 3D nano-ESCA with advanced device technologies. This
work was published in Scientific Reports in 2018. As a joint research with Dr. Asakura
and Dr. Hosono at AIST group, we also combined 3D nano-ESCA with optical
microscope images, and studied Li ion diffusion mechanism in a m sized SnO2 wire andlocal Li insertion/extraction process in a Li4Ti5O12 (LTO) negative electrode active
material under operando (electrochemically operating) condition. The result on LTO was
published in Journal of Electron Spectroscopy and Related Phenomena. As collaboration
with Department of Materials Engineering, the University of Tokyo, we performed
operando nano-spectroscopy on device structures of transition metal dichalcogenide
tunnel field effect transistors (TMD-TFETs) and found difference in gate and drain
voltage dependence of the potential distribution between MoS2 on highly- and low- doped
p+-WSe2. Furthermore, we also conducted academia-industry collaboration research onprocess-induced defects in SiC trench MOSFETs based on the previous results publishedin e-JSSNT in 2018.
(3) Ambient-pressure X-ray photoelectron spectroscopy (AP-XPS)This station allows Operando observation of catalysts under reaction conditions using
AP-XPS. The AP-XPS station was constructed in 2014 by an external funding (JST ACTC project), and was opened to external users in 2018.
The AP-XPS system is equipped with a differentially pumped electron analyzer

(SPECS, PHOIBOS 150 NAP) and an ambient-pressure gas cell. XPS measurements canbe performed both under ultrahigh vacuum and in near-ambient gas pressure up to 20
mbar. Catalytic activity is monitored by mass spectrometer, and simultaneous evaluationof adsorbate and catalyst electronic states is performed by AP-XPS. In addition to theexperiments at the free port station using synchrotron soft X-ray, off-line experiments
using a twin-anode X-ray source can be performed.
A variety of research projects are now in progress at 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, (vii) developmentof time-resolved AP-XPS.
(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 ultrahigh-resolution (E/ΔE > 10,000) and undervarious environmental conditions (gas, liquid, and solid). The number of applications to
the HORNET station is increasing. In 2018, we have performed 15 collaborative researchusing the HORNET station: two S-type proposals, five proposals for molecular systems,
four for condensed matter physics, and four for electrochemistry of batteries. In molecularsystems, the behavior of water in a variety of circumstances was studied, such as wateraround disaccharide, water at the interface of biocompatible polymers, and hydration toplasma treated carbon nanotube and boron nitride. Jet system was installed in order todiscuss pure vibrational spectra without the contribution from an X-ray filter. In
condensed matter physics, characteristic experiments were carried out using theadvantages of RIXS. Charge excitation due to hole doped at oxygen sites inLa2−xSrxNiO4+δ, La2−2xSr1+2xMn3O7, and La1-xSr1+xMnO4 was investigated in comparison
with La2−xSrxCuO4 using elementary selectivity of O K-edge RIXS. Photon-in/photonout process of RIXS enabled to probe electronic structure change due to metal-insulatortransition in Ca2RuO4 induced by electronic field. Angle-resolved RIXS allowed us tomeasure the dispersion of elementary excitation in VO2 and resonant inelastic X-ray
diffraction from La1/3Sr2/3FeO3. In electrochemistry of batteries, we have continuously
studied mechanism of charge/discharge process and explored new electrode materials by
ex-situ and operand RIXS. In order to clarify the effect of charge transfer between metaland oxygen, oxygen-free electrolyte battery cell is being developed. In 2018, several
results for condensed matter physics were published, such as Fe L-edge RIXS of FeS

model complexes and low-spin ferrous and ferric iron complexes, V and Mn L-edge RIXS
of half-metallic ferrimagnet Mn2VAl, Cr L-edge RIXS of ruby, and Co L-edge RIXS of
LaCoO3.

(4) Free-port station
The station is equipped with a focusing mirror chamber, and users can connect their own
experimental chambers. In 2018, the following experiments were performed: time
resolved soft X-ray diffraction of Ba3CuSb2O9; time resolved X-ray absorption
spectroscopy of Eu compounds with valence fluctuations; N K-edge near-edge X-ray
absorption fine structure of 2D organic hetero-bilayer Cat-TTF/Im-SAM on Au;
evaluation of spatial coherence of synchrotron radiation using coherent diffraction
imaging technique; ambient-pressure X-ray photoemission spectroscopy ofH2adsorption/absorption in Pd alloys and CO2 hydrogenation reactions on Ni-based
catalysts; element-selective analysis of 3D atomic structures of quasi-crystal
BaTiO3(111)/Pt(111) thin film with photoelectron holography technique using Display
type ELlipsoidal Mesh Analyzer (DELMA).

Status of spin- and angle-resolved photoelectron spectroscopy withlaser 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. For example, one looks at the exchange
splitting of ferromagnetic materials. Also, recently, strongly spin-orbit coupled materials, such as
Rashba spin-split systems and topological insulators have been intensively studied. We developed a
SARPES apparatus with a vacuum-ultraviolet (VUV) laser at Laser and Synchrotron ResearchLaboratory in the Institute for Solid State Physics, named LOBSTER (Laser-Optics-BasedSpin-vecTor Experimental Research) machine. The LOBSTER machine is 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 FY 2015.
Figure 1 represents an overview of the LOBSTER machine [1]. The apparatus consists of ananalysis chamber, a sample-bank chamber connected to a load-lock chamber, and a molecular beam
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 excited by 6.994-eV photons, yielded by 6th harmonic of 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 be heatedby 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 beinstalled. At the carousel chamber, 16 samples can be stocked in the UHV environment.
In FY 2018, eight research papers were published. Among these, one of the most significant papers
is the discovery of a weak topological insulator (WTI) state in a bismuth iodide, reported by Prof.
Kondo group [2]. In this study, the authors were performed ARPES and SARPES with high-energy
and spatial resolutions using the VUV laser for the bismuth iodide. They demonstrated aquasi-one-dimensional Dirac surface state. They also found that a crystal transition from the β-phaseto the α-phase drives a topological phase transition from a nontrivial WTI to a normal insulator atroughly room temperature. The authors concluded that the weak topological phase can open a newpathway for developing novel spintronic devices that benefits from highly directional, dense spincurrents that are protected against backscattering.

Reference[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] R. 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. Dudin, T. K. Kim, M. Hoesch, S. Shin, R. Arita, T. Sasagawa andT. Kondo, Nature 588, 518-522 (2019).

Upshift in the Soft X-ray Science of Synchrotron Radiation
Date: 2018/11/30 (Fri.) , 2018/12/01 (Sat.)
Place: Lecture Room (A632), 6th Floor, ISSP, the University of Tokyo

Program
2018/11/30
13:00

13:15

Opening Address

Hatsumi Mori (ISSP, the University of Tokyo)

Guest Address

Atsushi Oku (MEXT)

Guest Address

Wataru Utsumi (QST)

Guest Address

Tadahiro Hayasaka (Tohoku University)

To Map the Next Generation SR Facility in the Academic Community in IMRAM,
Tohoku Univ.
Atsushi Muramatsu (Institute of Multidisciplinary Research for Advanced
Materials, Tohoku University)

13:45

A Role of Next Generation Synchrotron Radiation Facility to Build a New Range of
Innovation
Masaki Takata (PhoSIC / Institute of Multidisciplinary Research for Advanced
Materials, Tohoku University)

14:15

Activities of the University of Tokyo for the next-generation synchrotron radiation
facility
Taka-hisa Arima (Department of Advanced Materials Science, University of Tokyo)

14:45

Future of coherent diffractive imaging pioneered by the next-generation synchrotron
radiation source
Yukio Takahashi (Graduate School of Engineering, Osaka University)

15:15

Photo & Coffee break

15:30

Facilitating protein microcrystallography by automated data processing
KeitaroYamashita (The University of Tokyo / RIKEN SPring-8 Center)

16:00

Expectation for Next Generation Synchrotron Radiation Facilities:
From the Viewpoint of Physical Chemistry and Surface Science
Jun Yoshinobu (The Institute for Solid State Physics, The University of Tokyo)

16:30

The potential of next generation light sources to promote the soft X-ray science
Yusuke Tamenori (Japan Synchrotron Radiation Research Institute (JASRI) /Tohoku University)

17:00

Contributing advanced technology through precision manufacturing
Yuko Akabane (TDC Corporation)

18:00Banquet2018/12/01
9:30

Transfers of the Techniques from SPring-8 BL07LSU
Iwao Matsuda (The institute for Solid State Physics, the University of Tokyo)

10:00

Current status of development of micro- and nano- focusing mirrors for soft x-rays
Hidekazu Mimura (Graduate school of engineering, The University of Tokyo)

10:30

A two-color BL brings new colors of a SR facility
Hitoshi Abe (Institute of Materials Structure Science, High Energy Accelerator
Research Organization / Dept. of Materials Structure Science, Sch. of High Energy
Accelerator Science, SOKENDAI)

11:00

Development of soft x-ray polarization switching for studying spin dynamics
Hiroki Wadati (Institute for Solid State Physics, University of Tokyo)

11:30

Lunch

13:00

Combining Nano-focus, Operando, Machine Learning, Enough for further progress?
Hirokazu Fukidome (Research Institute of Electrical Communication,
Tohoku University)

13:30

Catalysis science by operando soft X-ray spectroscopy:
Present status and future prospect at next generation synchrotron radiation facility
Susumu Yamamoto (The Institute for Solid State Physics, The University of Tokyo)

14:00

Solid state physics and its future development with nano-spin ARPES
Takeshi Kondo (ISSP, The University of Tokyo)

14:30

Poster Session

15:00

Coffee Break

15:30

Crystal structure prediction by data assimilation
Synge Todo (Department of Physics, University of Tokyo / Institute for Solid State
Physics, University of Tokyo / MaDIS, National Institute for Materials Science)

16:00

Recent status and future of a scanning transmission X-ray microscope in
UVSOR-III)
Takuji Ohigashi (UVSOR Synchrotron Facility, Institute for Molecular Science /Sokendai)

16:30

Development for high-speed magnetic imaging by fusion of coherent soft X-ray andinformation technology)
Yuichi Yamasaki (NIMS-MaDIS, JST-PRESTO, RIKEN-CEMS)

17:00

Strategy for resonant inelastic soft X-ray scattering using next-generationsynchrotron radiation)
Yoshihisa Harada (Institute for Solid State Physics, University of Tokyo)

17:30

Closing Address

Nobuhiro Kosugi (JSSRR)

The conference was organized on November 30 and December 1 to promote scientific andtechnological innovations of soft X-ray synchrotron radiation, motivated by theannouncement on the next-generation facility by the Minister of Education, Culture, Sports,
Science and Technology on July 3, 2018. It has successfully brought together more than 200
participants, including the presidents and the outstanding researchers of synchrotron
radiation institutes and societies from all over Japan. The presentations at the ISSP lectureroom were broadcast live to a conference room in SPring-8 through the internet. On thefirst day, the program focused on science and technology to be evolved at the new facility,
while, on the second day, it featured experimental methods and information technologies tobe developed toward researches with the light source.
With the next-generation soft X-ray synchrotron radiation, measurements are expected
to be made with multi-dimensional data acquisitions or with ultra-high resolutions thathave never been possible at the existing facilities. The attendance interdisciplinarily argued
research topics to respond to needs in academic and industrial fields today. There was also
vigorous discussion on the cutting-edge informatics to be applied in the data analysis. We
were confident that the workshop was very timely and that all the arguments would become
seeds of the novel science and technology.
The conference was hosted by the Institute for Solid State Physics (the University ofTokyo), Synchrotron Radiation Research Organization (the University of Tokyo), Tohoku
University, and User Community of VUV・SX high-brilliant light sources.
(I. Matsuda, T. Arima, Y. Harada, H. Wadati, T. Kondo, S. Shin)4. Seminar

Date: May 30, 2018
Title: Spin-texture in image potential state and optical spincurrent control at surface
Speaker: Dr. Ryuichi Arafune（National Institute for Materials Science）
Date: June 19, 2018
Title: Electron dynamics at nanoscale organic surfaces and interfaces studied bytime-resolved photoelectron microspectroscopy
Speaker: Dr. Masahiro Shibuta（Graduate School of Science and Technology, Keio University）
Date: December 19, 2018
Title: Micro-Nano bubble Properties and the Acceleration Effect on Gas Hydrate Formation
Speaker: Dr. Tsutomu Uchida（Graduate School of Engineering, Hokkaido University）

OBSERVATION OF Li DIFFUSION AND ELECTRONIC-STRUCTURE
DISTRIBUTION IN THE ACTIVE MATERIAL OF ALL-SOLID-STATE
Li-ION BATTERY BY OPERANDO PHOTOELECTRON
SPECTROMICROSCOPY
Keishi Akada1, Takaaki Sudayama2, Daisuke Asakura2,3, Hirofumi Matsuda2,3,
Naoka Nagamura4, Masaharu Oshima5, Eiji Hosono2,3, Yoshihisa Harada1,31Institute for Solid State Physics, The University of Tokyo,
2
Research Institute for Energy Conservation,
National Institute of Advanced Industrial Science and Technology (AIST),3
AIST-UTokyo Advanced Operando-Measurement Technology Open Innovation Laboratory(OPERANDO-OIL),
4
Research Center for Advanced Measurement and Characterization,
National Institute for Materials Science (NIMS),
5
Synchrotron Radiation Research Organization, The University of Tokyo

For a sustainable modern society, Li-ion-batteries (LIBs) play a more important role as anenergy storage of electric vehicles. Understanding the operation mechanisms of LIBs isnecessary to improve their properties, such as energy density, safety operation, and thermal
stability, etc. Therefore, an elucidation of lithiation/delithiation phenomena by detailed
electronic-state analysis under potential control is essential.
For this purpose, there are many reports about
operando measurements on the electronic properties of (a)the constituent elements. However, most of the operando
synchrotron x-ray spectroscopy techniques typicallyprovide average information in the region of tens or
hundreds of m which is larger than the particle size ofthe active materials. A spectroscopic microscopy withhigh spatial resolution is particularly important to further
understand the charge/discharge mechanisms. This isbecause the method can detect changes in various
electronic structures of elements closely correlated with(b)lithiation/delithiation in single particle.
Here, we have developed operando photoelectron
spectromicroscopy to obtain spatially resolved
information to the electronic structure of a few μm single
crystal LIB active materials. For this purpose, we havedeveloped an all-solid-state LIB cell that can operate
under ultra-high-vacuum conditions, and measured
operando photoelectron spectra of LiCoO2 (LCO)
cathode material and Li4Ti5O12 (LTO) anode material in
charge/discharge state by using SPring-8 BL07LSU
3DnanoESCA having sub-100 nm spatial resolution [1].
Fig. 1 shows the operando photoelectron
spectromicroscopy system for the all-solid-state LIBs. In
preparation of operando cell, an LCO slurry was dropped
onto micro-grid/Au-mesh, and placed on a solid
electrolyte after slurry drying. The operando cell wasFig. 1 (a) Schematic details of allsolid-state LIBs for Operando
photoelectron
spectromicroscopy
system of 3DnanoESCA. (b)Optical image of operando cell on
sample holder with five terminals.

fabricated in an Ar-filled glovebox using Li metal as an anode material, and all of component
were fixed by ceramics jigs which has a hole for exposing LCO particles. A photoelectron
intensity mapping could be obtained by scanning a soft X-ray beam, which is focused on asample by using a Fresnel zone plate.
In an experiment, we obtained the Co 3p and Li 1s spectral mapping on LCO single crystal
after Li extraction. Integrated spectra form each facet shows that atomic ratios of Li: Co weredifferent depending on the facets. This result indicates that Li extraction varies with the facets,and guaranteed the reliability of our operando photoelectron spectromicroscopy[2].
Furthermore, this operando measurement was applied to Li4Ti5O12 anode material, which
has excellent performance for rapid charging and cycle stability. Fig. 2 (a-c) shows spectral
mapping of LTO single particle comparing with optical and SEM images. As Fig. 2 (d), we
compared Ti 3p changes after charging and discharging on the LTO particle, and found thatSEI was formed and Li 3p was hidden during Li insertion. During LTO charge/discharge,
lithium insertion and extraction proceed between two coexisting phases, spinel Li4Ti5O12 andthe rock salt Li7Ti5O12 (Li-rich phase). In additional experiments conducted after thisbeamtime, we found that the Ti 3p peak shows chemical shifts as a result of phase change
during the Li desertion [3].

(a)
(d)

(b)

(c)

Fig. 2 Optical microscopy (a), photoelectron intensity mapping (b), and SEM (c) imagesof LTO single particle. (d) Ti 3p XPS spectra from LTO particle in (b).

REFERENCES[1] K. Horiba, Y. Nakamura, N. Nagamura, S. Toyoda, H. Kumigashira, M. Oshima, K.
Amemiya, Y. Senba, and H. Ohashi, Rev. Sci. Instrum. 82, 113701 (2011).
[2] K. Akada, T. Sudayama, D. Asakura, H. Kitaura, N. Nagamura, K. Horiba, M. Oshima, E.
Hosono, and Y. Harada, Submitted.
[3] K. Akada, T. Sudayama, D. Asakura, H. Kitaura, N. Nagamura, K. Horiba, M. Oshima, E.
Hosono, and Y. Harada, J. Electron Spectros. Relat. Phenomena 233, 64 (2019).

X-RAY PHOTOELECTRON SPECTROMICROSCOPY OF LITHIUM
STORAGE IN SnO2 SINGLE-PARTICLE MICROWIRE ANODE
MATERIAL
Keishi Akada1, Takaaki Sudayama2, Daisuke Asakura2,3, Hirofumi Matsuda2,3,
Naoka Nagamura4, Masaharu Oshima5, Eiji Hosono2,3, Yoshihisa Harada1,31Institute for Solid State Physics, The University of Tokyo,
2
Research Institute for Energy Conservation,
National Institute of Advanced Industrial Science and Technology (AIST),3
AIST-UTokyo Advanced Operando-Measurement Technology Open Innovation Laboratory(OPERANDO-OIL),
4
Research Center for Advanced Measurement and Characterization,
National Institute for Materials Science (NIMS),
5
Synchrotron Radiation Research Organization, The University of Tokyo

To develop innovative materials for lithium-ion batteries (LIBs), which are rapidly
increasing in demand, intensive research has been carried out in the research ﬁelds of cathodeand anode materials. While novel materials have been eagerly sought for cathodes,
anode materials have been focused on the use of metallic Li and alloy materials such as
silicon and tin oxide (SnO2). Compared with commonly used graphite-based anode materials,
alloy-based materials are expected to increase in capacity by several to dozens of times.
However, the low cycle characteristics of alloy materials are a problem, because a large
volume change occurs with lithiation/delithiation during charge/discharge processes.
To overcome this problem, we propose in situ photoelectron spectromicroscopy with highspatial resolution in SnO2 wire during lithiation. We analyze the detail electronic states ofeach element in the electrode, and reveal the charge/discharge mechanism such as lithium
diffusion and volume/shape change. This study aims to develop a cycle degradation control
technique based on the understanding these phenomena. Therefore, we conducted an in situ
measurement during lithiation into single particle anode material by using 3DnanoESCA[1].
By the proposed method, the lithiation reaction in SnO2 single crystal can be visualized with
spectral mapping, and the effect from crystal orientation and facet on lithiation and diffusion
will be clarified [2,3]. Since detailed changes of chemical states are revealed fromphotoelectron spectroscopy measurement, discrimination between conversion reaction of
SnO2 and alloying reaction of Sn and Li is also expected. The findings obtained by the
proposed experiments will greatly contribute to the future development of Li-ion batteries.
In the sample preparation, we embedded SnO2 wires on metal Li in Ar-filled glovebox. This
sample was put on the heater, which was connected to the terminal of the sample holder. This
sample and sample holder was transferred into the 3DnanoESCA chamber without air
exposure, and the SnO2 wire was observed with photoelectron mapping. The sample washeated to promote the Li reaction, and the morphological and chemical changes wereobserved from mapping and photoelectron spectroscopy. Observed SnO2 wires were observed
before and after beamtime without air exposure using high resolution optical microscope andcompared with the results of 3DnanoESCA.
Fig. 1 (a, b) shows obtained images of optical microscopy and photoelectron intensity
mapping of SnO2 after Li reaction enhanced at 50°C. Li diffused from the left side of theimages and reacted with SnO2, and the reacted area was strongly expanded in the left region.
The photoelectron spectra (Fig. 2) of Sn 3d showed that the Sn4+ peak on SnO2 wire (red) was
shifted to Sn0 peak on the reacted region (green), which means SnO2 wire was reduced andchanged to metal Sn and/or Sn-Li alloy. The Li 1s also showed that components of Li2O and
LixSn are detected in the reacted region. These results clearly shows that the conversion
reaction and the alloying reaction proceeded in the reacted region.

Fig. 1 Optical (a) and photoelectron intensity mapping(b) images of partially reacted SnO2 wire.

Fig. 2 Pin-point XPS spectra of Li 1s and Sn
3d from upper wire and reacted region on
photoelectron mapping in Fig. 1 (b).

REFERENCES[1] K. Horiba, Y. Nakamura, N. Nagamura, S. Toyoda, H. Kumigashira, M. Oshima, K.
Amemiya, Y. Senba, and H. Ohashi, Rev. Sci. Instrum. 82, 113701 (2011).
[2] K. Akada, T. Sudayama, D. Asakura, H. Kitaura, N. Nagamura, K. Horiba, M. Oshima, E.
Hosono, and Y. Harada, Submitted (n.d.).
[3] K. Akada, T. Sudayama, D. Asakura, H. Kitaura, N. Nagamura, K. Horiba, M. Oshima, E.
Hosono, and Y. Harada, J. Electron Spectros. Relat. Phenomena 233, 64 (2019).

ELECTRONIC STRUCTURE ANALYSIS OF CATHODE MATERIALSFOR LI-ION BATTERIES BY OPERANDO SOFT X-RAY EMISSION
SPECTROSCOPY USING AN AQUEOUS ELECTROLYTE SOLUTION
Daisuke Asakura1,2, Takaaki Sudayama1, Jun Miyawaki3,4, Yoshihisa Harada2,3,4, andEiji Hosono1,2
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

Improvement of the performances of electrode materials for Li-ion batteries (LIBs) arehighly important to further develop electric and hybrid-electric vehicles. To develop novel
electrode materials having high performances, understanding the charge-discharge
mechanisms of the electrode materials from a viewpoint of the electronic structure isindispensable. Soft x-ray spectroscopy, which directly reveals the 3d orbital of transition
metals, has recently attracted much attention for the electronic-structure analyses of theelectrode materials.
In this study we demonstrate operando soft x-ray emission spectroscopy (XES) for LiFePO4
with an aqueous electrolyte solution. LiFePO4 is a typical cathode material for LIB1 andworks as a cathode even for an aqueous electrolyte solution as well as organic electrolyte
solutions used for general LIB. While a high voltage cannot be obtained for the aqueous LIBs
because of the narrow voltage window for H2O, the redox reaction in the host framework of
LiFePO4 is expected to be almost the same with the case when an organic electrolyte solution
is used.
We developed an in situ cell consisting of the LiFePO4 cathode, a Pt-wire counter electrode,
Ag/AgCl reference electrode, and a 1 M LiNO3/H2O electrolyte solution by modifying the insitu cell for fuel cell catalysts2. The charge-discharge experiment was performed with cyclic
voltammetry (CV) with a scan speed of 0.5 mV/s. The XES spectra were recorded at the four
points on the second cycle (OCV 1 → charge → discharge → OCV 2) shown in Fig. 1.
The operando XES experiments were carried out using HORNET XES spectrometer3 at
BL07LSU of SPring-8. We selected 708
eV as the excitation energy (Ein) in which
the Fe2+ character should be enhanced for
the Fe L3-edge absorption spectrum. The
charge-discharge
experiments
wereperformed by cyclic voltammetry. The
operando XES measurements were
performedforthesecondcharge-discharge cycle.
The Fe 2p-3d-2p resonant XES spectra
revealed great difference between theslurry (initial state) LiFePO4 powder andopen-circuit-voltage (OCV) state afterFig. 1: The CV curve for the operando XES measurement.
the first cycle (OCV 1, -0.07 V) (Fig. 2). The potentials for the charged and discharged states were
For the slurry, the dd-excitation peaks are maintained by a potentiostatic operation. For OCV 1 andclearly observed from 704 to 707.5 eV 2, the potentials were almost unchanged during recording
and the charge-transfer (CT) excitation is the corresponding XES spectra.

considerably suppressed. These results are ascribed to be the Fe2+ high-spin state as shown ina previous report4. For OCV 1, broad feature centred on ~703 eV should overlap on the Fe2+
peaks.
For the charged state (+0.4 V), the profile is substantially changed from that of the OCV 1.
Especially, the intensity for 704.5-707.5 eV greatly decreased, indicating a disappearance of
the Fe2+ component possibly due to the oxidation reaction to Fe3+ state with Li extraction.
Then, the profile for the charged state should be of the Fe3+ state. For the discharged state(-0.7 V), the dd-excitation peaks for the Fe2+ state as with the slurry appeared again,
suggesting a reduction reaction to the Fe2+ state by discharge (Li re-intercalation). The
spectrum for OCV 2 (-0.1 V) is similar to that for discharged state.
In summary, operando XES
measurements for LiFePO4
with an aqueous electrolyte
solution were performed torevealtheFe
3delectronic-structurechangeduring the charge-discharge
process. The great difference
between the slurry and OCV 1
and the XES profile of charged
state possibly reflecting the
Fe3+ state indicate that theredox inactive Fe3+ component
should be formed by the firstcycle. For the second cycle, theredox reaction is reversible tosome extent. In future, theelectronic-structure parameters
for the Fe2+ and Fe3+ states will
be determined accurately byusing multiplet calculations.
The relationship between theelectronic
structureandelectrochemical properties will
also be discussed.
Fig. 2: Operando Fe 2p-3d-2p XES spectra for LiFePO4.

REFERENCES[1] A. K. Padhi et al., J. Electrochem. Soc. 144, 1188 (1997).
[2] H. Niwa et al., Electrochem. Commun. 35, 57 (2013).
[3] Y. Harada et al., Rev. Sci. Instrum. 83, 013116 (2012).
[4] D. Asakura et al., Chem. Phys. Chem. 19, 988 (2018).

AN OPERANDO SOFT X-RAY SPECTROSCOPY STUDY OF ACATHODE MATERIAL FOR LI-ION BATTERIES TO INVESTIGATE
THE REACTION OF EACH REDOX PEAK
Daisuke Asakura1,2, Takaaki Sudayama1, Hirofumi Matsuda1,2, Jun Miyawaki3,4,
Yoshihisa Harada2,3,4, and Eiji Hosono1,2
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

The redox potential of electrode materials for Li-ion batteries (LIBs) is the most importantparameter to realize high-voltage LIB. For the cathode materials, the redox potential should
be high as long as the electrolyte solution used in the battery is not decomposed. For example,
LiCoO2 (LCO) and LiMn2O4 (LMO) which are typical cathode materials exhibit redox
potential of ~4 V vs. Li/Li+. In contrast, LiFePO4 (LFP) has a redox potential of 3.5 V vs.
Li/Li+ based on the Fe2+/Fe3+ redox couple, while the chemical/thermal stability andcharge-discharge cycle performances of LFP are higher than those of LCO and LMO. To
elevate the redox potential of LFP, it is well known that substitution of Mn for Fe iseffective.1A potential plateau indeed appears at 4 V vs. Li/Li+ by the Mn doping, which isthought to be of the Mn2+/Mn3+ redox couple. On the other hand, there are several reportssuggesting that redox-inactive Mn exists.2 In this study, we confirmed the redox reactions of
LiMn0.5Fe0.5PO4 (LMFP) by using operando soft X-ray spectroscopy.
For the operando experiment, we used our operando cell3 with an oxygen-free current
collector for O K-edge measurement. LMFP thin-film layer was directly fabricated by
sputtering on the oxygen-free current collector layer on Si3N4 window (150 nm) which
separate the vacuum from the electrolyte solution. The operando soft X-ray absorption
spectroscopy (XAS) for the LMFP thin film were carried out at BL07LSU of SPring-8.4
Partial-fluorescence-yield (PFY) detection mode using a silicon-drift detector was employed
for the XAS measurement. The charge-discharge experiment was performed with cyclic
voltammetry (CV) (Fig. 1). The operando XAS spectra were recorded at the five points on thesecond cycle as shown in Fig. 1: ① open-circuit voltage (OCV), ② 4.0-V charge, ③ fullcharge, ④ 3.7-V discharge, and ⑤ full discharge.
Figure 2(a) shows the operando XAS
spectra at the Mn L edge. The XAS
spectrum for ① OCV is attributed to Mn2+
high-spin state. This profile did not change
for ② 4.0-V charge. For ③ full charge,
the Mn L3 peaks became smaller and abroad structure centered on 643 eV
appeared. Moreover, the peak at 652.5 eV
for the Mn L2 edge was slightly enhanced.
These changes suggest a small amount of
Mn3+ emerged by charge, while the Mn2+
component considerably remained. The
XAS profile returned to the Mn2+ state for Fig. 1. CV curve for LMFP. ①－⑤ indicate potentials④ 3.7-V discharge and maintained after where the operando XAS was performed.

⑤ full discharge, indicating the oxidation peak centered on 4.2 V and the reduction peak
centered on 3.9 V in Fig. 1 are of partial redox reaction on the Mn. On the other hand, the Fe
L-edge XAS largely changed for ① → ② and ④ → ⑤ (Fig. 2(b)). These reactions are
attributed to reversible Fe2+ ⇔ Fe3+ redox reaction around 3.5 V similar to LFP.

Fig. 2. Operando XAS results (a) at the Mn L-edge and (b) Fe L-edge. The numbers correspond to thepotentials in Fig. 1.

The O K-edge XAS profile gradually changed during the charge-discharge process (Fig. 3).
In particular, the pre-edge region below 532 eV was enhanced for ③ full charge. Taking the
Mn L-edge XAS results into account, the Mn 3d-O 2p hybridized orbital should be redox
active around 4 V. In
addition, the peak at 533 eV
comes from the electrolyte
solution.
In summary, we measured
the operando XAS at the Mn
L-edge, Fe L-edge and O
K-edge for LMFP. The redox
orbital around 4 V is not
attributed to localized Mn 3d
orbital, but hybridized Mn
3d-O 2p orbital. In the near
future, we will perform theoperandosoft
X-ray
emission spectroscopy toFig. 3. Operando XAS results at the O K-edge.
further understand the redox
reaction.
REFERENCES[1] For example, A. Yamada et al., J. Electrochem. Soc. 148, A747 (2001).
[2] For example, L. F. J. Piper et al., J. Phys. Chem. C 117, 10383 (2013).
[3] D. Asakura et al., Electrochem. Commun. 50, 93 (2015).
[4] Y. Harada et al., Rev. Sci. Instrum. 83, 013116 (2012).

PROBING OXYGEN-REDOX IN BATTERY CATHODES
Masashi Okubo
Department of Chemical System Engineering, School of Engineering, The University of Tokyo

Current lithium-ion battery (LIB) technology powers most of today’s portable electronics, inpart, owing to the high energy efficiency with which LIBs operate through intercalation
chemistry. However, the tremendous industrial demand resulting from the market penetrationof electric vehicles has significantly raised the requirement for the development of rechargeable
batteries with higher energy density.
One of the main means of increasing the energy density of batteries is the development of thepositive electrode (cathode), in which ion (de)intercalation is charge-compensated by the redox
reactions of transition metals. Quantitatively, in the conventional charge-compensation regime,
which relies on the redox capability of transition metals, the theoretical capacity is maximized
for the chemical composition of AMO2 (A: alkali metal, M: transition metal). However, recent
experimental observations have shown that A-excess transition-metal oxides, A1+xM1−xO2, candeliver large extra capacities exceeding the theoretical limit of the M-redox reaction because ofan additional oxygen-redox capacity. For example, Li1.2Ni0.13Co0.13Mn0.54O2 gives a large
specific capacity of 270 mAh g-1 with a large contribution from an oxygen-redox reaction (~150mAh g-1). Therefore, exploiting additional oxygen-redox reactions of oxides would
significantly contribute to surpassing the current energy-density limit of batteries.
We investigated the reversible oxygen-redox capacity of a layered sodium manganese oxide
Na2Mn3O7. Na2Mn3O7 (Na4/7[1/7Mn4+6/7]O2 in conventional NaxMO2 notation) consists of Na+
and [1/7Mn4+6/7] layers that stack alternately (Figure 1). We expect that Na2Mn3O7 has
nonbonding 2p orbitals of oxygens neighboring , leading to an extra oxygen-redox capacity
upon Na+ extraction at a higher potential.
The powder X-ray diffraction (XRD) pattern of the as-prepared compound confirms the
successful synthesis of Na2Mn3O7. The galvanostatic charge/discharge curves of Na2+xMn3O7
recorded between 1.5 and 3.0 V vs. Na/Na+ (Figure 1) confirm the reversible electrochemical
reaction of Na2Mn3O7 ↔ Na4Mn3O7 based on Mn4+/Mn3+. In addition, we confirm that Na+ canalso be deintercalated from Na2Mn3O7. The galvanostatic charge/discharge curves for first few
cycles between 3.0 and 4.7 V vs. Na/Na+ at a C/20 rate show that Na2−xMn3O7 delivers an extra

Figure 1. Left: Crystal structure of Na2Mn3O7. Right: Charge-discharge curves of Na2Mn3O7 in a
sodium-ion cell.

reversible capacity of approximately 75 mAh g-1, corresponding to the (de)intercalation of 1.0
Na+ per formula unit (Na2Mn3O7 ↔ NaMn3O7) at an average potential of 4.1 V vs. Na/Na+.
Considering that Mn4+ in Na2Mn3O7 cannot undergo further oxidation during Na+
deintercalation, the origin for this capacity is the reversible oxygen-redox reaction.
X-ray absorption spectroscopy (XAS) at the Mn L-edge and oxygen K-edge was conductedto confirm the oxygen-redox activity. The XAS spectra were measured in the bulk sensitive
partial fluorescence yield (PFY) mode with a probing depth deeper than 100 nm, as well as in
the surface sensitive total electron yield (TEY) mode with a probing depth of approximately 5
nm. Before charging, the XAS spectral shape of the Mn L3-edge in the PFY mode is almost
identical with that reported for Mn4+ in Li2MnO3. Maintaining its initial shape during the entire
charge/discharge processes indicates that Mn in bulk does not contribute to the redox reaction.
However, the XAS spectra of the Mn L3-edge in the surface sensitive TEY mode indicate theformation of Mn3+ at the surface when discharging below 4.0 V vs. Na/Na+. Therefore, at thesloping charge/discharge profile region below 4.0 V vs. Na/Na+, the surface redox reaction of
Mn4+/Mn3+ is responsible for the capacity.
The XAS oxygen K-edge spectrum (PFY) before charging shows two pre-edge peaks that
correspond to the excitation from 1s to unoccupied oxygen 2p orbitals hybridized with Mn eg↑,
t2g↓, and eg↓. If Mn4+ ions were oxidized during Na+ deintercalation, a new absorption peakcorresponding to the excitation from 1s to the unoccupied O 2p−Mn t2g↑ state should appear at
the lowest energy region (ca. 528 eV). However, the absorption increases at the high energy
region of 532 eV after charging. The emergence of this new absorption at the high energy region
upon charge is a typical behavior of oxygen-redox electrodes.
It is important to note that the oxygen K-edge XAS spectral change is reversible uponcharge/discharge, confirming the reversibility of the oxygen-redox reactions. The oxidized
oxygen is stabilized by a π-type interaction between the oxygen 2p and Mn t2g orbitals. In
addition, the large difference in the effective ionic radii of Na+ (1.16 Å) and Mn4+ (0.67 Å)
should suppress damaging Mn migration while coulombic attraction between Na+ and 
maintains the stacking of [1/7Mn4+6/7] layers after desodiation.
In summary, a Mn deficient layered compound Na2−xMn3O7 (Na4/7[1/7Mn4+6/7]O2) showed
highly reversible oxygen-redox
reaction at approximately 4.1 V
vs. Na/Na+. Nonbonding 2p
orbitals of oxygens neighboring
Mn vacancies
weredetermined to be essential forthe reversible oxygen-redox
reaction delivering an extra
capacity of 75 mAh g-1. This
work provides a compelling
futureresearch
direction
toward reversible oxygenredox cathodes for high energy
densitybatteries
through Figure 2. O K-edge X-ray absorption spectra for Na2Mn3O7 uponcharge/discharge.
orbital/structural engineering.
REFERENCES[1] B. Mortemard de Boisse, et al., M. Okubo & A. Yamada, Adv. Energy Mater. (2018) 1800409.

REDOX DRIVEN SPIN TRANSITION IN SOLID
Masashi Okubo
Department of Chemical System Engineering, School of Engineering, The University of Tokyo

Electronic spin plays a crucial role in determining the physical and chemical properties oftransition-metal compounds. In particular, d4−d7 electronic configurations of first-row
transition-metals in an octahedral coordination have either high spin (HS) or low spin (LS)states, governed by the competition between the ligand-field splitting and electron paring
energy. When two spin states are thermally accessible, the spin state changes (also known as
spin crossover or spin transition) owing to external stimuli such as temperature, pressure, light,
and chemical reactions. Because the alteration of spin states drastically modulates frontier
orbitals that are fully responsible for chemical reactivity, coupling phenomena between spintransitions and chemical reactions are of great interest in chemistry. In an electrochemicalsystem, the number of d electrons at a redox center is variable. Therefore, a stable spin state
may occasionally be altered after oxidation/reduction, leading to a spin transition.
Redox-driven spin transitions are important processes, for example in biochemistry, and havealso been studied for possible applications for molecular devices.
In battery ceramics electrode materials containing transition-metals M, electrochemical ion(de)intercalation modulates the valence of M, which in turn changes the magnitude of theligand-field splitting and the number of d electrons. In analogy with liquid statetransition-metal complexes, it is likely that M in battery electrode materials exhibits a hitherto
unreported solid-state electrochemical spin transition, which might considerably influence theoperating potential of the electrodes. However, despite considerable interest in the redox
behavior of M in battery electrodematerials, materials featuring a RDST has
not been explored to date. One suitablematerial for exploring this new concept isthe layered transition-metal oxide NaMO2
where MO6 octahedra form a triangular
two-dimensional slab and Na ions reside in
the interlayer space. NaMO2 has been
investigated as a promising cathodematerial for sodium-ion batteries with alarge theoretical capacity. Importantly,
NaMO2 maintains a layered structure withvarious Mn+ owing to the large ionic size
difference between Na+ and Mn+. Thesefeatures make the NaMO2 system suitablefor investigating the electrochemical
properties of various redox couples
M(n+1)+/Mn+ in an identical solid-statematrix.
Taking advantage of the remarkablestructural tolerance of NaMO2 towards thevariation of the electronic structure, we
explored the possibility of spin transitions
in NaxMO2 during charging/discharging Figure 1. (a) Charge/discharge curves forreactions.
Using
computational O3-NaxTi0.5Co0.5O2 and NaxTi0.5Ni0.5O2. (b) Ex situ
calculations assuming various d4−d7 X-ray diffraction patterns for NaxTi0.5Co0.5O2.
configurations in NaxMO2, we predict Mn+

species that can cause the HS-LS transition in a battery cell and verify these predictions in
multiple experiments.
We verified the redox-driven spin transition for Co3+/Co2+ experimentally. Generally,
NaxMO2 exhibits reversible electrochemical Na-ion (de)intercalation with a small polarization
owing to high Na-ion diffusivity and high electron mobility. For example, NaTi0.5Ni0.5O2
shows reversible charge/discharge curves over a narrow voltage region. Conversely, thecharge/discharge curves for NaxTi0.5Co0.5O2 show a large voltage hysteresis of approximately
2.8 V for the Na+ (de)intercalation (Figure 1a). This phenomenon is not caused by sluggish
conversion/phase segregation because ex situ X-ray diffraction experiments confirmed acommonly-observed smooth reversible transformation between the O3 and P3 phases upon
Na-ion (de)intercalation (Figure 1b). Therefore, we attribute this large voltage hysteresis tospecific energetics of the electron configuration.
To clarify the change in the electronic-structure of NaxTi0.5Co0.5O2 upon desodiation(charging) and subsequent sodiation (discharging) processes, we measured L-edge X-ray
absorption spectroscopy of the transition metals M. No significant change of the Ti L-edge
spectra was observed upon charge/discharge. The spectral shapes for all samples were well
reproduced by the configuration interaction (CI) calculation of Ti4+ in octahedral TiO6.
Clearly, Ti is not redox active during charging/discharging NaxTi0.5Co0.5O2. We further
confirmed the redox inactivity of Ti by X-ray photoelectron spectroscopy measurements of
the Ti 2p region. Conversely, the Co L-edge spectra for NaxTi0.5Co0.5O2 showed notable andreversible changes upon charge/discharge. The spectra for NaTi0.5Co0.5O2 andNa0.5Ti0.5Co0.5O2 were well reproduced by CI calculations of Co2+ HS and Co3+ LS inoctahedral CoO6, respectively (Figure 2). Therefore, the stable spin state of Co reversibly
changed during the charging/discharging of NaxTi0.5Co0.5O2.
In summary, we discovered a solid-state
redox-driven spin-state transition of
Co3+/Co2+ in layered transition metaloxidesupon
reversible
sodiumintercalation
reactioninanelectro-chemical cell. Our work points tothe considerable influence of thespin-state variance of transition metals
with d4−d7 configurations on theirsolid-state electrochemistry. Further
ex-ploration in various host structures
might reveal the missing link between thespin transition and electrochemical
modula-tions. In a practical sense,
redox-driven spin transitions cause a large Figure 2. Ex situ Co L-edge absorption spectra forvoltage hysteresis (> 1 V) upon O3-NaxTi0.5Co0.5O2.
electrochemicalcharg-ing/discharging
processes and a large loss in energy efficiency. Thus, Co3+/Co2+, and possibly Fe4+/Fe3+, inoxides should be used with care as the redox couples in battery electrodes.
REFERENCES[1] E. Watanabe, et al., M. Okubo & A. Yamada, Chem. Mater. (2019) 31, 2358.

ENHANCEMENT OF PHOTORESPONSIVITY OF FULLERENE IN
PHTHALOCYANINE /FULLERENE ORGANIC SOLAR CELL
Kenichi Ozawaa, Susumu Yamamotob, Marie D’angeloc, Yuto Natsuid, Naoya Terashimad,
Kazuhiko Masee,f, Iwao MatsudabaDepartment of Chemistry, Tokyo Institute of Technology, bThe Institute for Solid State Physics, The
University of Tokyo, cSorbonne Université, Institut des NanoScience de Paris, dDepartment ofAdvanced Physics, Hirosaki University, eInstitute of Materials Structure Science, High Energy
Accelerator Research Organization, fSOKENDAI (The Graduate University for Advanced Studies)

INTRODUCTION
Organic photovoltaic (OPV) devices are one of leading solutions for realizing ubiquitouslight-harvesting systems. Although much effort has been devoted to improve the OPV
performance, the light-electricity conversion efficiency is no more than 15% [1]. An effective
electron-hole separation before an electron-hole recombination is a key to realize a highefficiency. Thus, it is important to understand the behavior of the photoexcited electrons andholes especially at heterojunctions between p- and n-type organic semiconductors.
In the present study, layered systems fabricated by phthalocyanine (CuPc and TiOPc), ap-type organic semiconductor, and C60, an n-type organic semiconductor, on a single
crystalline rutile TiO2(110) surface were investigated by pump-probe time-resolved soft X-ray
photoelectron spectroscopy (TRXPS) to clarify the photoexcited carrier dynamics in thelayered organic thin films [2]. Photoresponsivity of C60 towards the ultraviolet (UV) light was
found to be enhanced when the phthalocyanine layer covers the C60 layer.
EXPERIMENTAL
The TRXPS measurements were done at BL07LSU of SPring-8 [3]. A 402-nm UV laser
was used to generate the photoexcited carriers in the organic films, and the excited states weremonitored through synchrotron-radiation (SR)-excited C 1s peaks of C60 and phthalocyanine.
The SR was provided with an H-mode operation of SPring-8 with a pulse width of about 50
ps, which corresponded to the time resolution of the TRXPS system. We also carried out
conventional X-ray photoelectron spectroscopy (XPS) measurements at BL-13B of the
Photon Factory [4] utilizing an SES200 electron energy analyser (Gamma Data/Scienta).
A 0.05 wt% Nb-doped rutile TiO2 crystal with (110) orientation was used as a substrate.
The (110) surface was cleaned in an ultrahigh vacuum (UHV) chamber by cycles of Ar+
sputtering and annealing at 950 K in O2 atmosphere. C60, CuPc and TiOPc were evaporated
on the TiO2 surface at room temperature using Knudsen cells. The thicknesses of the organic
layers were adjusted to monolayer-equivalent thicknesses, i.e., 0.8 nm for C60 and 0.34 nm forthe phthalocyanine molecules, by controlling evaporation rate and time.
RESULTS AND DISCUSSION
Fig. 1 shows C 1s XPS spectra of the C60/TiO2(110) and CuPc/C60/TiO2(110) systems. The
C 1s spectrum of the former system is composed of a single component, whereas both C60 and
CuPc contributions are confirmed by peak fitting. Even though the C60 layer is beneath the
CuPc layer, the C 1s spectrum for the CuPc/C60/TiO2(110) system is dominated by the C60
component, reflecting the densities of the composite C atoms in the CuPc and C60 layers.
Upon the UV laser irradiation, the C 1s spectrum moves slightly towards the higher binding
energy side (Fig. 2). Amounts of the peak shift are 13 and 14 meV at a delay time, a timedifference between the pump UV laser pulse and the probe SR pulse, of 0.1 ns for
CuPc/C60/TiO2(110) (Fig. 2) and TiOPc/C60/TiO2(110) (not shown here but given in Ref. [2]),respectively. Since the C 1s spectrum is dominated by the C60 component, the observed

spectral shift is caused mainly by the shift of the C60 component. The direction of the shift
implies cationization of C60 after photoabsorption. We speculate that an electron excited into
an unoccupied state of C60 is transferred to the conduction band of TiO2. Thus, the peak shift
indicates the electron-hole separation.
As the delay time is prolonged, the shift is diminished to ~8 meV for both systems (Fig. 3).
Solid lines in Fig. 3 are best-fitted results using Δ1 exp(− t/τ1) + Δ2 exp(− t/τ2), where Δ and τ
are the magnitude of the shift and the decay time constant, respectively. τ for
CuPc/C60/TiO2(110) are 95 ns and >100 μs and those for TiOPc/C60/TiO2(110) are 20 ns and>100 μs. This indicates that there are two excited states of C60; one has a lifetime in ananosecond order and the other has a very long lifetime (a microsecond order). Existence ofthe long-lifetime component is beneficial to the efficient OPV because it means suppressionof the electron-hole recombination.
Also shown in Fig. 3 is a delay-time dependence of the C60 C 1s peak shift for the
C60/TiO2(110) system. The C 1s peak moves to the higher binding energy side upon the UV
laser irradiation. The magnitude of the shift is approximately 7 meV irrespective of the delay
time. If the magnitude of the shift is proportional to the density of excited C60, comparison of
the three systems leads us to conclude that, although the phthalocyanine overlayer partially
blocks the incoming UV photons, photoresponsivity of C60 is enhanced by the overlayer. A
pohotoresponsivity enhancement is possible when an energy is effectively transferred fromexcited phthalocyanine to C60 via either a Förster mechanism or a Dexter mechanism [5].
Such an intermolecular energy transfer is an important process in, for example, dye-sensitizedsolar cells, in which a repeated stack of the C60 and phthalocyanine layers may be effective toachieve a high efficiency.

C60
CuPc

C60/TiO 2(110)

C 1s
ON

OFF

100
0
−100
Relative Energy (meV)285
Binding Energy (eV)Pump Laser
402 nmDelay time
t = 0.1 ns

Laser ON
Laser OFF286
298h = 385 eV

285
284
Binding Energy (eV)20
C60 C 1s Shift (meV)CuPc/C60/TiO 2(110)Intensity (arb. units)h = 600 eV

C 1s

CuPc / C60 / TiO2(110)
TiOPc / C60 / TiO2(110)10

C60 / TiO2(110)

0

0.1

1

10
100 1000
Delay Time (ns)282

FIGURE 1. C 1s spectra of
CuPc/C60/TiO2 and C60/TiO2.
Lines drawn by dots areexperimental results, and solidlines are best-fitted results ofpeak fitting using Voigt
functions.

FIGURE 2. C 1s spectra of
CuPc/C60/TiO2 without UV laser
irradiation and at 0.1 ns after
the laser pulse irradiation.
Magnified view around thespectral peak region is shown in
the inset. A slight shift of thepeak position is induced by theUV irradiation.

REFERENCES[1] M. Green et al., Prog. Photovoltaics Res. Appl. 25, 668−676 (2017).
[2] K. Ozawa et al., J. Phys. Chem. C 123, 4388−4395 (2019).
[3] S. Yamamoto et al., J. Synchrotron Radiat. 21, 352−365 (2014).
[4] A. Toyoshima et al., J. Phys.: Conf. Ser. 425, 152019 (2013).
[5] M. T. Lloyd et al., Mater. Today 10, 34−41 (2007).

FIGURE 3. Summary of
the laser-induced shift of
the C60 C 1s peak as afunction of the delay time.
Dots are the experimental
results and solid lines are
obtained by fitting thedata using a biexponentialfunction.

PHOTOELECTRON NANO-SPECTROSCOPY OF REACTIVE IONETCHING-INDUCED DAMAGES TO THE TRENCH SIDEWALLS AND
BOTTOMS OF 4H-SIC TRENCH-MOSFETS
Masaharu Oshima1, 5, Daisuke Mori2, Aki Takigawa2, Akihiko Otsuki2, Naoka Nagamura3,
Shun Konno4, Yoshinori Takahashi4, Masato Kotsugi4, Hiroshi Nohira5, Keishi Akada1 andYoshihisa Harada1
1
The Institute for Solid State Physics, The University of Tokyo, 2Fuji Electric Co., 3NIMS, 4Tokyo
University of Science, 5Tokyo City University
Silicon carbide has attracted great attention in order to be applied to high-power devices fordevices that depict low power consumption, low on-resistance, and high blocking voltage,
because of its superior material properties such as high voltage endurance and saturation
velocity. Recently, 4H-SiC trench-MOSFETs have been regarded as one of the mostpromising device structures to reduce on-resistance. Furthermore, the non-polar 1-100 plane(m-plane) of 4H-SiC can offer the highest channel mobility among various SiC crystal planes
and can be used as a channel plane for the trench sidewall in trench-MOSFETs. [1]In this study, SiC trench structures having a width of 0.6 m and a depth of 2.0 mfabricated by reactive ion etching (RIE) using a gas mixture of SF6, Ar, and O2 were cleaved
to expose the sidewall for the channel region of a trench MOSFET and analyzed by 3D
nano-ESCA [2] using a 100 nm soft-X-ray beam at SPring-8 BL07LSU (Fig. 1). The AFM
image of RIE-tranch sidewall surface is shown in Fig. 2.
It is observed that around 2 nm-thick homogeneous carbon-rich layer containing 1%-2 % F
is formed on the SiC sidewalls. This may be caused due to the re-deposition of RIE reaction
products, CF4, and SiF4 under appropriate conditions to fabricate approximately verticaltrench walls, as shown in Fig. 3. Further, a carbon-rich layer having a thickness of about 2.4
nm is also formed on the bottom of the SiC trench, suggesting the possibility of selective
etching of Si from the SiC substrates. CFx species are likely to form polymer-like protective
layers during the RIE process, resulting in the enhancement of the etching selectivity, and the
addition of Ar to the etching gas is thought to enhance the physical reaction or sputtering
effect. The overall chemical reaction can be represented as follows:
4 SF6 + 4 O2 + 3 SiC → 4 SO2↑ + 3 SiF4 ↑ + 3 CF4 ↑(1).
It should be noted that the position of the Si 2p dominant peak that is associated with the
SiC component remains constant regardless of the trench depth, as shown in Fig. 4. Thissuggests homogeneous band bending due to the RIE defects, which may explain the reason
for no variation being observed in the gate oxide/SiC interface trap density values. The Si 2p
binding energy of 101.1 eV on the sidewall corresponds to a band bending of 1.50 eV.
Theoretical calculations using DFT reportedly revealed that the EH7 trap observed by DLTS
[3] can be attributed to the presence of positively charged carbon vacancies (VC+) whoseenergy level is 1.73 eV below the CBM [4]. Based on these experiments and calculations theobserved band bending of 1.50 eV may be attributed to the dominant EH7 trap or to thepositively charged carbon vacancy (VC+). [5]We also analysed SiC trench sidewall which was treated with thermal annealing andoxidation processes, and found that the almost same band bending remained unchanged evenby these processes. Further study is needed to realize nearly flat band condition.
REFERENCES[1] J. Senzaki et al., IEEE Electron Device Lett. 23, 13 (2002).

[2] K. Horiba et al., Rev. Sci. Instrum. 82, 113701 (2011).
[3] L. Storasta et al., J. Appl. Phys. 96, 4909 (2004).
[4] L. Torpo et al., J. Phys.: Cond. Mat. 13, 6203 (2001).

[5] M. Oshima et al., e-Journal of Surf. Sci. and Nanotech. 16, 257 (2018).

Fig. 1. 3D nano-ESCA at SPring-8 BL07LSU

Fig. 2. AFM image of SiC trench sidewall(1mx1m)

Fig. 3. F 1s photoelectron spectra with the deconvoluted components from four different

positions (X= 0.1, 0.7, 1.2, and 1.8 m from the surface) of the SiC trench sidewall, and aschematic model of RIE trench structure.

Fig. 4. Si 2p photoelectron spectra in a narrow scan mode with the deconvoluted components
from four different positions (X= 0.1, 0.7, 1.2, and 1.8 m from the surface) of the SiC trench
sidewall, and a schematic band diagram for the m-plane SiC sidewall caused by RIE.

STUDY ON INTERFACIAL HYDROGEN-BOND STATES IN
ELECTRON-PROTON-CORRELATED ORGANIC HETEROBILAYERS
Hiroyuki S. Kato a, Susumu Yamamoto b, Akira Ueda b, Shinya Yoshimoto b, Hatsumi Mori b,
Jun Yoshinobu b, Hiroki Wadati b, Iwao Matsuda bab

Graduate School of Science, Osaka University,
The Institute for Solid State Physics, The University of Tokyo

Introduction
Recently, new organic functional materials that show the proton (H+) dynamics coupledwith electrical conductivity have been reported [1,2]. In the organic crystal consisting ofcatechol-fused tetrathiafulvalene derivative (Cat-TTF), the change in the position of H+ in thehydrogen bonds (HBs) induces charge redistribution of the π electrons in the molecules,
which results in the switching of electrical conductivity [1,2]. Although the novel“proton-electron” concerted functionality of Cat-TTF has been achieved in 3D crystals, it
should be necessary to reduce the dimensionality to 2D when one considers its application inorganic electronic devices. In the previous study, hence, we designed the proton-electron
correlated bilayers and created them in a way of self-assembly [3]. The bilayer consists of thecatechol-fused bis(methylthio)tetrathiafulvalene (H2Cat-BMT-TTF) molecular layer as a H+
donor
and
the
imidazole-terminatedalkanethiolate
self-assembled
monolayer(Im-SAM) as a H+ acceptor (Fig. 1). When thepairs of H+ donor and acceptor provide strong
HBs, the H+ transfer along the HBs becomeseasier in the bilayer and thus the conductivityof the Cat-TTF layer may become more
controllable. In this study, therefore, we aimed
to elucidate the characteristics of theinterfacial HB states in the heterobilayer for
creating the 2D proton-electron correlated Fig. 1. Schematic chemical structures of thesystems on a substrate [4].
self-assembled heterobilayer consisting of the
Cat-TTF layer and Im-SAM on an Au substrate.

Experimental
The organic heterobilayer on an Au surface was prepared by sequential two-step immersionof the Au substrate deposited on cleaved mica (Au film thickness of 200 nm, Phasis) inindividual solutions of the organic compounds. First, the clean Au substrate was immersed ina deoxygenated ethanol solution containing 1 mM of 1-(11-mercaptoundecyl)-imidazole(Im-(CH2)11-SH; 96%, Sigma-Aldrich) for ≥ 20 h. This produced a well-packed Im-SAM on
Au. Secondly, the Im-SAM/Au sample was immersed in a deoxygenated chloroform solution
containing 1 mM of the synthesized H2Cat-BMT-TTF for ≥ 20 h. All immersion processes
were carried out under dry N2 atmosphere in a glove box at room temperature. Further detailsof sample preparation can be found in the previous publication [3].
The element- and site-specific molecular interactions in the organic bilayer were
investigated by NEXAFS. The NEXAFS experiments were carried out at soft X-ray beamline
BL07LSU at SPring-8. The spectra were recorded in total electron yield (TEY) by measuring
the sample drain current. The incidence angle of the p-polarized soft X-ray was set to 54.7°
from the sample surface, i.e., “magic angle”. The incident soft X-ray beam was defocused to~60 µm (H) × ~30 µm (V) to avoid beam damage of the organic molecules. All NEXAFS
spectra were measured in UHV (base pressure of ~5 × 10−6 Pa) at 300 K. The NEXAFS
spectra were normalized with the incident photon flux measured by the drain current of the
Au mesh and then, a linear background determined by the slope at the nitrogen preedge region

(396−399 eV) was subtracted. Further details of NEXAFS
measurements can be found in our recent publication [4].
Results and Discussion
By the design of our heterobilayer, the OH group of
catechol and imino N in the imidazole group may make HB(Fig. 2). In fact, the interfacial HB states in the heterobilayer
are characterized by a broadened and highly red-shifted bandof O-H stretching vibrations observed by infrared reflection
absorption spectroscopy [3]. This is the clear evidence ofstrong HBs of the H+ donors. However, it was not clear that
imino N works as the H+ accepter in HBs. Hence, the N
K-edge NEXAFS was conducted to specify the complete Fig. 2. Schematic chemical
picture of interfacial HB states [4]. The element specificity of structures and a hydrogen bondat the interface between the
NEXAFS allows elucidation of the nature of HB at the H+ Cat-TTF layer and Im-SAM.
acceptor side because N atoms exist only in the Im-SAM.
Figure 3 shows the N K-edge NEXAFS spectra of Im-SAM/Au and H2Cat-BMT-TTF
adsorbed on Im-SAM/Au. The sharp peaks at 400.0 eV and 401.8 eV are ascribed to the N 1s
→ 1π* transition of the imino N (N3) and amino N (N1) atoms, respectively, of the imidazole
ring in Im-SAM. In comparison between the spectra before and after the H2Cat-BMT-TTF
adsorption, the π* peak of imino N (N3) shifts from 400.0 eV to 400.3 eV, while that ofamino N (N1) remains at the same energy. This shows that the adsorption of
H2Cat-BMT-TTF changes only the chemical environment of imino N, by making HBs. Basedon the deconvolution analysis, the ratio of the shifted component to the total imino N isdetermined to be 0.41. These results depict the interfacial HB states in the heterobilayer asthat closely half of imino N makes strong HB with the OH groups of H2Cat-BMT-TTF.

Fig. 3. N K-edge NEXAFS spectra of Im-SAM and H2Cat-BMT-TTF adsorbed on Im-SAM: (a) Wholeabsorption, (b) enlarged π* peak regions (raw data), and (c) enlarged π* peak regions (raw data and fit data)[see details in Ref. 4]. The NEXAFS spectra were fit by up to six Gaussian peaks for the excitation of a coreelectron to π* and σ* states, and two step functions for the excitation of a core electron to a continuum. Two
step functions originate from two different N atoms in Im-SAM; imino N (N3) and amino N (N1) atoms.

REFERENCES[1] T. Isono, et al., Nat. Commun. 4, 1344 (2013).
[2] A. Ueda, et al., J. Am. Chem. Soc. 136, 12184 (2014).
[3] H.S. Kato, et al., Langmuir 34, 2189 (2018).
[4] S. Yamamoto, et al., e-J. Surf. Sci. Nanotechnol. 17, 49 (2019).

OPERANDO OBSERVATION OF METHANE ACTIVATION ON
HETEROGENEOUS CATALYSTS BY AMBIENT-PRESSURE XPS
Takanori Koitaya1,2, Susumu Yamamoto3, Iwao Matsuda3, Jun Yoshinobu3, and Toshihiko
Yokoyama1
12Institute for Molecular Science, National Institutes of Natural Sciences
Japan Science and Technology Agency, Precursory Research for Embryonic Science and Technology
3
The Institute for Solid State Physics, The University of Tokyo

Chemical transformation of methane to valuable chemicals is an attracting, but challenging
research topic. Activation of inert methane molecules on metal or metal-oxide surfaces is an
important process for catalytic conversion [1]. In order to detect reaction of physisorbed
methane molecules, (near) ambient-pressure condition is indispensable, since dissociative
sticking probability of methane is very small [2]. In this study, the reaction of methane withpreadsorbed oxygen on Pd(110) and a PdO thin film was investigated by XPS under ultrahighvacuum and ambient-pressure XPS (AP-XPS).
XPS experiments were performed using the AP-XPS apparatus in BL07LSU [3]. The
Pd(110) sample was cleaned by sputtering and annealing in the preparation chamber (basepressure = 3 × 10-10 mbar). The oxygen-preadsorbed Pd(110) (O-Pd(110)) was prepared byexposure of 22 mbar oxygen on Pd(110) at room-temperature for 1500 s. The PdO thin film
on Pd(110) (PdO-Pd(110)) was formed by heating the sample to 770 K in the presence of 1
mbar oxygen.
Figure 1(a) shows a series of Pd 3p and O 1s spectra of the O-Pd(110) surface as a functionof methane exposure. Methane molecules were exposed on the surface at room-temperature.
By the reaction of methane with preadosorbed oxygen, the adsorbed amount of oxygen was
gradually decreased. Concomitantly, peaks of oxygen-adsorbed Pd atoms in Pd 3d core-level(Fig. 2b) decreased in intensity. Fig. 1(c) shows change in the coverage of oxygen atoms. The
oxygen coverage was steeply decreased at low methane exposure, indicating that reaction
probability of methane is higher at high oxygen coverage than those at lower oxygen
coverage.
Fig. 2 (a) and (b) show Pd 3p, O 1s and Pd 3d spectra of PdO-Pd(110) surface under 0.4
mbar methane atmosphere as a function of sample temperature. The methane gas wasintroduced into the gas-cell at sample temperature of 300 K, followed by heating in thepresence of methane. The reaction of methane did not occur on the PdO thin film at room
temperature. The sample was reduced by heating above 450 K. These results indicate thatmethane reactivity is much lower on the PdO thin film formed on Pd(110) than on theoxygen-preadsorbed Pd (110) surface.

(a)

(b)

Oxygen atom coverage (ML)

1.0

(c)

0.8

0.6

0.4

0.2

0.0

0

2000

4000

6000

8000 10000 12000 14000

Methane exposure (L)Fig. 1 (a) Pd 3p and O 1s, and (b) Pd 3d core-levels of oxygen-preadsorbed
Pd(110) as a function of methane exposure. Methane was dosed on thesurface at room temperature. (c) Oxygen coverage estimated from O 1s
intensity.
(a)

536

534

532

530

Binding energy (eV)

PdO

Intensity (arb. unit)

温度（K)
524
507
486
465
443
416
386
355
331
323
329

Intensity (arb. unit)
538

(b)

PdO

温度（K)
527
513
492
472
451
426
396
365
338
326
324

528

338

336

334

Binding energy (eV)Fig. 2 (a) Pd 3p, O 1s, and (b) Pd 3d AP-XPS spectra of the PdO thin film on
Pd(110) as a function of sample temperature. A series of spectra was measured in thepresence of 0.4 mbar methane.

REFERENCES
[1]
[2]
[3]

R. Horn and R. Schlögl, Catal. Lett. 145, 23 (2014).
R. A. Campbell, J. Szanyi, P. Lenz, and D. W. Goodman, Catal. Lett. 17, 39 (1993).
T. Koitaya et al., Top. Catal. 59, 526 (2015).

SYNERGETIC EFFECT AT METAL-OXIDE INTERFACE ON
METHANE ACTIVATION MEASURED BY AMBIENT-PRESSURE XPS
Takanori Koitaya1,2, Susumu Yamamoto3, Iwao Matsuda3, Jun Yoshinobu3, and Toshihiko
Yokoyama1
12Institute for Molecular Science, National Institutes of Natural Sciences
Japan Science and Technology Agency, Precursory Research for Embryonic Science and Technology
3
The Institute for Solid State Physics, The University of Tokyo

The interfacial interaction between metal and metal oxide significantly affects reactivity ofheterogeneous catalysts [1]. The metal particle on the oxide support is often decorated or
encapsulated by the metal-oxide thin film through the strong metal-support interaction(SMSI) [2]. Metal-oxide thin films deposited on metal substrates are thus a good model
system to examine the SMSI effect in catalysis. Recently, we have studied CO2 hydrogenation
on Zn-deposited Cu surfaces, and revealed that surface oxidation states depend on reaction
conditions, such as sample temperature and gas composition [3]. Therefore, in situ
observation of catalysts during reaction is indispensable for elucidation of chemical states ofworking catalysts.
Ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) is now widely applied formany reaction systems for real time observation of electronic structures of catalysts andreaction intermediates on the surface under reaction condition [4]. In particular, soft X-ray
AP-XPS is suitable for investigation of chemical states of a thin film deposited on the surface,since it is surface-sensitive technique due to low kinetic energy of photoelectron. In this study,
redox properties of the Ir-deposited Pd(110) surface were investigated by AP-XPS.
The sample preparation and AP-XPS measurements were carried out in ultrahigh vacuum
chambers (base pressure: 3 × 10-10 mbar). Iridium was deposited on the Pd(110) surface atroom temperature using electron-beam evaporator (Scienta Omicron, EFM 3). The coverageof Ir was estimated to be 1.7 ML. After the preparation of the Ir-deposited Pd(110) model
catalyst, AP-XPS measurements of redox reactions were conducted in the gas-cell at room
temperature.
Figure 1 shows Pd 3d and Ir 4f AP-XPS spectra measured under 1 mbar oxygen at sample
temperature of 300 K as a function of elapsed time. The Pd 3d spectra show gradual growth ofan oxygen-adsorbed Pd peak (O-Pd). Oxidation of the substrate to PdO was not observed inthis reaction condition. In contrast, the Ir 4f spectra show oxidation of deposited Ir from a
metallic state to IrO2 oxide. Note that total intensity of the Pd 3d peaks decreased as theoxidation of Ir proceeds. This indicates that Ir atoms were segregated on the surface, and an
IrO2-Pd(110) interface was formed in the presence of 1 mbar oxygen.
Figure 2 shows a series of Pd 3d and Ir 4f AP-XPS spectra of the IrO2-Pd(110) interface
under 0.4 mbar methane at 300 K. By the reaction with methane, atomic oxygen on Pd atom
was removed, and the IrO2 thin film was reduced to a metallic state. Thus, the Ir-depositedPd(110) surface is oxidized by O2 and reduced by methane at room temperature. In the case of
the bare Pd(110) surface, oxidation and formation of a PdO thin film occur at much higher
temperature (about 600 K). In addition, the PdO thin film was reduced to metallic Pd by
heating to 450 K in the presence of 0.4 mbar methane. These experimental results indicatethat the redox reactivity is significantly improved by the deposition of Ir atoms on the Pd(110)
surface.

Figure 1. (a) Pd 3d and (b) Ir 4f core-level spectra of Ir-deposited Pd(110)measured under 1 mbar oxygen atmosphere as a function of elapsed time.
During the measurement, the sample was kept at 300 K.

Figure 2. (a) Pd 3d and (b) Ir 4f core-level spectra of IrO2-Pd(110) in thepresence of 0.4 mbar methane as a function of elapsed time. The sample
temperature was 300 K.

REFERENCES[1] S. Shaikhutdinov and H. J. Freund, Annu. Rev. Phys. Chem. 63, 619 (2012).
[2] Q. Fu and T. Wagner, Surf. Sci. Rep. 62, 431 (2007).
[3] T. Koitaya et al., ACS Catalysis 9, 4539 (2019).
[4] D. E. Starr, Z. Liu, M. Hävecker, A. Knop-Gericke, and H. Bluhm, Chem. Soc. Rev. 42, 5833
(2013).

CHEMICAL STATE ANALYSIS OF INTERMEDIATE WATER
BOUNDED BY BIOCOMPATIBLE POLYMERSMasahiro Kunisu1, Masaru Nakada1, Koichi Ozaki1, Junpei Yahiro1,
Yitao Cui2, Kosuke Yamazoe2, Jun Miyawaki2 and Yoshihisa Harada2
1

Toray Research Center, Inc.,

2

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

Biocompatibility is one of the important performance indicators for polymers used in
medical materials. It is thought that adsorbed water on the polymer surface contributes tothe expression of biocompatibility. At the biocompatible polymer interface, there areroughly three types of water classified as free water, intermediate water, and antifreezingwater. It is considered that the intermediate water plays an important role in theexpression of biocompatibility. However, the local structural and dynamical properties ofintermediate water are still unknown. In this project, it aims at studying water contained inbiocompatible polymer in detail, using soft X-ray absorption spectroscopy (XAS) by O
K-edge core excitation and (resonant) soft X-ray emission spectroscopy (XES/RIXS) by
successive emission from oxygen valence to the excited core states after XAS.
Polyvinyl pyrrolidone (Poly Vinyl Pyrrolidone (C6H9NO)n: PVP) and polyethylene
glycol (Poly Ethylene Glycol (C2H4O)n: PEG) were used as the biocompatible polymer.
For PVP and PEG, an aqueous solution was prepared as a measurement sample at apolymer concentration of 5 wt% - 65 wt%, along with absolutely dry polymer powders as
an estimate of background. The O K-edge XAS measurements were performed in theenergy range from 520 to 550 eV. The XES measurements were performed underionization excitation energy of 550 eV. In the RIXS analysis resonant excitation to thepre-edge (535 eV), main-edge (537 eV), and post-edge (541 eV) in the XAS spectrum ofwater were used. Moreover, measurement was also performed with excitation energy near
the resonance to the C = O carbonyl *state (near 532 eV). The measurement of
the polymer aqueous solution was carriedout using an originally developed solution
cell at room temperature and underambient pressure environment by
circulating the solution.
Figure 1 shows XAS spectra of PVP
aqueous solution with a polymer
concentration of 5 wt% and 50 wt%, PVP
powder, and pure water. Peaks at 532.7
eV for the aqueous PVP solution and at
532.2 eV for the PVP powder are
considered to be attributed to C = O
Figure 1. O K-edge XAS spectrum of PVP(carbonyl). In the PVP aqueous solution,
aqueous solution with a polymer concentration of
5 wt%(yellow) and 50 wt%(green), PVP as the concentration decreased, the peakpowder(red), and pure water(blue).
intensity also decreased, and a clear

correlation was found between the concentration of the polymer aqueous solution and thepeak intensity.
In this experiment, XES / RIXS analysis was also performed. In the RIXS analysis, theenergy shown by the arrow in Fig. 1 was used as the excitation energy. In the near future,
we plan to proceed the analysis of the obtained results and compare them with theprevious results to investigate in more detail the characteristics and electronic state ofwater contained in the biocompatible polymers.

SIZE AND TEMPERATURE DEPENDENCE OF
HYDROGEN-STORAGE IN Pd NANOPARTICLES
1

Seo Okkyun1, Jiayi Tang2, Takanori Koitaya3, Susumu Yamamoto4, Chulho Song1,
Iwao Mastuda4, Jun Yoshinobu4, Osami Sakata1

National Institute for Materials Science, 2Departnment of Engineering, University of Hyogo,
3
Institute for Molecular Science, 4The Institute for Solid State Physics, The University of Tokyo

Introduction
Research into the interaction between metals and hydrogen is very important for thedevelopment of potential applications, such as advanced materials for hydrogen storage andpurification. Among these catalysts, Pd nanoparticles (NPs) are known as the classic models
for the clarification of hydrogen-storage properties of metal nanoparticles [1-5]. It is reported
that the hydrogen absorption/desorption processes during the hydrogen-storage leads to alarge expansion of the Pd lattice, which is called as α-β phase transition [2]. In addition,hydrogen atoms can be trapped strongly in the Pd NPs compared with Pd bulk catalysts
because of the larger surface area to volume ratio [3,4]. Recently, many studies have been
conducted on the structures effects on the catalyst performance of Pd NPs, however, in-situ
observations of hydrogen adsorption/desorption mechanisms on Pd nanoparticles withspecific surface depending on nanoparticle size under the ambient pressure are lacked. In thisstudy, the valence band as well as the core-level photoelectron spectra of Pd NPs were
investigated under the ambient hydrogen atmosphere to characterize the size dependence ofhydrogen absorption process.
Experimental
The Pd cube shaped NPs samples with size of 12 and 20 nm are prepared in aqueous
solution by reducing Na2PdCl4 with citric acid in the presence of PVP [6]. The size andcrystalline structure of Pd NPs were characterized by TEM and XRD measurements. The Pd
microparticles with 1 μm size were measured as a reference sample. The hydrogen absorption
were observed under H2 atmosphere at 2.0 mbar at 300, 373, and 473 K by using ambientpressure X-ray photoelectron spectroscopy system at BL07LSU. Pure H2 (> 99.99999%) gas
was introduced into an ambient-pressure gas cell, where the base pressure was below 5 × 10-10
Torr. An electron analyzer (SPECS, Phoibos 150) was assembled with a three-stage
differential pumping system. The Pd 3d core-level and valence band spectra were observed ata photon energy of 1200 eV.
Results and Discussion
Figures 1 (a)-(c) show the Pd 3d5/2 spectra measured before hydrogen exposure and duringhydrogen absorption at 2.0 mbar at 300, 373, and 473 K for 12 nm Pd NPs, for 20 nm Pd NPs,and Pd microparticles, respectively. Pd 3d5/2 spectra observed before hydrogen exposure on
three samples were fit well with two components, which are located at 334.8 eV and 335.4 eV.
The component at lower binding energy (BE) is assigned as Pd atoms from the bulk (Pd bulk),
another one at higher BE is assigned as Pd carbide (Pd-C). After hydrogen exposure at 2.0
mbar at 300 K, one new component appeared at BE of 335.1 eV on the Pd NPs surface. It canbe assigned as the Pd hydride (Pd-H), which is generally located at lower BE relative to Pd-C.
On the other hand, there is almost no Pd hydride formed on Pd microparticles duringhydrogen exposure at 300 K. Moreover, the amount of Pd hydride formed on 12 nm Pd NPs ismore than that on 20 nm Pd NPs, indicating the hydrogen absorption rate is higher withdecreasing the particle size. The hydrogen absorption was enhanced on both Pd NPs and Pd
microparticles by increasing the temperature. In the meantime, the Pd carbide was reduced
because of the carbon diffusion into the bulk with the increase of temperature, whichproduced more absorption sties for hydrogen atoms. We suggested that the increase of theabsorption sites enhance the hydrogen absorption rate. Figures 2 (a)-(c) show the valenceband spectra observed on the Pd NPs and Pd microparticles under the same condition. The
valence band spectra near Fermi edge was increased after hydrogen exposure on Pd NPs,
whereas there was no changes on Pd microparticles. First-principle simulations of the valenceband will be carried out to compared with the experiment results to explain the detailed
electronic properties during hydrogen adsorption/absorption.
REFERENCES[1] R. Griessen et al., Nature Mater. 15 (2016) 311-317.
[2] A. Baldi et al., Nature Mater. 13 (2014) 1143-1148.
[3] C. Wadell et al., Chem. Phys. Lett. 603 (2014) 75-81.
[4] S. Syrenova et al., Nature Mater. 12 (2013) 905-912.
[5] R. Bardham et al. Nature Mater. 15 (2016) 311-317.
[6] G. Li et al., Nature Mater. 13 (2014) 802-806.

Fig.1 Pd 3d5/2 spectra measured before H2 exposure and during hydrogen absorption at 300 K, 373 K
and 473 K on (a) Pd nanoparitles (12 nm), (b) Pd nanoparitles (20 nm) and (c) Pd microparticle.

Fig.2 valence band spectra measured before H2 exposure and during hydrogen absorption at 300 K, 373
K and 473 K on (a) Pd nanoparitles (12 nm), (b) Pd nanoparitles (20 nm) and (c) Pd microparticle.

ELUCIDATION OF BAND ALIGNMENT ATSEMICONDUCTOR/ELECTROLYTE INTERFACEBY X-RAY PHOTOELECTRON SPECTROSCOPY
Yuki Imazeki1, Masahiro Sato2, Takahito Takeda1, Masaki Kobayashi1, Susumu Yamamoto3,
Iwao Matsuda3, Jun Yoshinobu3, Yoshiaki Nakano1, and Masakazu Sugiyama1,2
1

Department of Electrical Engineering and Information Systems, The University of Tokyo

1

Research Center for Advanced Science and Technology, The University of Tokyo
3

The Institute for Solid State Physics, The University of Tokyo

Photocatalytic water splitting is one of the most attractive technologies for storing sunlight.
However, today’s photocatalysts (photoelectrodes) still suffer from low efficiency or poor
stability [1], and to overcome these problems, the physical basis of photoelectrochemical
reactions has to be understood in more detail. Band alignment (, i.e., the relative position
between the conduction band minimum (CBM) or the valence band maximum (VBM) of thesemiconductor and the redox potentials in the electrolyte at the semiconductor/electrolyte
interface,) determines whether the reaction of interest proceeds or not, and therefore is the key
factor for photoelectrochemical reactions. Nevertheless, from an atomistic point of view, little
is known about the photocatalyst(semiconductor)/electrolyte interface, and the relationshipbetween the band alignment and the geometric structure at the interface remains unrevealed.
The recent development of ambient pressure X-ray photoelectron spectroscopy (AP-XPS)
has made it possible to probe the geometric as well as the electronic structure of thesemiconductor/electrolyte interface under nearly in situ conditions [2]. On the other hand, we
have been exploring the band alignment at the semiconductor/electrolyte interface under thesame environment as in the progress of photoelectrochemical water splitting [3]. The purposeof this research is to bridge the band alignment in the surface vicinity of n-GaN photoanodebetween the situation in vacuum to the condition of water splitting, using AP-XPS with
controlled coverage of water molecules on the n-GaN surface.
An n-type GaN layer was grown epitaxially on a silicon substrate using metalorganic
vapor-phase epitaxy (MOVPE) with a doping concentration of ca. 4×1018 cm-3. X-ray
photoelectron spectroscopy (XPS) of the GaN surface was conducted using SPring-8
BL07LSU beamline in order to quantify the band bending in the surface vicinity of the GaN.
The amplitude of band bending was evaluated on the basis of the binding energy of Ga 3d
peak with respect to the Fermi level.
Prior to XPS measurements, the surface of the GaN layer was treated with N2 plasma in a
vacuum environment and the periodic arrangement of surface atoms was confirmed by
low-energy electron diffraction (LEED). In order to observe the change in band bending
under light irradiation, ultraviolet light from Xe lamp was introduced onto the GaN surface
through the quartz window of the specimen chamber. Water molecules were attached to the
GaN surface by introducing water vapor (1.9 mbar in partial pressure) into the chamber while
keeping the surface temperature to either 289 or 302 K.
Band bending under the dark condition was evaluated first. Under high-vacuum condition,
band bending was 0.43 eV. Adsorption of water molecules on the GaN surface at 289 K
reduced band bending to 0.032 eV. By increasing the surface temperature to 302 K,
corresponding to the reduction in the amount of adsorped water molecules, band bending wasslightly increased to 0.060 eV. Such reduction is band bending induced by the adsorption ofwater molecules is consistent with the previous report [4]. It seems that water molecules
passivated the surface states of n-GaN and un-pinned the Fermi-level from the surface states.
As depicted in Fig. 1, such almost flat-band conditions brought about by water adsorption

seems is in contrast to the huge band bending which has been observed for the n-GaN surface
adjacent to 1M NaOH aqueous solution [3]. This gap is a target of future investigation. By
increasing the amount of water adsorbed on the GaN surface, further structural change in the
GaN surface is anticipated.
H
H O

H

H
H O
GaN
In contact toelectrolyte

H

GaN
In vacuum
PotentialO
GaN
Water adsorption(< 1ML)
Potential

Band bending
0.4 V
Surface states

Potential

Almostflat band

H+/H2

Band bending
1.7 V

Surface states
O2 /H2On-GaN

Fig. 1n-GaN

1M NaOHaq.

n-GaN

Band alignment in the surface vicinity of n-GaN photoelectrode under dark.

The next analysis is the impact of light irradiation on the band bending with and withoutwater adsorption on the n-GaN surface. Under the light intensity of this experiment (notquantitatively calibrated), the amplitude of band bending was decreased by 10 - 20 meV, towhich the impact of water adsorption is not clear. For the n-GaN adjacent to 1M NaOH
aqueous solution, much larger change in band bending has been observed, but further
discussion needs the XPS measurement with more intense and variable light irradiation,
which is the target of the future trial.
The initial results described here provides a perspective for the future experiment: it will bepossible to analyze the band bending of n-GaN photoelectrode as a function of both theamount of adsorbed water molecules and light intensity.
(a)

(b)

Fig. 2 The shift in Ga 3d peak position under dark and Xe-lamp illumination: (a) undervacuum, (b) with water adsorption at a GaN surface temperature of 289 and 302 K. The
increased binding energy under light irradiation indicates the increased distance between the
Fermi level and the Ga 3d level in the topmost surface of GaN, i.e., the reduction in band
bending.
REFERENCES[1] J. Liu et al., Science, 347, 970, 2015.
[2] M. Lichterman et al., Energy Environ. Sci., 8, 2409, 2015.
[3] Y. Imazeki et al., ECS Transactions, 77, 25-30, 2017.
[4] P. Lorenz et al., Phys. Status Solidi C, 7, 169–172, 2010.

STRUCTURE ANALYSIS OF BATIO3 /PT (111)/SI
QUASCICRYSTAL THIN FILM BY PHOTOELECTRONHOLOGRAPHY
Shun Fukami a, Hiroki Momono a, Masafumi Ogi a, Peng Dawei a, Yusuke Hashimoto a,
Hiroyuki Matsuda a, Munetaka Taguchi a, Stefan Förster b, Wolf Widdra b, Hiroshi Daimon aaNara Institute of Science and Technology(NAIST),b

Martin-Luther-Universität Halle-Wittenberg

Stefan et al 1) focused on the BaTiO3 (111) / Pt (111) thin film and succeeded to makeOxide Quasi-Crystal (OQC) on Pt (111) substrate by specific annealing steps. Theircharacteristic two-dimensional structures are revealed by STM and LEED technique. OQC not
only has a 12-fold quasi-periodic structure called Stampfli-Gähler tiling, but it also changes toa more regular approximate crystal structure under an another annealing condition. Furthermore,
the wetting mechanism on the substrate indicates that the OQC film has potential as a brandnew device. However, there is still room for clarification of the local structure of each element.
We have conducted elemental selective experiments of two-dimensional oxide thin filmusing photoelectron holography technique. These experiments used BL07LSU beam line,
which produces strong linear and circular polarizations in the soft x-ray region. Meanwhile, weused Display type ELlipsoidal Mesh Analyzer (DELMA) 2), a unique device, to perform highenergy resolution photoelectron diffraction measurements.

[11 ]Fig.1 shows O 1s core-excited diffraction pattern from BaTiO3 (111) islands. The pattern
was operated by three-fold symmetry operation and mirror-symmetry operation. Distinctive
diffraction rings derived from two-dimensional structures were confirmed in ultra-thin film
pattern. This feature directly indicates that the local structure around O atoms have changed
dramatically by the additional annealing treatment. Fig. 2 shows a local atomic model aroundoxygen constructed from the O1s using holographic transformation. From this result, we clearlyshow that two-dimensional photoelectron holography is a very effective tool for the analysis ofultrathin films.

Fig.1

O 1s core photoelectron diffraction
pattern from BaTiO3 (111) .

O

Fig.2 Local atomic structural model around O.

This work was performed with the approval of the Japan Synchrotron Radiation Institute,
Proposal No. 2014A7462, 2014B7476, 2015A7488, 2017B7554 and 2018A7569, andsupported by Japan Society for the Promotion of Science Grant-in-Aid for Scientific Researchon Innovative Areas: Grant Number 26105001.

REFERENCES[1] Stefan Förster, Klaus Meinel, René Hammer, Martin Trautmann & Wolf Widdra, Nature 502, 215218 (2013).
[2] K. Goto, H. Matsuda, M. Hashimoto, H. Nojiri, C. Sakai, F. Matsui , H. Daimon, L. Tóth, T.
Matsushita, e-J. Surf. Sci. Nanotech. 9, 311-314 (2011).

PHOTOINDUCED DYNAMICS IN Ba3CuSb2O9 WITH Cu 3d ORBITAL
LIQUIDS MEDIATED BY OXYGEN 2p HOLES
Kou Takubo,1,2 Kohei Yamamoto,1 Yujun Zhang,1,3 Yasuyuki Hirata,1 Hiroki Wadati,1,3
Huiyuan Man,1 Satoru. Nakatsuji,1 Takashi Mizokawa4
1

The Institute for Solid State Physics, The University of Tokyo
2
Department of Chemistry, Tokyo Institute of Technology
3
Graduate School of Material Science, University of Hyogo
4
Department of Applied Physics, Waseda University

Quantum spin liquids in Mott insulators usually emerge in geometrically frustrated magnets.
Less commonly, unfrustrated magnets can show a variety of quantum spin liquids if charge
and/or orbital degrees of freedom are involved. Ba3CuSb2O9 harbors orthorhombic phase andhexagonal phase. Jahn-Teller distortion of Cu2+O6 octahedra is absent only in the hexagonal
Ba3CuSb2O9 suggesting a Cu 3d spin-orbital liquid [1,2]. Many previous experiments
indicated that the spin-orbital liquid phase in the hexagonal Ba3CuSb2O9 is associated with adynamical character of fluctuation in the orbital sector. However, the real microscopic originof this behavior was still not clarified in these studies. A puzzle is that the same material
seems to exist in the two different modifications. The orthorhombic phase behaves quite
normally: it shows a cooperative JT distortion, no special dynamic effects, etc. On the other
hand, the hexagonal phase has all these strange features argued above.
In order to clarify the microscopic origin for this different behavior, we measured x-ray
absorption spectroscopy (XAS) and time-resolved resonant soft x-ray scattering (Tr-RSXS) at
BL07LSU of SPring-8 for two phases of Ba3CuSb2O9. Single crystals of Ba3CuSb2O9 weregrown under oxygen atmosphere from the
BaCl2-based flux [2]. The samples were cleaved
along the (001) plane in situ to avoid the surface
contamination. The XAS spectra were recorded bothusing the total electron yield (TEY) and total
fluorescence yield (TFY) modes. The Tr-RSXS
measurement was performed using the pump-probe
technique with a time-resolution of ~50 ps [3]. A
second-harmonic Ti:sapphire laser pulse (hv=3.1 eV,
repetition rate = ~ 1 kHz, width = 50 fs) was adopted
as the pump light, which was introduced from thelaser station of BL07LSU.
Figure 1 shows TFY-XAS spectra at the O K and
Cu L edges of Ba3CuSb2O9. The Cu L3 main peak at
930.2 eV is accompanied by the charge transfer
satellite at about 933 eV. The intensity of thesatellite seems to be rather large in the hexagonalphase compared to the orthorhombic phase. The
main and satellite peaks are assigned to thetransitions of 2p63d9 to 2p53d10 and of 2p63d9L to
2p53d10 [4]. Here L represents a hole on the oxygen
2p orbitals. The Cu 2p XAS indicates that the
ground state of the hexagonal phase includes more Fig. 1 Typical XAS spectra at the (a) O K
and (b) Cu L edges for the hexagonal and3d9L than the orthorhombic phase. On the other hand, orthorhombic phases of Ba3CuSb2O9
the structure at 530 eV observed at O K edge spectra taken in the TFY mode.

of Fig. 1(a) can be assigned to the transition from 1s to 2p which is hybridized into the upper
Hubbard band. The peak at 528.2 eV is observed only in the hexagonal region and can be
assigned to the Zhang-Rice singlet band seen in the hole doped high-Tc cuprates: thetransition from oxygen 1s to unoccupied oxygen 2p (corresponding to “3d9L to 3d9”) [5].
Figure 2 shows the dynamics of
Tr-RSXS at Q=(002) with hv=930.2
eV on the Cu L resonance after thepump pulse at 3.1 eV. A coherent
oscillation with period of ~165 ps(~6 GHz) is observed in the
hexagonal phase while no oscillation
seems to be not observed in theorthorhombic phase. In addition, thex-ray scattering signals probed at
923.0 eV below the absorption edge
do not show appreciable change after
the pump pulse. The observed
coherent oscillation of ~6 GHz israther slow compared to various
optical phonons in the system which
are usually coupled to charge or Fig. 2 Time-revolution of the Tr-RSXS signals at Q=(002) for
orbital orderings. Such a slow the hexagonal and orthorhombic and phases after the
dynamics in the hexagonal phase is pump-pulse of
consistent with the expectations in
the previous reports [6,7].
To summarize, x-ray absorption spectroscopy on Ba3CuSb2O9 reveal that the hexagonalphase with spin and orbital liquid is characterized by emergence of oxygen 2p hole in thehighly insulating state. The coherent oscillation in the pump-probe resonant x-ray scattering
measurement suggests that the spin-charge-orbital fluctuation in the hexagons of Cu sites canbe controlled by optical laser.
REFERENCES[1] S. Nakatsuji et al., Science 336, 559 (2012).
[2] H. Man et al., J. Phys.: Condens. Matter 30, 443002 (2018).
[3] K. Takubo et al., Appl. Phys. Lett. 110, 162401 (2017).
[4] T. Mizokawa et al., Phys. Rev. Lett. 67, 1638 (1991).
[5] C. T. Chen et al., Phys. Rev. Lett. 66, 104 (1991).
[6] Y. Ishiguro et al., Nature Comm. 4, 2022 (2013).
[7] Y. Han et al., Phys. Rev. B 92, 180410(R) (2015).

AMBIENT-PRESSURE XPS STUDIES OF CO2 HYDROGENATION ON
PD-BASED BIMETALLIC ALLOY CATALYSTS
Jiayi Tang1, Takanori Koitaya2, Susumu Yamamoto3, Okkyun Seo4, Kozo Mukai3, Iwao
Mastuda3, Jun Yoshinobu3
1

Departnment of Engineering, University of Hyogo,2 Institute for Molecular Science, 3 The Institute for
Solid State Physics, The University of Tokyo, 4 National Institute for Materials Science

Introduction
Reducing CO2 emission, capturing, storage and utilizing of CO2 as a feedstock tosynthesize useful chemicals not only contribute to solve “greenhouse effect” but also toprovide a great challenge in exploring new methods and opportunities for catalytic andindustrial development. Although various technologies for CO2 reduction has been considered,
the recycling of CO2 by hydrogenation to high-energy-density liquid fuel is considered to be
the most attractive approach in terms of economy, ecology, and safety. In order to further
improve the activity, selectivity and stability of catalyst, the origins of catalytic properties,
transition metal Pd is an excellent candidate to activate hydrogen molecules, leading to acatalyst surface with highly reduced state, which can facilitate the hydrogenation process.
Alloy sites have been considered as the active sites in the CO2 hydrogenation, such as
Pd-based bimetallic catalysts [1-4]. The study of bimetallic surfaces has gained considerable
interest because it is difficult to know the correlations between electronic and chemical
properties of the intermixed bimetallic surfaces and the catalytic reaction. In this study, theambient pressure X-ray photoelectron spectroscopy were carried out to investigate thechanges of electronic properties as well as the reaction mechanisms during the CO2hydrogenation over Pd-based alloy catalysts.
Experimental
The fcc- and B2- phased PdCu40 and PdAg23 alloys (ϕ12.4 mm, 0.1 mm thickness) wereobtained from Tanaka Kikinzoku Kogyo K.K.. The bulk compositions were 60 wt% Pd and
40 wt% Cu for PdCu alloy, and 77 wt% Pd and 23 wt% Ag for PdAg alloy. The clean surfaceof PdCu and PdAg alloys are prepared by several cycles of Ar+ sputtering (0.5 keV) for 15
min and annealing at 670-675 K for 2 min. The CO2 hydrogenation were observed under H2and CO2 atmosphere by using ambient pressure X-ray photoelectron spectroscopy systemwith base pressure of ~5.0 ×10-10 Torr at BL07LSU, SPring-8. The temperature dependence
experiments were performed under H2 pressure of 0.4 mbar and CO2 pressure of 1.6 mbar at
300 K, 310 K and 320 K. The gas pressure dependence was performed under H2 pressure of
0.4 mbar and CO2 pressure of 0.8, 1.6 and 2.4 mbar at 300 K. The C 1s, O 1s, Pd 3d and Ag
3d core-level were measured at photon energy of 680 eV, and Cu 2p spectra were measured at
photon energy of 1200 eV.
Results and Discussion
C 1s spectra measured on the clean surface and under H2 pressure of 0.4 mbar and CO2
pressure of 1.6 mbar at 300 K on the PdCu(fcc) alloy are shown in Fig.1. The graphene-likecarbon C-C(sp2) located at binding energy (BE) of 284.4 eV and CO contaminations whose
BE is 286.2 eV still remained on the clean surface even after several cycles of Ar+ sputteringand annealing. The component located at BE of 287.5 eV appeared after flowing H2 and CO2,
which can be assigned as the intermediate species formate (H2+2CO2→2HCOO) during theCO2 hydrogenation. The amount of formate slightly increased with exposure time. Figure 2
shows the C 1s spectra observed on the clean surface and during CO2 hydrogenation process
under the H2 pressure of 0.4 mbar and CO2 pressure of 1.6 mbar at 300 K on the PdCu(B2)surface. It shows the same reaction behavior on the B2-phased PdCu surfaces as that onfcc-phased PdCu surfaces, where the formate
slightly increased with exposure time. The
amount of formate formed on the surface after
exposure time of 82 min are almost same between
fcc phase and B2 phase. It indicates that theactivity of CO2 hydrogenation is relatively high toother catalyst, such as ZnCu alloy. The
phase-structure has little effect on CO2hydrogenation activity on the PdCu alloys. Figure
3 shows the C 1s spectra measured on the cleansurface and under H2 atmosphere of 0.4 mbar andCO2 atmosphere of 1.6 mbar at 300 K on the
PdAg alloy. After H2 and CO2 exposure, theformate was formed on the surface, and increased
with time. Besides, the additional adsorption
species C=O species, CHx and C-C(sp3) appearedon the surface, whose BEs are 287.2 eV, 285.0 eV Fig.1 C 1s spectra measured on the clean
and 284.2 eV, respectively. We observed that CO surface and after H2 exposure at 0.4 mbar andCO2 at 1.6 mbar at 300 K on PdCu(fcc) alloy.
formation is largely suppressed, however, formate
formation activity is almost same on the PdAg
alloy compared to the reaction on the PdCu alloys.
The temperature dependence of the formate
formation on PdCu(fcc) alloy is shown in Fig.4.
The formate formation rate is higher at lower
temperature, which indicates the desorption offormate is occurring during the CO2hydrogenation by increasing the temperature.
REFERENCES[1] X. Jang et al. Catalysis Communications 118 (2019)10.
[2] X. Nie et al. ACS Catal. 2018, 8, 4873.
[3] S. Bai et al. J. Am. Chem. Soc. 2017, 139, 20.
[4] K. Mori et al. J. Am. Chem. Soc. 2018, 140, 8902.

Fig.4 Temperature dependence of the formation of
HCOO with exposure time on the PdCu(fcc) alloy.

Fig.2 C 1s spectra mesured on the cleansurface and under H2 exposure at 0.4 mbar andCO2 at 1.6 mbar at 300 K on PdCu(B2) alloy.

Fig.3 C 1s spectra measured on the cleansurface and under H2 exposure at 0.4 mbar andCO2 at 1.6 mbar at 300 K on PdAg alloy.

SOFT X-RAY SPECTROSCOPY OF SOLUTIONS WITH CARBON
NANOTUBES TREATED VIA PLASMA IN LIQUID
1

Kazuo Terashima1,2, Noritaka Skakibara1,2, Kenichi Inoue1, Shion Takahashi1, Taku Goto1,2,
Tsuyohito Ito1, Keishi Akada2,3, Jun Miyawaki1,3, Yukiya Hakuta2, Yoshihisa Harada1,2,3
Department of Advanced Materials Science, Graduate School of Frontier Sciences, The University ofTokyo
2AIST-UTokyo Advanced Operando-Measurement Technology Open Innovation Laboratory(OPERANDO-OIL), National Institute of Advanced Industrial Science and Technology (AIST)
3

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

Flexible and wearable electronic devices have attracted much attentions. Substrates for suchdevices are often required to have both flexibility and functional properties. Composite
materials, which consists of inorganic fine particles providing functionalities and polymer
materials providing flexibility, are promising candidates for the substrate. However, ingeneral, the flexibility is greatly reduced as the mixing ratio of inorganic fine particles
increases to reach desired functionalities.
To overcome such trade-off, we treated inorganic fine particles by plasma in solution. The
resulted composite materials, such as plasma-treated BN particles with slide-ring materials,
showed improved toughness and high thermal conductivity comparing with the composite
with non-treated BN particles [1]. One of the reasons for the good performance might be highdispersibility of inorganic particles in the composites [2]. The high dispersibility should beachieved with well-dispersed suspension during the process.
Carbon nanotubes are well known to have high thermal and electrical conductivities andthus applied for high thermal and electrical conductive composites. It is also known thatplasma treatment in solution could improve dispersibility in solution, thus might results inbetter performances in the composites made e.g. with solution. In this study, by soft X-ray
emission spectroscopy (XES) we are analysing the solutions with carbon nanotubes treated byplasma in liquid to reveal how the plasma treatment could improve the dispersibility.
Soft X-ray emission spectroscopy (XES) of the solutions were performed at SPring-8
synchrotron radiation facility using the BL07LSU HORNET station. In the station, theoriginal system for performing XES of liquid with a very thin (150 nm) Si3N4 window isequipped. Thus we have applied this spectroscopic system for investigating our solution with
CNTs. The probe was O 1s, which should represent the structure of H2O molecules in the
CNT-dispersed solution. The first purpose of this study is to detect changes of water structureassociated with CNTs.
The soft X-ray emission spectrum of
the hydrophilized CNTs aqueous solution
was compared with the untreated CNTs
dispersion. The hydrophilization was
performed with plasma in solution
containing 4 wt% hydroquinone and 0.5
wt% multi-walled CNTs (average
diameter and length were 20-30 nm and
0.5-2 μm) in 70 ml deionized water Fig. 1 Schematic of plasma treatment of CNTs insolution.
(Fig. 1). The treatment time was 30 min.

After the treatment, theremaining hydroquinone wererinsed away with ethanol anddried CNTs were re-dispersed
in deionized water to form thesolution investigated.
The XES spectra of purewater,
solutionswithplasma-treated CNTs, andsolutions with raw CNTs wererecorded and compared. CNT
concentrations were modified
from 0.1 to 1 w% forplasma-treated one. Since the Fig. 2 Schematic diagram of XES applied in this study.
untreated CNTs could not be
dispersed in water well, the concentration of raw CNTs were essentially unknown although
we modified the initial concentration at 0.5 w% .
The spectra of pure water and the solution with raw CNTs were almost identical, while thespectra with plasma-treated CNTs with concentration higher than 0.5 w% clearly showed
different feature around 3a1 region at 521–525 eV, indicating the modification of hydrogenbonding network by plasma-treated CNTs. While the mechanism for the spectral change stillneed to be revealed with theoretical study, the difference could be seen only with highconcentration of CNTs, which could not be achieved with untreated CNTs.
REFERENCES[1] T. Goto, M. Iida, H. Tan, C. Liu, K. Mayumi, R. Maeda, K. Kitahara, K. Hatakeyama, T. Ito, Y.
Shimizu, H. Yokoyama, K. Kimura, K. Ito, Y. Hakuta, K. Terashima, Appl. Phys. Lett. 112,101901 (2018).
[2] T. Goto, M. Iida, H. Tan, C. Liu, K. Mayumi, R. Maeda, K. Kitahara, K. Hatakeyama, T. Ito, Y.
Shimizu, H. Yokoyama, K. Kimura, K. Ito, Y. Hakuta, K. Terashima, J. Japan Inst. Met. Mater.
82, 403-407 (2018). [in Japanese]Photoelectron spectral imaging of p+-WSe2/n-MoS2 hetero junctions for
TFET device structures
Naoka Nagamura1,2, Keigo Nakamura3, 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

The significant increase of data traffic causes the increase of energy consumption, so powersaving of electronic devices is an urgent issue. Recently, two-dimensional tunnel field effecttransistors (2D TFETs) have drawn attention as the low power switching FETs which canrealize the low subthreshold swing compared to conventional MOSFETs. TFET devices need
the p+/n heterojunction with large band-discontinuity. Here we have found that p+-WSe2
doped chemically by WOx can be stabilized in air by transferring it on h-BN, then we
succeeded in observing TFET characteristics in the p+-WSe2/MoS2(4 ML) heterostructure [1].
In a usual p/n heterojunction of semiconductors, a depletion
layer is generated and continuous band bending of
micro-meter order occurs at the interface. However, ourphotoluminescence spectroscopy measurements suggest thatthe charge transfer at the p+-WSe2/MoS2 heterojunction forms
valence band offset of the MoS2 sheet and band-discontinuity
at the interface.
1 Schematic image of aIn this study, we have attempt to characterise the local Fig.
+p
-WSe
2/MoS2 TFET device
charge transfer at interfaces in the p+-WSe2/MoS2 TFET
structuredevice structure as shown in Fig. 1 by scanningphotoelectron emission microscopic (SPEM)
measurements using a three-dimensional
nanoscale electron-spectroscopy chemical
analysis (3D nano-ESCA) system installed at
BL07LSU of SPring-8.
2D materials used in the sample arefabricated by exfoliation of bulk crystals. WOx
side of WSe2 film is transferred onto h-BN ona SiO2/n+-Si substrate after forming
WSe2/WOx by O3 annealing. In this process, Fig. 2 (a) O 1s core level photoelectron intensity
p+-WSe2 is stabilized. Then, MoS2 with mapping of the device structure. (b) Opticalimage of the device structure.
suitable thickness (1-5 ML) is chosen by its
optical contrast and the p+-WSe2/MoS2 heterostructures
were fabricated via dry transfer method using PDMS under
the alignment system. Ni/Au was deposited as source/drainelectrodes by EB lithography.
Fig. 2 shows a photoelectron intensity mapping of the
p+-WSe2/MoS2 TFET sample. Comparing to the opticalimage, the device structure can be clary identified. Mo 3dand S 2s core level photoelectron spectra were measured
locally at two different regions; MoS2 on h-BN and
MoS2 on p+-WSe2. Peak shift which is caused by charge Fig. 3 (a) Mo 3d and S 2s coretransfer between MoS2 and p+-WSe2 was not detected as level spectra of pin-point analysis.
Measurement points are presented
shown in Fig. 3.
Background signal owing to the 0th order diffracted in Fig. 2(a) as circles.

beam of the focusing optics, the Fresnel zone plate, prevent to detect such a small peak shift.
So, we have induced a new focusing optical stage with a center beam stop and dual Fresnel
zone plates for wide-energy-range experiments. The significant improvement in special
resolution and signal-to-noise ratio will be expected in the measurement in 2019.
We are also working on the feasible study of thepractical use of machine learning technique for thedata analysis of spectral imaging obtained by 3D
nano-ESCA. To reduce computational cost for
high-throughput analysis in multi-parametermeasurements in 3D nano-ESCA, compared withprevious studies using a Bayesian peak separation
with the exchange Monte Carlo method [2], weused an expectation-maximization (EM) algorithmand confirmed drastic acceleration of peak fitting in Fig. 4 Schematic image (left) and photocomparison to the manual approach [3]. Now we (right) of the new optical stage including
aim for applying this method to on-site data Fresnel zone plates.
analysis during experiments.
REFERENCES[1] J. He et al. Adv. Electron. Mater. 4, 1800207 (2018).
[2] K. Nagata et al., Neural Networks 3, 82 (2012).
[3] T. Matsumura et al. Sci. Technol. Adv. Mater. (2019) in press.
(DOI: 10.1080/14686996.2019.1620123)

TIME-RESOLVED PHOTOELECTRON HOLOGRAPHY STUDY OF
SILICENE
Kouichi Hayashi1,2, Artoni Kevin R. Ang1, Koji Kimura1, Yuichiro Fukatsu1, Akito Asano1,
Takahiro Tsukada1, Yohei Okuda3, Takahiro Yonezawa4, Antoine Fleurence4, Yukiko
Yamada-Takamura4
1

Structural Physics Laboratory, Nagoya Institute of Technology, Nagoya, Japan
Frontier Research Institute for Materials Science, Nagoya Institute of Technology, Nagoya, Japan
3
Okayama University, Okayama, 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
Dirac fermions, similar to its carbon based counterpart in graphene. The development ofsilicene based devices have 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 electronicinteractions with the substrate. It was recently demonstrated that silicene with minimal
substrate interactions can be grown on a ZrB2/Si(111) substrate through the surface
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
have shown 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 different local structures around these Si
atoms using photoelectron holography (PEH). Furthermore, dynamic investigations will bedone using time-resolved PEH.
[Experiment]
To do this, the silicene samples were grown in-situ in the angle-resolved time-of-flight(ARTOF) end station of BL-07. A ZrB2/Si(111) substrate was prepared elsewhere and wasdegassed after introduction into the UHV chamber. This was followed by annealing at 800°C,
where the silicene layer forms spontaneously through the segregation of the Si atoms from thesubstrate [1]. The silicene surface was evaluated using LEED and core level photoemission.
For the photoemission experiments, the photon energy used was hν = 300 eV. The angular
distribution and the energy of the photoelectrons were recorded using the ARTOF detector. In
this experiment, the acceptance angle was ±13°. To obtain a larger section of the hologram
pattern, the sample’s orientation with respect to the detector was varied.
In the time-resolved experiments, a 1.5 eV, 208 kHz
laser was used as the pump and 300 eV X-rays from thesynchrotron was used as the probe.
[Results]
The LEED and Si 2p spectra confirmed the formation
a well-defined silicene layer. The Si 2p spectra (Fig.
1b), show two peak components resolved by peakfitting, similar to 2 of the 3 reported peak fitting
components [1,3]. In our preliminary PEH experiments
in a high resolution hemispherical analyser in KEK-PF,
the same sample preparation was used and resulted inwell resolved components in the Si 2p spectra, identicalto those reported [1,3].

Fig. 1 a) Structural model of epitaxial
silicene, b) Si 2p peak fitting
components, and simulated andexperimental holograms from the c) SiA
and d) SiB peak components.

These peak components, α and β, correspond to the Si atoms in the A and B positions, as
noted in Fig. 1a. Using the difference in the peak positions of these components, the hologram
patterns for each component was extracted by pixel-by-pixel peak fitting of the 3D data I(kx,ky, E). Figure 1c and 1d shows the simulated and experimental holograms from the Si A andSiB atoms. The prominent features of these patterns are the diffraction rings (formed byscatterers located in-plane) and the forward focusing peaks (FFP) (formed by scaterrers
located out-of-plane, which in this case is the Si C atom). In particular, the FFP from the Si B
atom (θ ≈ 48°) is well reproduced.
For
thedynamic
measurements,
thetime-resolved XPS spectrain Fig. 2a) show a small
shifts (~10 meV) in the
peak
position
after
irradiation with the pumplaser. The FFPs (Fig. 2b)from the time-resolved SiB
hologram patterns (Fig. 2c- 2f) also show small shifts
towards higher polar angle
positions, this indicatesthat there is a small
decrease in the height of
the SiC atom from the Fig. 2 a) Time-resolved Si 2p spectra, b) polar line profiles along the FFP
from the time-resolved PEH patterns, and the SiB time-resolved holograms
SiA-SiB plane.
at delay times of: c) 0.1 ns, d) 1.0 ns,
The time-resolved Si 2p e) 10 ns and f) 100ns.
spectra indicate that there
are no significant changes in the chemical environment of the Si atoms in the silicene layer,
while the FFPs suggest small shifts in the height of the SiC atom. These structural changes
could be due to localized heating from the laser, causing expansion of the layer.
High-temperature STM work done in JAIST shows modulation of Si C atom height, which
seems to agree with the changes in the FFPs.
However, in the time-scales investigated here, the system was not observed to relax back toequilibrium, and further experiments with longer time scales are necessary (~ms scale).
Furthermore, a detailed understanding of the structural changes occurring upon laser
irradiation is still needed.
[Results]
In this study, site-selective photoelectron holography was performed on silicene. The FFPs
from the simulation are well reproduced in the experiment. Time-resolved photoemission
experiments show small shifts in the SiC position, possibly as a result of localized heating by
the laser. Additional experiments with longer time-scales will be performed to fully
understand what happens to the buckled Silicene layer upon heating. Such information will bevital for future silicene based devices.

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] R. Friedlein, et al., J. Chem. Phys. 140, 184704 (2014).

SOFT X-RAY INELASTIC SCATTERING AND DIFFRACTIONTECHNIQUES APPLIED FOR THE STUDY ON THE ROLE OF
WATER AND HYDRATION IN BIOPOLYMERS, POLYSACCHARIDESAND ELECTROLYTES
Jun Miyawaki, Kosuke Yamazoe, Yitao Cui and Yoshihisa Harada*
Synchrotron Radiation Laboratory, The Institute for Solid State Physics, The University of Tokyo

With the rapid increase in its energy resolving power, RIXS has become one of the
cutting-edge tools to investigate the electronic structure of materials. We have developed our
RIXS station to accept various sample circumstances and seamless spectrometer rotation for
momentum dependence experiments. The aim of this project is to provide abundant potential
use of RIXS/XES and its combination with soft X-ray diffraction. Here we show three typesof outcomes obtained in this project in 2018.
1) Folding induced dehydration observed in PNIPAM
We have probed the electronic structure of poly(N-isopropylacrylamide) solution (PNIPAM,
inset in Fig. 1), a stimuli-sensitive polymer that undergoes clouding (around 33.7 ℃) uponheating and is regarded as a simple model of proteins, using soft X-ray absorption/emission
spectroscopy. Figure 1(a) shows the temperature dependence of the O K-edge X-ray
absorption spectra of PNIPAM
aqueous solution. The peak around532 eV is assigned as C=O doublebond but has at least twocomponents, a 531.5 eV peakfavoured at 30 ℃ and a 532.4 eV
peak favoured at 36 ℃. Figs. 1 (b)and (c) show O K-edge RIXS
spectra at the 531.5 eV and 532.4 eV
resonances. In addition to two peaks
from the C=O σ and π components,
one more peak appears at the 532.4
eV resonance. The new peak would
be related to the formation ofhydrogen bonds between watermolecules and PNIPAMs, thus
Figure 1. (a) Temperature dependence of soft X-ray
absorption spectrum of PNIPAM aqueous solution. (b)(c)reflecting the dehydration process
Temperature dependence of the resonant soft x-ray
through the precise temperatureemission spectrum, respectively. The hydrogen bonddependent RIXS.
between water and PNIPAM is shown above panel (c).
2) Li-H2O interaction observed in hydrate melt electrolytes
The structure of a lithium salt hydrate melt (LSHM) was investigated by O K-edge X-ray
absorption and RIXS spectroscopy. Hydrate melt is a metal salt that forms liquid by additionof only a small amount of water. How exactly H2O bonds with other anions/cations to form aliquid and how Li behaves in the hydrate melt are still open questions. Figure 2 shows O
K-edge RIXS spectrum of liquid LSHM with the chemical formula Li(TFSI)0.7(BETI)0.3･
2H2O which has high performance as an aqueous electrolyte for Li-ion batteries. In order toextract actual local structural information of water in the LSHM from the experimental RIXS
spectra, density-functional theory based molecular dynamics
simulations were carried out and the RIXS spectra were
calculated for the ensemble of clusters extracted from themolecular dynamics simulations coupled with non-resonant
semiclassical Kramers-Heisenberg formalism. Elementary
structures were identified through the comparison between
the experimental and theoretical spectra, which clearly
distinguished interactions between Li-H2O, Li-anion, andH2O-anion present in the hydrate melt. It is found that Li is
not necessarily moving jointly with other water molecules in
Figure 2. O K-edge RIXS spectra the hydrate melt but is rather breaking old and forming newof pure H2O water and hydrate
Li-O bonds as it is progressing through the hydrate melt.
melt Li(TFSI) (BETI) ·2H O.
0.7

0.3

2

3) Resonant Inelastic Soft X-ray Diffraction of La1/3Sr2/3FeO3Scattered X-ray is separated into elastic and inelastic scattering according to the amountof energy loss. The momentum dependence of elastic scattering, which has no energy loss,
gives information about structural order by X-ray diffraction, and that of inelastic scattering,which has energy loss, gives the energy-momentum dispersion of elementary excitations,such as orbiton, magnon, and phonon. Here, at a particular momentum q which satisfies the
Bragg condition and where X-ray diffraction peak (enhanced intensity of elastic scattering) isobserved, the intensity of the inelastic
scattering is expected to be enhanced
assisted by the X-ray diffraction: “Inelastic
X-ray diffraction” is expected to be observed.
Thus, in order to demonstrate “inelastic
X-ray diffraction”, we have investigated theq dependence of the intensity of the inelastic
scattering of La1/3Sr2/3FeO3 around its
magnetic diffraction.
Figure 3 shows the results of resonant
soft X-ray diffraction of
Figure 3. Resonant elastic X-ray scattering (red) inelastic
and resonant inelastic X-ray diffraction (blue) La1/3Sr2/3FeO3
measuredat
100 K
scans measured at Fe L-edge RIXS. The
integration range for the inelastic scattering is (magnetically ordered) and 210 K (nomagnetic order). A peak at q≈0.472 Å−1 ofindicated by two green lines in the right figure.
elastic scattering at 100 K is ascribed to themagnetic diffraction. However, an extra peak was observed at q≈0.447 Å−1 for inelastic
scattering. At 210 K, the peak at q≈0.447 Å−1 disappeared accompanied by the disappearanceof magnetic diffraction peak. These results strongly suggest that the observed extra peak at
q≈0.447 Å−1 is the enhancement of the inelastic scattering assisted by X-ray diffraction, andthus inelastic X-ray diffraction was successfully demonstrated.

OPERANDO OBSERVATION OF BURIED INTERFACES OF DEVICESBY USING SOFT X-RAY PHOTOELECTRON MICROSCOPY IN
COMBINATION WITH CHARGE NEUTRAL CONDITION
Hirokazu Fukidome, Naoka Nagamura2, Masaharu Oshima3
1

Research Institute of Electrical Communication, Tohoku University
2

NIMS

3

ISSP, The University of Tokyo

The scanning-type soft x-ray photoelectron spectromicroscopy, so-called, 3D nano-ESCA is
the powerful tool in elucidating chemical and electronic natures of surfaces with thehigh-spatial resolution. To be noted, we have newly developed created operando observation
system, i.e. the system that enables to observe surface electronic and chemical states ofdevices in operation. In 2018-2019, we have published the articles about device observation
using 3D nano-ESCA, for Graphene FETs [1, 2] and GaN-HEMT [3] by using 3D
nano-ESCA.
These works have been high evaluated in terms of both basic research to elucidate materials
functioning and applied device researches. As a consequence, in addition to paper
publications, we have applied for more than 10 patents, as registered in SPring-8 researchrecord, with companies (Sumitomo Electric Industreies, and ShinEtsu Chemicals) which has
collaborated with us about device operation mechanistic study with the aid of 3D nano-ESCA,
in the industry-government-academia collaboration projects in recent years, such as SCOPE
of Ministry of internal affairs and communication, NEDO of Ministry of Economy, Trade andIndustry, and Grant-in-Aid for Specially Promoted Research of MEXT.
However, soft x-ray photoelectron nanospectroscopy has the limitation that it is hard toprobe buried interfaces because of electron escape depth of several nanometers. We should
surmount this limitation for devices or functionalized materials that has complicatedstructures to deliver maximal performances.
In this study, we tackle to solve the above-mentioned issue by using the charge-neutrality
condition, in combination with precise advanced device technologies.
The charge-neutrality condition is the key in this study for solving the issue. By using thiscondition with the aid of the advanced device technologies, the soft x-ray photoelectron
nanospectroscopy can probe electronic states of both buried interfaces and surfaces (Figure 1).
The model for this is GaN-HEMT [3].
The operation of GaN-HEMT isdegraded by the so-called current
collapse phenomena. The main cause of
the current collapse phenomena is
electron trapping at surface traps. In the
direction perpendicular to the surface,
the charge neutrality condition holds, i.e.
the sum of charges is zero at every point.
Then, if there are negligible density ofcharges except surface trapped electron
and carrier density in the channel, thecarrier density in the channel, which is
the key factor in device performances, isdetermined by measuring surface
trapped electron density by using
Figure 1 Schematics of this study

operando photoelectron nano spectroscopy with 3D nano-ESCA [3].
However, in real devices, the charge trapping except at the surface and in the channel
cannot be ignored. For instance, the buffer layer between GaN and SiC substrates for
GaN-HEMT contains trapped electrons. This causes so-called virtual back-gate effects whichleads to long-term instabilities of GaN-HEMT. Coversely, controlling enough the buriedinterface electrically enables us to probe the electronic states of buried interface by using soft
x-ray photoelectron spectroscopy.
In the experiments in 2019 B, we have tried to establish the method to probe buriedinterfaces by using soft x-ray. For that purpose, we used GaN-HEMT with an additional sidegate as a model sample, in collaboration with Sumitomo Electric Industries. A typical result isshown in Figure 2.
The application of the side-gate bias
withdraws mobile (trapped charges) inthe buffer layer and leaves opposite
charges of trapping sites. As shown in
the previous report [3], the blue-shift ofthe spectral centroid of the surface GaN
indicates the electron trapping at thesurface. However, as shown in Fig. 1,
we predicted that the surface trappedelectron decrease by leaving negative
charges(trappingsites)
withwithdrawing mobile holes due to theside-gate application. We speculates thecause to the temporal change of surface
condition irrelevant to the side-gate bias
application that overwhelms the shift
due to the side-gate bias application.
Unfortunately, however, we cannot haveenough evidence to support our
speculation.
Figure 2 A typical result of this study.
REFERENCES[1] N. Nagamura, H. Fukidome, K. Nagashio, K. Horiba, T. Ide, K. Funakubo, K. Tashima, A.
Toriumi, M. Suemitsu, and M. Oshima, Influence of interface dipole layers on the performance ofgraphene field effect transistors, Carbon, 152 (2019) 680-687. Impact factor：7.466[2] H. Fukidome, K. Funakubo, N. Nagamura, K. Horiba, Y. Tateno, M. Oshima, and M. Suemitsu,
Modulation of Electronic States near Electrodes in Graphene Transistors Observed by Operando
Photoelectron Nanospectroscopy, Sensors and Materials, (2019), accepted.
[3] K. Omika, Y. Tateno, T. Kouchi, T. Komatani, S. Yaegassi, K. Yui, K. Nakata, N. Nagamura, M.
Kotsugi, K. Horiba, M. Oshima, M. Suemitsu, and H. Fukidome, Operation Mechanism ofGaN-based Transistors Elucidated by Element-Specific X-ray Nanospectroscopy, Scientific
Reports, 8 (2018) 13268.

REVEALING THE MECHANISM OF THE METAL-INSULATOR
TRANSITION IN VO2 PROBED BY MOMENTUM RESOLVED RIXS
H. Fujiwara1, T. Kashiuchi1, K. Nishimoto1, N. Nishikawa1, A. Irizawa2,
J. Miyawaki3,4, and Y. Harada3,4
1

Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka 560-8531, Japan
Institute of Scientific & Industrial Research, Osaka University, Ibaraki, Osaka 567-0047, Japan
3
Institute for Solid State Physics (ISSP), The University of Tokyo, Japan
4
Synchrotron Radiation Research Organization, The University of Tokyo, Japan
2

Introduction
VO2 is a text book compound which shows the first-order metal-insulator transition (MIT) at
340 K [1]. Since the transition temperature is controlled by applying the voltage, this material
is a good candidate for the application of a resistive memory in the new field of the oxide
electronics [2]. Since the crystal structure changes across the transition accompanied byforming the V dimer along the c-axis in the insulating phase, Peierls transition is one of thepossibilities to explain the MIT. On the other hand, the Mott transition due to the strong electron
correlation in the V 3d states has also been discussed. Moreover, the recent study using thephotoemission and X-ray absorption spectroscopy pointed out the orbital-assisted MIT scenario,
in which the electron correlation is enhanced by forming the one-dimensional chain due to thec-axis dimer [3,4]. Therefore, the mechanism of the MIT has been under debate. To reveal themechanism of the MIT in VO2, we have performed the resonant inelastic soft-x-ray scattering
at V 2p edge to probe the excitation from the occupied to the unoccupied 3d states (ddexcitation) by changing the scattering angle parallel to the c axis.
Experimental
RIXS measurements were performed at the SPring-8 BL07LSU HORNET end-station, wherea high-resolution soft x-ray spectrometer are installed [5]. Total energy resolution was set toabout 150 meV at V 2p edge, and the experimental temperature was set to 300 K and 390 K to
study the dd-exciation in the insulating and insulating phases, respectively. Moreover, themomentum resolved measurements were performed at 300 K. The single crystalline VO2 was
fractured in situ to expose the clean (001) surface. For the momentum resolved measurements,
the geometry of the spectrometer is set to keep the scattering vector parallel to the (001)
direction.

Figure 1 shows V 2p XAS spectra obtainedby the horizontally and vertically polarized
photons with electric field vector E paralleland perpendicular to the crystal c axis at 300
K in the insulating phase. The spectra show
the clear linear polarization dependence,
which is consistent with previous report [3].
This dichroism suggests the anisotropic
charge distribution of the V 3d states due tothe dimer formation of the V ions along theIntensity (a.u)

Results and discussions

VO2
T = 300 K (insulating)
V 2p XAS
µE⊥c
µE//c
A

512

514

516

518

Photon Energy (eV)520

Figure 1V 2p XAS spectra of VO2 recorded with E //cand E ⊥c configurations.

crystal c-axis. Especially the shoulder structure atincoming photon energy (hnin) of 515.4 eV (A)clearly shows the linear dichroism, suggesting thatthe dd-excitations in the RIXS spectra obtained at A
reflect the electronic structure along the V dimer,
since the intermediate states in the optical process of
RIXS is same as XAS.
Figure 2 shows V 2p RIXS at hnin = 515.4 eV
obtained with a scattering angle 2q of 90° and 120°.
The dd-excitations around 0.5 eV in the energy loss
scale are observed in both 90° and 120° spectra.
Interestingly, the additional low energy shoulder isshown up around 0.3 eV in the 90° spectra,
suggesting the existence of the low-energy excitation
dispersing along the c axis dimer. To reveal the originof the MIT, the detailed momentum resolved
measurements are required in both insulating andmetallic phases.

Figure 2
Scattering angle dependence of the V 2p
RIXS spectra obtained at hvin = 515.4
eV.

Summary
We have performed the momentum resolved RIXS of VO2. By tuning the incoming photon
energy at the low energy sholder of the V 2p XAS, we have successfully observed the ddexcitation in the insulating phase. The RIXS spectra show the scattering angle depdence at thelower loss-energy side of the peak structure due to the dd-excitation due to the low energy
excitations dispersing along the c-axis dimer.
REFERENCES[1] F. J. Morin, Phys. Rev. Lett., 3,34, (1959).
[2] M. Nakano, K. Shibuya, D. Okuyama, T. Hatano, S. Ono, M. Kawasaki, Y. Iwasa and Y. Tokura,
Nature 487, 459 (2012).
[3] M. W. Haverkort, Z. Hu, A. Tanaka, W. Reichelt, S. V .Streltsov, M. A. Korotin, V . I. Anisimov,
H. H. Hsieh, H.-J. Lin, C. T. Chen, D. I. Khomskii, and L. H. Tjeng, Phys. Rev. Lett. 95, 196404(2005).
[4] T. C. Koethe, Z. Hu, M. W. Haverkort, C. Schler-Langeheine, F. Venturini, N. B. Brookes, O.
Tjernberg, W. Reichelt, H. H. Hsieh, H.-J. Lin, C. T. Chen, and L.H. Tjeng, Phys. Rev. Lett. 97,
116402 (2006).
[5] Y. Harada, M. Kobayashi, H. Niwa, Y. Senba, H. Ohashi, T. Tokushima, Y. Horikawa, S. Shin,and M. Oshima, Rev. of Sci. Instrum. 83, 013116 (2012).

ELECTRONIC-STRUCTURE MAPPING OF ELECTRODE
MATERIALS BY OPERANDO PHOTOELECTRON
SPECTROMICROSCOPY USING AN ALL-SOLID-STATE
LI-ION-BATTERY
Eiji Hosonoa,b, Takaaki Sudayamaa, Daisuke Asakuraa,a, Keishi Akadac,c, Naoka Nagamurac,
Koji Horibae, Masaharu Oshimaf, Yoshihisa Haradab, b, faNational Institute of Advanced Industrial Science and Technology (AIST), Research Institutefor Energy Conservation, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japanb
AIST-UTokyo Advanced Operando-Measurement Technology Open Innovation Laboratory,
5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8565, Japanc
Institute for Solid State Physics (ISSP), The University of Tokyo,
5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8581, Japand
National Institute for Materials Science, Research Center for Advanced Measurement andCharacterization, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japane
Photon Factory, Institute of Materials Structure Science, High Energy Accelerator Research
Organization (KEK), 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japanf
Synchrotron Radiation Research Organization, The University of Tokyo, 7-3-1
Hongo, Bunkyo-ku, Tokyo, 113-8656, Japan

Li-ion-batteries (LIBs) play an important role as an energy storage of electric vehicles toestablish a sustainable society. Understanding the charge-discharge mechanism of LIBs isessential to improve their properties, such as high energy density, long cycle life, safety
operation, and thermal stability. Many operando researches have been done on the electronic
properties of the constituent elements using, e.g. hard X-ray absorption spectroscopy, while
the character of the most functional 3d electrons in LIBs have not yet been fully understood.
Recently Asakura and Hosono et al. have successfully introduced operando soft X-ray
absorption / emission spectroscopy as a novel method to understand the functional metal 3d
states during Li-intercalation/deintercalation process in LIB cathode materials [1].
However, the X-ray beam size is usually larger than the particle size of active materials in
LIB electrodes. Most of the synchrotron X-ray spectroscopy techniques provide averaged
information of electrodes within several-tens or several-hundreds m scale limited by the
X-ray beam size. A spectromicroscopy with a high spatial resolution is of particularimportance because that method can detect changes of various electronic structures oftransition metals and oxygen, which are correlated closely with the
Li-intercalation/deintercalation, in a single particle. The local information of the materials is
important for further understanding the charge/discharge mechanisms.
We propose the use of operando photoelectron spectromicroscopy to add spatially resolved
information to the electronic structure of single-crystal LIB cathode materials. We developed
an all-solid-state LIB cell that can operate under ultra-high vacuum condition and obtained
operando photoelectron spectra of LiCoO2 cathode at charge/discharge states using
3DnanoESCA station at BL07LSU of SPring-8 [2].
Figure 1 shows the Co 3p and Li 1s spectra of single crystalline LiCoO2 in the all-solid-state
LIB cell for operando measurement. The peak position of each Co 3p spectrum from initial,
charge, and discharge condition is not changed. It is considered that strong hybridization
between Co 3d and O 2p orbitals causes the charge-polarization at the oxygen side. We shouldmeasure the narrow scan of O 2p for detail discussion. However, oxygen components in thedry polymer as solid-state electrolyte is overlapped with the oxygen atom in LiCoO2. We will
create oxygen free measurement system.

Co 3pkCounts [a.u.] (au)Intensity

20

15'Li_1s0027(#4)''Li_1s0034(#4)''Li_1s0048(#4)'

discharge

10

charge5initial
80

75

Li 1s

70

65

60

55

50

Binding Energy / eV)
Figure 1 Co 3p and Li 1s spectra of LiCoO2.

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. A part ofthis work was supported by TIA collaborative research program “Kakehashi”
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] 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).

VALENCE-CHANGE MAPPING OF TRANSITION METALS BY
OPERANDO PHOTOELECTRON SPECTROMICROSCOPY USING AN
ALL-SOLID-STATE LI-ION-BATTERY
Eiji Hosonoa,b, Takaaki Sudayamaa, Daisuke Asakuraa,a, Keishi Akadac,c, Naoka Nagamurac,
Koji Horibae, Masaharu Oshimaf, Yoshihisa Haradab, b, faNational Institute of Advanced Industrial Science and Technology (AIST), Research Institutefor Energy Conservation, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japanb
AIST-UTokyo Advanced Operando-Measurement Technology Open Innovation Laboratory,
5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8565, Japanc
Institute for Solid State Physics (ISSP), The University of Tokyo,
5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8581, Japand
National Institute for Materials Science, Research Center for Advanced Measurement andCharacterization, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japane
Photon Factory, Institute of Materials Structure Science, High Energy Accelerator Research
Organization (KEK), 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japanf
Synchrotron Radiation Research Organization, The University of Tokyo, 7-3-1
Hongo, Bunkyo-ku, Tokyo, 113-8656, JapanFor realizing a sustainable low-carbon society, development of high-performance lithium-ion
battery (LIB) for electric vehicles is one of the most important research subjects. In thepresence of trade-offs among the performances such as energy density, power performance,
cycle performance, and safety property, it is difficult to fulfill all the requirements for those
properties. Comprehensive researches and developments are highly important because the
combination of the materials among the cathode, anode, and electrolyte is crucial for the
improvement of total performances of LIB. Understanding of the redox reactions andstructural/electronic properties for the materials in LIBs is necessary to design/create
innovative materials for LIBs. Thus, cutting-edge analyses are inevitable in addition to thematerials development.
(a)

(b)

(c)

(d)

20 µm

5 µm

Figure 1 (a)(b) optical microscope images and (c)(d)corresponding XPS images of working electrodeSynchrotron radiation X-ray spectroscopy is a useful technique to understand the crystal
structure and electronic structure of LIB materials. We have been studying thecharge-discharge mechanisms in LIBs by the electronic-structure analysis based on soft X-ray
spectroscopy. The operando soft X-ray emission spectroscopy under the charge-discharge
operation is an important technique to investigate the redox reactions of electrode materials[1]. Furthermore, spatially resolved electronic-structure information is of particularimportance because the redox reaction by charge-discharge reaction should depend on theposition like a facet of an electrode material and the interfaces between the electrode materials
and electrolytes.
We report operando photoemission spectromicroscopy of single crystalline Li4Ti5O12 (ananode material) [2] by 3DnanoESCA [3] with all-solid-state LIB. The soft X-ray
photoemission spectromicroscopy experiments has been conducted at BL07LSU of SPring-8.
Figures 1 shows (a) optical microscope image of the surface of working electrodeconstructed by Cu mesh, Li4Ti5O12, carbon as conductive additive, and polymer solid-stateelectrolyte, along with (b) the enlarged images of the white squared area in (a), while Fig. 1(c) and (d) are the corresponding XPS images. The 3DnanoESCA obtains corresponding XPS
images of working electrode to images of optical microscope.
ACKNOWLEDGEMENT
This work was conducted under the international joint research program for innovative energy
technology by METI, Japan and TIA collaborative research program “Kakehashi”.
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] 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).
[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).

THE ELECTRONIC STRUCTURE OF HYDRATED WATER IN
BIOCOMPATIBLE POLYMERSDaiki Murakami1,2, Kosuke Yamazoe3, Tomoya Ueda2, Toshiki Sonoda2,
Jun Miyawaki3, Yoshihisa Harada3, Masaru Tanaka1,2
1Institute for Materials Chemistry and Engineering, Kyushu University, Japan
2
Graduate School of Engineering, Kyushu University, Japan,
3
Institute for Solid State Physics, The University of Tokyo, Japan

Introduction
The role of water molecules in biological systems have long been discussed. Tanaka et al.
showed by differential scanning calorimetry (DSC) that water in biocompatible polymer,
poly(2-methoxyethyl acrylate) (PMEA) could be classified into three types; non-freezingwater, intermediate water, and free water, while water in less biocompatible analogues lacks
the intermediate water. [1] This tendency has been verified for other biocompatible polymers
such as poly(ethylene glycol) (PEG) and poly(2-methacryloyloxyethy phosphorylcholine)(PMPC). Lots of experiments verified that the intermediate water is a key to control thebiocompatibility of polymers. However, the detailed structural feature of the intermediatewater is not clear so far.
In this work, we aimed to clarify the structural property of intermediate water by soft x-ray
absorption spectroscopy (XAS) and soft x-ray emission spectroscopy (XES). XAS and XES
experiments are powerful tools to determine the local hydrogen bond network of water. [2]Experimental
XAS/XES measurement was performed at BL07LSU HORNET station in SPring-8. We
prepared the PMEA spin-coated film on a SiC substrate. PMEA was synthesized by radical
polymerization method. To enhance the stability of the film, small amount of PMEA withterminal thiol group was grafted on the SiC surface before the spin-coating. Similarly, the filmof poly(n-butyl acrylate)(PBA) was prepared and used for comparison. The humid air with
controlled humidity was flowed on the sample to change the water contents in the polymer
films.

Figure 1. Sample preparation and experimental setup
Results and discussion
Figure 2 shows the XES spectra of PMEA and PBA under the humidity control. As
expected the emission intensity increased as the relative humidity increased. In the hydration
process of PMEA, first non-freezing water which shows the strongest interaction withpolymer chains appears. Intermediate water and free water should contribute in the order ofinteraction strength. [3] Thus we assigned the XES spectrum in the highest humidity(95%RH) to free water which shows a similar spectral profile with bulk water having two
peaks originated from tetrahedrally-coordinated and distorted hydrogen bonds. [4] In 50%RH,

non-freezing water appears as a broad profile peaked at the tetrahedrally-coordinatedcomponent, along with a sharp peak at 527 eV indicating adsorbed water. It is remarkable thatthe 75%RH spectrum showed still a broad profile with larger tetrahedrally-coordinated andlesser distorted components compared with bulk water. We speculate this unique spectrum isclosely relevant to the presence of intermediate water in PMEA. In PBA, this type of water
was not observed while highly distorted but tetrahedrally coordinated component [5]dominates at higher humidity, which would rather lead to a decreased biocompatibility.
The XAS spectra in the same humidity conditions were shown in Fig. 3. We notice thatwater in PMEA showed increased post-edge profile peaked around 540 eV in 75%RH, whichis relevant to the interpretation of the XES profile in 75%RH. In PBA, the profile peaked at
537 eV is again characteristic for the distorted but tetrahedrally-coordinated component [5].

Fig. 2 XES spectra of PMEA and PBA under the humidity control.

Fig. 3 XAS spectra of PMEA and PBA under the humidity control.
Conclusion
The biocompatible polymer PMEA has characteristic XAS/XES profile with larger
tetrahedrally-coordinated and lesser distorted components, while the less biocompatible
polymer PBA dominantly has highly distorted but tetrahedrally coordinated component. This
study provides a clue to reveal the detailed mechanism of biocompatibility and design highlybiocompatible polymers.
References[1] M. Tanaka et al. Polymer Journal, 2015, 47, 114.
[2] T. Fransson et al. Chem. Rev, 2016, 116, 7551.
[3] S. Morita et al. Langmuir 2007, 23, 3750.
[4] T. Tokushima et al. Chemical Physics Letters, 2008, 460, 387.
[5] K. Yamazoe et al. Langmuir 2017, 33, 3954.

SOFT X-RAY ABSORPTION AND EMISSION SPECTROSCOPY OF
THE INTERMEDIATE WATER IN BIOCOMPATIBLE POLYMERSDaiki Murakami1,2, Kosuke Yamazoe3, Tomoya Ueda2,
Jun Miyawaki3, Yoshihisa Harada3, Masaru Tanaka1,2
1Institute for Materials Chemistry and Engineering, Kyushu University, Japan
2
Graduate School of Engineering, Kyushu University, Japan,
3
Institute for Solid State Physics, The University of Tokyo, Japan

Introduction
Poly(2-methoxyethyl acrylate) (PMEA) is an outstanding biocompatible polymer widelyused in the medical field. We have revealed that the water molecules contained in PMEA canbe classified into three types: non-freezing water, free water, and intermediate water.
Non-freezing water does not crystallize below 0 oC due to a strong interaction with a polymer
chain. Free water crystallizes at approximately 0 oC like bulk water. Intermediate water does
not freeze at 0 oC but exhibits melting/crystallization at lower temperature due to the weakinteraction with polymer chains. It has been well recognized that the intermediate water
dominates the biocompatibility of polymers. [1] However, the structure and characteristics ofintermediate water remains unclear so far. In this work, we aimed to shed light on the states ofintermediate water using soft x-ray absorption/emission spectroscopy (XAS/XES), which is a
powerful tool to determine the local hydrogen bond network of water.

Experimental
We prepared the PMEA brush on SiC substrate bygrafting-to method. First we synthesized the PMEA withterminal thiol group (PMEA-SH, Fig. 1) by raftpolymerization. The molecular weight of PMEA-SH wasestimated to be 38,000 g/mol and molecular weight
distribution was 1.1 from Gel Permeation Chromatography.
The SiC substrate (Au coated) was dipped in
PMEA-SH/MeOH solution (200 µmol/l) for 1 hour, andFig. 1 Chemical structurethen rinsed with MeOH and dried. The graft density ofof PMEA-SH
obtained PMEA brush was c.a. 0.1 chains/nm2.
XAS/XES measurement was performed at BL07LSU HORNET Station in SPring-8. The
humid air with controlled humidity was flowed on the sample to change the water contents inthe brush sample.
Results and discussion
In the XES spectrum of bulk liquid water, basically two significant peaks are observed. One
is around 525.6 eV indicating the presence of ordered (tetrahedrally coordinated) hydrogenbonds, and the other is around 526.8 eV corresponding to distorted hydrogen bonds. [2] As
shown in Fig. 2(a), in the low humidity range below 65%RH, a broad profile peaked at the
tetrahedrally-coordinated component dominates, along with a sharp peak at 527.1 eV which
we consider water adsorbed on PMEA, Above 75%RH, slight increase in the distortedhydrogen bonds was observed and finally in 90%RH the spectrum is very similar to that ofbulk water.
For more detailed consideration, the difference spectra between the neighboring XES
spectra were calculated (Fig. 2(b)). We notice that the differential spectrum of 75%-65%

showed only a broad peak centered around the tetrahedrally coordinated hydrogen bond. We
expect this is the feature of non-freezing water, which is strongly bounded to the adsorbed
water on the polymer chain. In contrast, the difference spectrum of 90%-78% showed two
peaks, which is very similar to the spectrum of bulk water. This is assigned to free water,which weakly interacts with the polymer chain. The spectrum of 78%-75% is particularlyinteresting. It showed a broad peak centered at the middle of ordered and distorted peaks. In
XAS spectra (Fig. 3), we observed tetrahedrally coordinated behavior (increment ofabsorption intensity in post-edge region) of water at around 75%RH. These results indicatethat the hydrogen bond network of water in PMEA, especially around 75%RH, is obviously
different from that of free water and non-freezing water. We consider this is the uniquecharacteristics of intermediate water.
We successfully detected the intermediate water in PMEA, whereas the origin ofintermediate water is still unclear. Further investigation will be performed in our future workin order to clarify the essential feature of intermediate water and the role on biocompatibility.

Fig. 2 (a) XES spectra of PMEA under the humidity control.
(b) Difference between the spectrum in each humidity.

Fig. 3 (a) XAS spectra of PMEA under the humidity control.
References[1] M. Tanaka et al. Polymer Journal, 2015, 47, 114.
[2] T. Tokushima et al. Chemical Physics Letters, 2008, 460, 387

Dispersionless Charge Excitations for La2-xSrxNiO4+δ
probed by Oxygen K-edge Resonant Inelastic X-ray Scattering
Kohei Yamagami1, Kenji Ishii2, Yasuyuki Hirata 1, Keisuke Ikeda1, Jun Miyawaki1,
Yoshihisa Harada 1, Shun Asano3, Masaki Fujita3, and Hiroki Wadati1
1

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

In order to investigate the correlation between the charge stripe state and high temperature
superconductivity for La2-xSrxCuO4 (LSCO), the identification of the characteristics of thecharge stripe state for layered-perovskite type Ni oxides La2-xSrxNiO4+δ (LSNO) is requiredsince the charge stripe states for LSCO and LSNO are caused by the doped holes [1,2]. Since
single spin-flip excitations are forbidden, charge excitations are mainly observed by not only
Ni K-edge resonant inelastic X-ray scattering (RIXS)[3,4] but also the O K-edge RIXS. To
directly observe the charge excitations due to the doped holes, in this study, we carried out O
K-edge RIXS, which was already performed for LSCO and successfully revealed dispersive
charge excitations [5], for LSNO.
Momentum resolved O K-edge RIXS for LSNO was measured at HORNET station in
SPring-8 BL07LSU. The number of doped holes nh (≡ x + 2δ) of LSNO was 0.1 (no charge
stripe state) and 0.33 (with the charge stripe state), respectively. The figure shows thecomparison among O K-edge RIXS spectra for nh = 0.1 and 0.33 at incident energy (hvin) of
528.7 eV. Four constant loss energy peaks were successfully observed, indicated by labels a-din that figure. Peaks b and c correspond to charge excitations observed in the opticalconductivity spectra [6,7]. Momentum resolved RIXS spectra indicate that the clear energy
dispersions of charge excitations were not observed, reflecting the localized charge
excitations. This dispersionless of charge excitations in LSNO are different from that of
LSCO where the charge excitations have clear energy
dispersion.
These localized charge excitations of doped holes
are consistent with the insulating behavior in
conductivity [6] and related to the size of polaron
reported by the optical conductivity measurement [8].
The simple mean-field theory applied to a three-band
Hubbard model predicts that the stabilization energyof the polaron originating from electron-phonon
coupling is correlated with the charge transfer energy
between nickel and oxygen ions [9] and it could be an
origin of wider hole density of charge-stripe orderedphase in the nickelates than that in the cuprates [10].
REFERENCES[1] J. M. Tranquada et al., PRB 54, 12318 (1996).
[2] H. Kimura et al., PRB 67, 140503(R) (2003).
[3] S. Wakimoto et al., PRL 102, 157001 (2009).
[4] L. Simonelli et al., PRB 81, 195124 (2010).
[5] K. Ishii et al., PRB 96, 115148 (2017).
[6] T. Katsufuji et al., PRB 54, 14230 (1996).
[7] T. Ido, et. al., PRB 44, 12094 (1991).
[8] X-X. Bi and P. C. Eklund, PRL 70, 2625 (1993).
[9] J. Zaanen and P. B. Littlewood, PRB 50, 7222 (1994).

Figure: O K-edge RIXS spectra for
LSNO (nh = 0.1 and 0.33). Opticalconductivity spectra for nh = 0.1 and
0.33 are shown in the bottom [6,7].

[10] K. Ishizaka et al., PRB 67, 184418 (2003).

Charge Excitations for single- and bi-layered Manganate
probed by Resonant Inelastic X-ray Scattering at Oxygen K-edge
Kohei Yamagami1, Kenji Ishii2, Jun Miyawaki1, Yoshihisa Harada 1, Hideki Kuwahara3,and Hiroki Wadati1
1

Institute for Solid State Physics, The University of Tokyo
Synchrotron Radiation Research Center, National Institutes for Quantum and Radiological Scienceand Technology
3
Department of Physics, Sophia University, Tokyo2

Effect of carrier doping into insulating transition-metal oxides is a central issue in strongly
correlated transition-metal systems and parent insulators are categorized into two types: Mott
insulators and charge-transfer insulators. In the latter, the energy of charge transfer from theoxygen 2p to the transition-metal d levels is smaller than the on-site Coulomb repulsion of thed electrons. Among them, the layered manganate LaSr2Mn3O7 (LSMO327), La0.5Sr1.5MnO4(LSMO214) has the checkerboard-type charge ordering within the insulating due to the doped
holes[1]. In particular, LSMO327 collapses/reorders charge ordering with decreasing
temperature, not seen in LSMO214. In order to investigate the charge excitation originatingfrom doped holes, being related to the charge ordering, we have performed the resonant inelastic
X-ray scattering (RIXS) at oxygen K-edge for LSMO327 and LSMO214.

Fig.1 Temperature and momentum transfer (q) dependence of O K-edge RIXS spectra for (a)LSMO214 and (b) LSMO327. In LSMO327, the temperature is raised and lowered between
30 K and 300 K.

The O K-edge RIXS of single crystal LSMO327 and LSMO214 was carried out at HORNET
station in SPring-8 BL07LSU. Figure 1 shows the O K-edge RIXS spectra for (a) LSMO214
and (b) LSMO327 at incident energy (hvin) of 529.5 eV with σ-polarization parallel to the MnO2
plane. Combining with the O K-edge X-ray absorption and hvin dependence of RIXS spectra,
we judge that the excitations at 0 eV, 1.3 eV, and 2.5 eV are elastic including in phonons andmagnons, fluorescence, and the charge excitation respectively. It is found that the charge
excitation has energy dispersionless corresponding localized character. This can be consideredthat there is presence of short-range correlation such as the electron-lattice coupling due to the
Jahn-Teller distortion. On the other hand, interestingly, the peak and intensity of fluorescencecomponent depends on the temperature. The multi-gaussian fitting results indicates that thistemperature behavior is associated with the electron resistivity, which reflects the electronic
states at unoccupied conduction bands within LSMO214 and LSMO327, not being seen in La2xSrxNiO4 in our previous O K-edge RIXS study.
Moreover, in LSMO327, the peak and intensity of fluorescence component depends on the
momentum transfer (q) at 125 K corresponding to the collapsing charge ordering. Since thesuperlattice reflection characteristic of orbital ordering has been observed around 125 K[2], we
consider that the fluorescence component includes in the degree of freedom for eg orbital. At
present, we are discussing about the origin of temperature and q change that appears in the
fluorescence components and the correlation with the orbital order being observed
simultaneously within LSMO214 and LSMO327.
References[1] J. Dho, et al., J. Phys.: Condens. Matter, 13, 3655 (2001).
[2] T. Kimura, et al., Phys. Rev. B 58, 11081 (1998).

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

RIKEN Center for Emergent Matter Science (CEMS)

4The Institute for Solid State Physics, University of Tokyo

5

Department of Materials Molecular Science, Institute for Molecular Science
6
7

Japan Synchrotron Radiation Institute (JASRI)

Graduate School of Material Science, University of Hyogo

8Department of Advanced Materials Science, University of Tokyo

Introduction:
Topological magnetic textures have attracted a great deal of attention and have been
extensively studied in the recent years. In a chiral magnetic material with broken spatial
inversion symmetry, a vortex-like stable magnetic structure called a magnetic skyrmion may
emerge under certain temperatures and magnetic field conditions. A triangle crystal state of
the magnetic skyrmion was first observed in the chiral magnet MnSi by using small-angle
neutron scattering, and it was later observed in real space via Lorentz-type transmission
electron microscopy. Since then, magnetic skyrmions have also been observed by using soft
X-ray techniques, such as small-angle scattering, X-ray holography, and transmission soft
X-ray microscope. Since a magnetic skyrmion behaves like an isolated particle in a magnetic
material and can be controlled by an external field, it is a promising candidate for information
media in next-generation spintronics devices. To understand its characteristics, such as its
response to an applied external field, observing skyrmions in real space with high temporaland spatial resolution is important.
In the present study, we have developed adiffractometer of small-angle resonant soft
x-ray magnetic scattering for performing thecoherent x-ray magnetic imaging of magnetic
textures at the undulator beamline BL07LSU,
SPring-8. The apparatus is equipped with avacuum chamber having a backgroundpressure of < 1 × 10-5 Pa, a CCD camera(2048x2048 pixel, Princeton Instrument), a
direct beam catcher, and a sample holder [Fig.
1]. In order to observe transmitted soft x-ray
scattering, we prepared thin plate sample
thinned by FIB method or magnetic thin films
fabricated on Si3N4 substrates.

Figure 1: Diffractometer for resonant
soft x-ray small-angle scatteringdeveloped at BL07LSU, SPring-8.

Results:
First, we measured the diffraction pattern from the pinhole in order to evaluate theperformance of the instrument and the coherence of the incident soft x-rays. Figure 2 shows
diffraction pattern and its line profile from a double pin hole of 0.5 μm in diameter. The
striped diffraction pattern is derived from coherent diffraction of a double pinhole separated
by 8 μm distance. As a result, it is suggested that the coherent length was at least 8 μm.

Figure 2: Coherent diffraction pattern (left) and its line profile (right) from a double
pinhole, fabricated on Ta deposited Si3N4 membrane. The size of the pinholes are 0.5 μm
in diameter, and their spacing is 8 μm. Red and blue lines indicate the observed andcalculated curves, respectively.
Subsequently, diffraction patterns from magnetic multilayer thin films FdGd/Co andfrom magnetic thin film Co/Pt were measured at Fe L-edge (~702 eV) and at Co L-edge(~776 eV), respectively. The samples were placed over the pinhole to cut out the coherent
light. When the light energies were changed to make it non-resonant, the diffraction patternwere observed to change, which suggested that magnetic scatterings reflecting ferromagnetic
domain structure were observed.

Figure 3: Magnetic diffraction pattern from magnetic thin films FeGd/Co and CoPt.
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 film. In the future, we aim toobserve 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.

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

Introduction
Operando spectroscopy of catalytic reactions is essential for our understanding of reaction
mechanisms as well as the rational development guidelines for novel catalysts. Ambientpressure X-ray photoelectron spectroscopy (AP-XPS) is one of the most powerful operando
spectroscopies, which permits the investigation of the electronic and chemical states ofadsorbates and catalysts under gas atmospheres at near-ambient pressure. However, thetypical temporal resolution of AP-XPS has been limited to ms, and thus the dynamic
processes faster than ms could not be studied. At SPring-8 BL07LSU, we developed
time-resolved X-ray photoelectron spectroscopy (tr-XPS) in ultrahigh vacuum by combining
ultrafast optical laser with X-ray in a pump-probe fashion. tr-XPS allows one to monitor thetime-evolution of electronic structures of samples in real time. The temporal resolution of
tr-XPS is ~50 ps, which is determined by 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 will permit themonitoring of photocatalytic reactions in real time; all the players in photocatalytic reactions
such as photo-excited carriers, adsorbates, substrates can be investigated by tr-APXPS.

Experimental
All the experiments were performed at the soft X-ray undulator beamline BL07LSU at
SPring-8 [1]. 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 [2]. 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 highrepetition 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, a new soft X-ray chopper was installed at SPring-8
BL07LSU. The repetition rate of probe X-ray was 208 kHz in the H-mode operation ofSPring-8, which was much larger than that of pump laser (1 kHz). Soft X-ray chopper can
reduce the repetition rate of probe X-ray to 1 kHz to match that of pump laser. Consequently,
time-resolved XPS measurements became possible with the same experimental setup as inconventional static XPS measurements. Figure 1 shows a picture of a soft X-ray chopper
installed at SPring-8 BL07LSU and its operation of pulse selection. Soft X-ray chopper
blocks all the X-ray pulses that are not used in pump-probe time-resolved measurements. Thisdecreases unnecessary burden on the sample and the detector. Thus, the photon flux can beincreased 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. As a result of excitation by pump laser (800 nm, 1kHz), we could observe the energy shift of Si 2p core level due to surface photovoltage effect.
In addition, we could detect the energy shift of CO2 gas phase peak in O 1s core level. Theenergy 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
preliminary experimental results of tr-APXPS demonstrate that this experimental method canprovide a wealth of information in photo-excited systems and will be very suited to study
various light-induced reactions including photocatalysts and plasmonic chemical reactions.

Figure 1. (a) Soft X-ray chopper installed at SPring-8 BL07LSU (b) Time structure of soft
X-ray pulses with soft X-ray chopper OUT and IN. The repetition rate of soft X-ray pulses
was decreased to 1 kHz.
REFERENCES[1][2]

S. Yamamoto et al., J. Synchrotron Rad. 21, 352-365 (2014).
T. Koitaya, S. Yamamoto, et al., Topics in Catalysis 59, 526-531 (2016).

Sabatier reaction on a Ni/SiO2 catalyst studied byoperando soft X-ray spectroscopies
Susumu Yamamoto1, Jun Miyawaki1, Jiayi Tang1,2, Takanori Koitaya1,3,4, Takuma Tokunaga1,
Yu Shibata1, Yuka Kosegawa1, Kosuke Yamazoe1, Kozo Mukai1, Shinya Yoshimoto1,
Yoshihisa Harada1, Jun Yoshinobu1, Iwao Matsuda1
1

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

3

Graduate School of Engineering, University of Hyogo

Department of Materials Molecular Science, Institute for Molecular Science
4

Japan Science and Technology Agency (JST),
Precursory Research for Embryonic Science and Technology (PRESTO)

Introduction
Sabatier reaction, CO2 + 4H2 → CH4 + 2H2O, is the hydrogenation reaction of CO2 into
CH4. This catalytic reaction now attracts great interest as a key step in the “Power-to-Gas”
technology that converts renewable electricity into gas fuels. Although the development ofmethane synthesis catalysts with higher catalytic activity and selectivity is now highly
demanded, the lack of direct experimental observation of working catalysts impedes ourunderstanding of reaction mechanisms as well as the rational development guidelines fornovel catalysts.
In this study, we aimed to investigate Sabatier reaction on a silica-supported nickel(Ni/SiO2) catalyst under reaction conditions using ambient-pressure soft X-ray photoelectron
and absorption spectroscopies (AP-XPS/XAS). In the operando observation of Ni/SiO2
catalyst, the catalytic activity was simultaneously monitored using mass spectrometer, and thechemical states of the working catalyst were clarified by soft X-ray spectroscopies.
Experimental
The experiments were performed at the soft X-ray undulator beamline BL07LSU at
SPring-8 [1]. 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. The details of the AP-XPS apparatus can be found in our previous
publication [2]. AP-XAS experiments were carried out at X-ray emission spectroscopy
endstation at BL07LSU. AP-XAS spectra were measured in total electron yield and in partial
fluorescence yield. Home-developed five stage differential pumping system allows thewindowless delivery of soft X-ray photons to a sample even in atmospheric pressures.

Results and discussion
The catalytic activity of Ni/SiO2 catalyst (64 wt% Ni) was studied as a function oftemperature (Figure 1). The Ni/SiO2 catalyst was exposed to the mixed gas of 0.7 mbar CO2
and 2.1 mbar H2, and the sample temperature was increased in a stepwise manner from 300 K
to 373 K, 473 K, and 573 K. The change in gas composition with temperatures was monitored
by mass spectrometer. When the sample temperature was increased from 373 K to 473 K and
573 K, the signals for the reactant gases (CO2 and H2) decrease in intensity; the signals for the
product species (CH4, CO, and H2O) increase in intensity. Therefore, both Sabatier reaction
and reverse water gas shift reaction occur on the Ni/SiO2 catalyst in the reaction conditions
studied here.
Next the chemical state of Ni nanoparticles during catalytic reactions was evaluated by Ni
2p XPS and Ni L-edge XAS spectra. The comparison with Ni reference spectra revealed thatthe chemical state of Ni particles was NiO before reactions and then converted into a mixed
state of metallic Ni and NiO during reactions. The correlation between the chemical states of
Ni and catalytic activities was thus experimentally established.

Figure 1. Catalytic activity measurements of Ni/SiO2 catalyst. Gas composition wasmonitored by mass spectrometer, while the Ni/SiO2 catalyst was heated in a stepwise mannerfrom 300 K to 373 K, 473 K, and 573 K in the mixed gas of 0.7 mbar CO2 and 2.1 mbar H2.

REFERENCES[1][2]

S. Yamamoto et al., J. Synchrotron Rad. 21, 352-365 (2014).
T. Koitaya, S. Yamamoto, et al., Topics in Catalysis 59, 526-531 (2016).

PHOTOEMISSION MICROSCOPY OF
MOTT INSULATOR Ca2RuO4 UNDER ELECTRIC FIELD
Y. Takasuka1, A. Hishikawa1, D. Ootsuki1, C. Sow2, S. Yonezawa2, Y. Maeno2,
F. Nakamura3, N. Nagamura4, T. Yoshida11Grad. Sch. of Human & Environ. Studies., Kyoto Univ.,
Dept. of Phys., Kyoto Univ., 3Dept. Edu. Creat. Eng., Kurume Inst. Tech., 4NIMS

2Introduction
A Mott insulator Ca2RuO4 is known to show an insulator-to-metal transition (MIT), which isdriven by chemical substitution, temperature, and applying hydrostatic pressure. Recently,
Nakamura et al. reported that the MIT is also induced by applying a dry-battery level
electric-field at room temperature [1]. Even more surprisingly, strong diamagnetism underelectric current is discovered [2]. The diamagnetism is stronger than that in othernonsuperconducting materials. In strongly correlated electron systems, the domain structure isoften observed in the vicinity of MIT. Indeed, the spatial growth of the current-induced
metallic domain was observed by infrared nano-imaging and optical-microscopy
measurements [3]. In this work, we have performed a soft x-ray photoemission microscopy inorder to investigate the spatial dependence of the electronic structure for Ca2RuO4.
Experiments
Single crystals of Ca2RuO4 were grown by a floating zone method. The gold wires with adiameter of 100 ㎛ were attached to the terminals both ends of the sample using silver epoxy.
The electric field was applied parallel to the ab-plane. The voltage-current characteristics
were measured in situ during the measurements by a two-probe method using a DC V-I
source/monitor (Keithley 2450). The photoemission microscopy was performed using athree-dimensional nanoscale electron-spectroscopy chemical analysis (3D nano-ESCA)
system that was installed at BL07LSU of SPring-8. The present data were acquired withlinearly polarized light at h = 1000 eV. The clean surfaces were obtained by in situ cleavage
in vacuum with a base pressure better than 8.2 × 10-9 Pa. All data were collected at room
temperature.
Results and discussion
Figure 1 (a) shows the IV curves at T = 300 K. The Ru 3d5/2 core-level photoemission spectra
under the electric field are displayed in Figs. 1 (c) and (d). The spectra are integrated within
the region (100 × 100 ㎛ 2) as shown in Fig. 1(b). Figure 1 (c) shows the current
dependence of Ru 3 d5/2 core-level spectra. The width of Ru 3d5/2 spectrum with current (I =50 mA) becomes broad as compared with that without current (I = 0 mA), which indicates theappearance of the screened peak at lower binding energy than the main peak. This suggeststhat the screening electrons increase by applying electric-field. To investigate the spatial
dependence of the electronic structure under current, we compared the spectra of the region A
with that of the region B as shown in Fig.1 (d). The spectrum of the region A is slightly
broader than that of the region B, suggesting that there is the inhomogeneous spatial
distribution of the screening electrons.

Summary
In order to investigate the spatial dependence of the electronic structure, we have performed
the soft x-ray photoemission microscopy for Ca2RuO4. We found the spatial dependence of
the Ru 3d5/2 core-level spectra under electric current, suggesting the spatial growth of thecurrent-induced metallic domain.

Fig. 1. (a) Current-voltage curve at T = 300 K. (b) Photo of Ca2RuO4 crystal. The squareindicates the measured region. Arrow shows the direction of current. (c) Ru 3d5/2 core-level
spectra with/without current in the region A of Fig. 1 (b). (d) Ru 3d5/2 core-level spectra of
the region A and B with I = 50 mA.

REFERENCES[1] F. Nakamura et al., Sci. Rep. 3, 2536 (2013).
[2] C. Sow et al., Science 358, 1084 (2017).
[3] J. Zhang et al., Phys. Rev. X 9, 011032 (2019).

OBSERVATION OF ELECTRONIC STRUCTURE FOR Ca2RuO4
UNDER ELECTRIC FIELD BY SOFT X-RAY ABSORPTIONSPECTROSCOPY

Teppei Yoshida1, Yukie Takasuka1, Daisuke Shibata1, Daiki Ootsuki1, Jun Miyawaki2,
Yoshihisa Harada2, Chanchal Sow3, Shingo Yonezawa3, Yoshiteru Maeno3,
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
Graduate School of Science, Kyoto University
4
Department of Education and Creation Engineering, Kurume Institute of Technology

2

Recently, the correlated electronic states under electric field are receiving growing interest.
It has been reported that a Mott insulator Ca2RuO4 shows an intriguing electric-field induced
insulator to metal transition with a nonlinear conductivity [1]. The threshold electric field Ethof ~ 40 V/cm for Ca2RuO4 is weakest among the Mott insulators. Particularly, a giant
diamagnetism under electric current has been reported by Sow et al [2]. Thus, it would bedesirable to understand how the applied low electric field changes the insulating state of
Ca2RuO4 and causes the giant diamagnetism.
The Mott insulator Ca2RuO4 undergoes the metal to insulator (MI) transition at TMI = 357 K
accompanied by structural deformation such as a rotation, tilt, and/or compression of the
RuO6 octahedra [3]. In the insulating phase of Ca2RuO4, two of the four Ru 4d electrons
occupy the ground dxy orbital and the two remaining electrons are located in the dxz/yz orbitals
owing to the framework of the crystal field [4]. According to studies by angle-resolved
photoemission spectroscopy and dynamical mean-field theory (DMFT), the paramagnetic
insulating state of Ca2RuO4 was interpreted as an orbital-dependent band-Mott insulator,
which is characterized by the coexistence of the band insulating state of dxy orbital and theMott insulating states of half-filled dxz/yz orbitals [5].
Here, we performed soft x-ray absorption spectroscopy (XAS) measurements on Ca2RuO4
under electric field at SPring-8 BL07LSU to investigate the current-induced evolution in theelectronic structure. High-quality single crystals of Ca2RuO4 were grown by using thefloating-zone method. The gold wires were attached to the terminals both ends of the sample
using silver epoxy. In order to monitor the sample temperature under an electric field, thesample was mounted directly on a Pt thermometer. The sample and the thermometer wereglued to sapphire substrate with silver epoxy and an insulating adhesive. The electric field
was applied parallel to the ab-plane and the voltage-current characteristics were measured insitu during the XAS measurements. The sample temperature is controlled by liquid helium
and the sample heating is less than 3 K during all measurements.
Figure 1 shows the in situ IV characteristics of the sample. The open squares denote thepoints at which the spectra were measured. Figures 1(a) and 1(b) are O 1s x-ray absorption
spectra of Ca2RuO4 under current taken at 300 K and 200 K, respectively. The two peaks
located at 528.5 and 529.1 eV are the t2g-derived spectra and are transitions to apical andin-plane oxygen sites [6]. On the other hand, the structures at ~531 and 534 eV are theeg-derived spectra and are assigned to be d3z2-r2 and dx2-y2 band, respectively [7]. With
increasing current, the spectral intensity for the apical oxygen sites decreases while that for
the in-plane oxygen sites increases. This trend is consistent with the previous XAS study
under electric field and is suggestive of the increase in the number of dxy holes with increasing
current [8]. As for the eg-derived region, the dx2-y2 band shifts to the lower energy while thed3z2-r2 band shifts to the higher energy with increasing current. The spectral change in the
t2g-derived spectra is relevant to the elongation of the RuO6 octahedra [6]. Also the band shifts
in the eg-derived region indicates the change in the crystal-field splitting. These resultssuggest that the current-induced change in the electronic structure is related to the structuralchange of the octahedra and may give a hint to understand the origin of the diamagnetism.

Figure 1. Current density J variationsof the electric field E at T = 300 K and
200 K. The open circles indicate thepoints at which the x-ray absorption
spectra were taken.

Figure 2. O 1s x-ray absorption spectra of Ca2RuO4 under current. The spectra are taken at 300 K (a)and at 200 K (b) at normal incidence.

REFERENCES[1] F. Nakamura et al., Sci. Rep., 3, 2536 (2013).
[2] C. Sow et al., Science, 358, 6366 (2017).
[3] O. Friedt et al., Phys. Rev. B 63, 174432 (2001).
[4] E. Gorelov et al., Phys. Rev. Lett. 104, 226401 (2010).
[5] D. Sutter et al., Nat. Commun. 8, 15176 (2017).
[6] T. Mizokawa et al., Phys. Rev. Lett. 87, 077202 (2001).
[7] H. J. Noh et al., Phys. Rev. B 72, 052411 (2005).
[8] M. Sakaki et al., J. Phys. Soc. Jpn. 82, 093707 (2013).

HYDROGENATION OF FORMATE SPECIESON A SINGLE ATOM ALLOY MODEL CATALYST Pd-Cu(111)
STUDIED BY AMBIENT PRESSURE XPS
Takuma Tokunaga, Susumu Yamamoto, Shion Nakano, Tomoki Matsuda, Kozo Mukai, Iwao
Matsuda and Jun Yoshinobu
The Institute for Solid State Physics, The University of Tokyo

1. Introduction
In this research, we have prepared a single-atom alloy Pd-Cu(111) surface as a model catalyst
for the hydrogenation of adsorbed formate species. Single-atom alloy catalyst (SAAC) has
recently been attracting attention [1]. SAAC is an alloy catalyst in which a very small amountof foreign metal atoms are embedded in the surface of a substrate metal; the foreign atom gives
an additional catalytic function keeping the main catalytic function of the substrate metal. In
order to elucidate the chemical species and electronic states of the surface, we have observed
the adsorption and reaction processes using ambient-pressure X-ray photoelectron spectroscopy(AP-XPS) and mass spectroscopy in operando conditions. High resolution XPS usingsynchrotron radiation can detect a very small amount (a few percent or less) of isolated metal
atoms embedded in the surface of a host metal substrate. By operando observation using mass
spectrometry and AP-XPS, the function of SAAC in the catalytic reaction would be elucidated.
Copper is a typical catalyst for methanol synthesis from CO and CO2 [2]. However, copper
is not very active for the dissociative adsorption of H2. Here, we prepared the SAAC sample inwhich Pd was deposited on a Cu single crystal surface. The host Cu substrate is suitable formethanol synthesis reaction and water gas shift reaction. When a small amount of Pd atoms aredeposited on the Cu (111) surface, hydrogen molecules are easily dissociated at the Pd site. The
produced hydrogen atoms spill over on the Cu surface; these hydrogen atoms may be highly
reactive [1]. We examined the reaction processes between hydrogen atoms and formate specieson the Pd-Cu(111) surface by AP-XPS and mass spectroscopy.
2. Experiments
The present SAAC sample was prepared on Cu(111) at 380 K by vacuum deposition of Pd in
an ultra-high vacuum chamber connected to the AP-XPS system at BL07 LSU, SPring-8.
Gaseous formic acid was introduced on the Pd-Cu(111) SAAC sample at low temperature(~100 K). This sample was heated at 300 K to produce formate species (HCOO) on the surface.
The adsorbed species and electronic states of the sample were investigated by high resolution
XPS. Under hydrogen introduction, AP-XPS spectra were continuously measured.
24kcps23

intensity [kcps]

3. Results and discussions
A small amount of Pd atoms were
deposited on the Cu(111) surface at 380
K in order to prepare the present SAAC
sample, and the surface wascharacterized by XPS. Figure 1 shows
Pd 3d XPS spectra of the Pd-Cu(111)
surface (hn = 680 eV). The singlecomponent Pd 3d3/2 and 3d5/2 peaks areobserved at 340.7 and 335.4 eV,
respectively. Thus, we confirmed thatclean
Pd deposition

22
21
20
19
18

345eV

340

335

330

Binding Energy [eV]

Figure 1 XPS spectra of Pd 3d region of cleanCu(111) and Pd-Cu(111) surfaces.

the Pd-Cu(111) SAAC was prepared. The Pd coverage was estimated to be about 0.04 ML.
Next, this Pd-Cu(111) sample was cooled at about 100 K and exposed to gaseous formic acidin the UHV chamber. By heating this sample at 300 K, most formic acid molecules weredesorbed and a small amount of formate species of about 0.1 monolayer was produced, which
is similar to the case of formic acid on Cu(111) [3]. The C1s and O1s of formate species on PdCu(111) were observed at 287.6 eV and 531.2 eV.
Then, the sample was moved to the AP cell for AP-XPS measurement in 2 mbar H2 atcontrolled temperature. During this process, simultaneous observation of AP-XPS and mass
spectroscopy was conducted. From the AP-XPS measurement of C1s and O1s (Figure 2), it was
found that the formate species hardly changed at 300 K to 350 K, but the formate species
decreased in intensity above 370 K. On the other hand, our thermal desorption experiments
have shown that decomposition and desorption of formate into CO2 and H2 occurs at 420 K on
Pd-Cu(111). When hydrogen (2 mbar) was introduced to the formate on Pd-Cu (111), thedecreased of formate species occurred at lower temperature. This might suggest thehydrogenation of formate occur and new chemical species desorb from the surface. However,
we could not detect new intermediate adsorbed species or significant change in mass
spectroscopy measurements. Thus, further experiments are necessary to determine if theformate is simply decomposed or reacted under ambient hydrogen.

300K(11m35s)
340K(1h22m58s)
350K(2h30m35s)
360K(3h01m30s)
370K(3h32m24s)
400K(4h26m34s)
after vac

40kcps16kcpsafter vac
400K(4m27m19s)
370K(3h53m14s)
360K(3h02m19s)
350K(2h42m23s)
340K(1h22m57s)
300K(11m35s)

30

intensity [kcps]

intensity [kcps]

35

25

14

12
20

15
10

10
290eV

288

286

284

282

534eV

Binding Energy [eV]

533

532

531

530

529

528

Binding Energy [eV]

Figure 2 C 1s (left) and O 1s (right) XPS spectra of formate/Pd-Cu(111) under 2 mbar H2
as a function of elevated temperature.
REFERENCES[1] Georgios Kyriakou, Matthew B. Boucher, April D. Jewell, Emily A. Lewis, Timothy J. Lawton,
Ashleigh E. Baber, Heather L. Tierney, Maria Flytzani-Stephanopoulos, and E. Charles H. Sykes,
Science, 335, 1209 (2012).
[2] L. Llyoyd, “Handbook of Industrial Catalysts”, Springer, 2011.
[3] Yuichiro Shiozawa, Takanori Koitaya, Kozo Mukai, Shinya Yoshimoto, and Jun Yoshinobu, J.
Chem. Phys. 143, 234707 (2015).

INVESTIGATING ANTIFERROMAGNETIC-FERROMAGNETIC
PHASE TRANSITION IN GdBaCo2O5.5 THIN FILM BY RESONANT
SOFT X-RAY MANGETIC DIFFRACTION

1

2

Yujun Zhang1,2*, Yasuyuki Hirata2, Kou Takubo2, Kohei Yamagami2, Kohei Yamamoto2,
Keisuke Ikeda2, Tsukasa Katayama3, Akira Chikamatsu3, Tetsuya Hasegawa3, Hiroki
Wadati1,2
Graduate School of Materials Science, University of Hyogo, 3-2-1 Kouto, Kamigori-cho, Ako-gun,
Hyogo 678-1297, Japan
Institute for Solid State Physics, University of Tokyo, 5-1-5 Kashiwanoha, Chiba 277-8581, Japan
3

1

Department of Chemistry, University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113-0033, Japan

Introduction
Entanglement of charge, spin, lattice and orbital degrees of freedom in transition metaloxide (TMO) systems have attracted much attention recently [1]. The electric and magnetic
properties of TMOs, which are dominated by the partially filled d orbitals, could be strongly
affected by the crystal field effect from the surrounding lattice and corresponding d-orbital
occupation. With a strongly localized feature of d electrons in TMO systems (especially 3d
TMO systems), variety of ordering phenomena like magnetic ordering, charge ordering andferroelectricity can coexist and significantly couple with each other. Tantalizing physicalphenomena with prospective applications, such as high-Tc superconductivity and colossal
magnetoresistance, have emerged during the investigation of strongly correlated TMO
systems.
Among 3d TMO systems, except the famous maganites and cuprates, cobaltites have also
drawn much research attention due to their specific spin-state transition of Co ions. In many
cobaltite systems, for instance the most simple case, perovskite LaCoO3, the competition
between crystal field splitting and Hund coupling can lead to various spin states of Co ions[2].
Take Co3+ as an example, with a strong octahedral crystal field, low-spin (LS, t2g6eg0, S=0)
state is energetically preferable and high-spin (HS, t2g4eg2, S=2) state with occupied eg orbitalis preferred when crystal field is weak. In spite of LS and HS states, there is also debate thatintermediate-spin (IS, t2g5eg1, S=1) state can also be realized in various cobaltites. The spinstate of Co ions is very sensitive to the external condition such as temperature, pressure,magnetic field and substrate strain, etc. It is intensely reported that the magnetic properties of
cobatites are dominated by the spin state of Co ions and complicated magnetic structures
could be observed in quite a few cobaltite systems.
Recently, REBaCo2Ox(5<x<6, RE is rare earth element) systems have been intensely
investigated [3-5] for their intriguing physical properties such as high oxygen conductivity,
metal-insulator transiton, giant magneto-resistance and spin-state ordering, etc.. The oxygen
concentration in these systems is always variable, which leads to the high oxygen mobility
within the lattice. When x=6, the cystal structure is standard double perovskite with RE andBa sitting at A site, while Co sitting at B site. The nominal valence state of Co is +3.5. Due tothe different ionic size of RE and Ba ions, A site is often ordered into layers with alternating
RE and Ba layers. When x=5, a structure with ordered oxygen vacancy layers can be formed.
When x=5.5, the oxygen vacancies order into columns along a axis, resulting in 4
non-equivalent Co sites coordinated by either normal oxygen octahedra or oxygen pyramids.
With this oxygen concentration, the nominal valence state of Co is +3. In this case spin-stateordering is frequently observed. For example it is reported that in PrBaCo2O5.5+x samples HS,
IS and LS coexist and contribute to a complicated magnetic structure and magnetic phasetransition [4].
On the other hand, phase transition with large resistivity change entangled with
antiferromagnetic(AFM)-ferromagnetic (FM) transition and spin-state transition were also

explored in GdBaCo2O5.5-x system[5]. Magnetic ordering with an index of (0 0 1/2) was
clearly observed by resonant x-ray magnetic diffraction (RXMD). However, there is no reportabout GdBaCo2O5.5-x thin films. In this work, we prepared high-quality epitaxial GdBaCo2O5.5(GBCO) thin film sample on SrTiO3(001) (STO) substrate and conducted the soft x-ray
RXMD measurement to investigate the AFM-FM phase transition behavior.
2

Experiment
We conducted soft x-ray RXMD experiments for GBCO thin film at BL07LSU of SPring-8.
Horizontal (π) and vertical (σ) linearly polarized soft x-ray at Co L edge was employed. The
measurement temperature was changed in range of 150 K to 260 K, where the AFM-FM
transition occurs.
3

Results and Discussion
Fig. 1 shows the energy scan of the (0 0 0.5) RXMD peak at Co L edge. Polarization
dependence of the diffraction clearly confirms the magnetic origin of this diffraction peaksince π polarization has more intensity than σ polarization at Co L3 edge, and the peakintensity strongly decreases when the temperature went across the phase transition
temperature (~230 K). In this energy region we can also observe the resonant scattering of Ba
M edge, as shown by the two sharp peaks in the figure. This peak shows much larger intensity
forσ polarization thanπ polarization, which indicates a charge contribution to the scattering
instead of magnetism. Note that the intensity of the peak at Ba M edge also decreases acrossthe phase transition temperature. This indicates that there is some concomitant structural peakwith the same Q. The peaks in energy scans does not go to 0 since there is small contribution
from the reflectivity of the sample.
Fig. 2 shows temperature dependent L scan of the RXMD peak. It is clear that the peakvanishes while temperature increase beyond the phase transition temperature.
Our experimental results confirmed the AFM-FM phase transition behavior in our GBCO
thin film samples. These results are important for discussion about the electronic structure andmagnetic ordering in GBCO thin films.
0.07

π (Hor.) 250 K
σ (Ver.) 250 K
π (Hor.) 200 K
σ (Ver.) 200 K
π (Hor.) 150 K
σ (Ver.) 150 K

0.3

Intensity (arb. units)

Intensity (arb. units)

0.4

0.2

0.1

0.0
770

780

790

800

810

0.06

(Hor.)
150 K
200 K
220 K
230 K
235 K
240 K
250 K
260 K

0.05
0.04
0.03
0.02
0.01
0.00
0.46

0.48

0.50

0.52

0.54

L

Photon Energy (eV)Fig.1. Energy scans of the (0 0 0.5) RMXD peak. Fig. 2. L scans of the (0 0 0.5) RMXD peak.
REFERENCES[1] E. Dagotto. Science 309: 257-262 (2005).
[2] M. W. Haverkort, et al. Phys. Rev. Lett. 97: 176405 (2006).
[3] T. Katayama, et al. J. Mater. Chem. C 6: 3445 (2018).
[4] P. Miao, et al. Phys. Rev. B 95: 125123 (2017).
[5] M. García-Fernández, et al. Phys. Rev. B 78: 054424 (2008).
[6] A. A. Taskin, et al. Phys. Rev. Lett. 90: 227201 (2003).

?
π (Hor.)?
σ (Ver.)?
XLD*10Spin- and angle-resolved photoemission study of MAX phase compound
V2AlC
Takahiro Ito1,2, Masashi Ikemoto2, Damir Pinek3, Koichiro Yaji4, Masashi Nakatake5,
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

5

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, Japan

Aichi Synchrotron Radiation Center, Aichi Science & Technology Foundation, Seto, Aichi 489-0985,
Japan

MAX 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]. To elucidate the electronic structure, especially, the existence of DP
and spin-polarized states, we have performed spin - and angle - resolved ARPES (SARPES)
with using a quasi - continuous - wave laser [5].
As a result, we have found the formation of the Dirac cone is consistent with DFT
calculation (Fig.1). Furthermore, the SARPES clearly demonstrates spin-polarized branches(S+ and S-) in the larger electron pocket (eL), where the spin directions are opposite to eachother and tangential to the Fermi surface along the ΓM line (Fig.2). Furthermore, missing of
the out-of-plane spin polarization as well as inversion symmetry breaking at the observed

Fig. 1: Band structure along ΓM lineof V2AlC. Gray solid (dashed) lines
are obtained from DFT calculation
along the ΓM (AL) line. The
definition of spin polarization axis in
Fig. 2 is indicated by arrow on the
DFT Fermi surface in the inset.

Fig. 2: SARPES spectra (S+:△; S−:▼) (a) and spin-polarization (b) of
V2AlC at the momentum cut fromp1 to p8 in Fig. 1.

spin-polarization with respect to the Γ point have been elucidated. There are at least two typesof mechanism to understand the origin of the observed spin-textures. One is the existence ofhelical spin texture around the M point with anti-parallel spin splitting due to Rashba effect[6]. The other is the formation of spin-polarized “drum-head surface states” connecting NL[7]. We expect that the present observation of the spin-textures on V2AlC might suggest thepotential of layered MAX phases as new series of topological materials.
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] V. Sunko et al., Nature 549, 492 (2017).
[7] G. Bian et al., Phys. Rev. B 93, 121113(R) (2016).

SPIN-ORBITAL TEXTURE IN QUASI-ONE-DIMENSIONAL GIANTRASHBA SYSTEM ON 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

Low-dimensional electronic structures with helical spin-polarization due to spin-orbit
interaction (SOI) have been extensively studied for not only basic physics but also applications
to spintronic devices [1-3]. Recently, it is revealed that the spin texture of many spin-polarized
two-dimensional (2D) surface states is not a simple helical one, but entangled with the orbital
characters and symmetry operations of the ground states [4-6]. In such case, the spinpolarization orientation of the photoelectrons drastically changes, even inverts its sign,depending on the polarization of the incident photons. So far, such spin-orbital entanglement
effects have been studied in 2D systems but not extended in one-dimensional (1D) or Q1D
systems yet.
In this study, we observed detailed spin-orbital texture of spin-polarized Q1D surface state
on Bi/InAs(110)-(2×1) by using laser-based spin-resolved photoemission system (laserSARPES) at ISSP. Since the surface state of this sample shows one of the largest sizes ofRashba-type spin splitting among Q1D systems [7], it is a suitable material to investigate therole of spin orbital entanglement in Q1D systems.
Figures 1 (a) and (b) show the band dispersions of the Bi/InAs(110)-(2×1) surface along Q1D
chain direction measured by conventional (spin-integrated) channel of the laser-SARPE setup
with p- and s- polarized photons, respectively. The pair of parabolic bands in both Figs 1 (a)and (b) lie slightly below the Fermi level. The photoelectron intensity of the parabolic band isstrong overall the parabola in Fig. 1 (a), whereas its intensity decays around the centre of thesurface Brillouin zone in (b), reflecting the difference in the orbital symmetry composing theparabolic band.
(a)

(b)

Figure 1. ARPES intensity plots of Bi/InAs(110)-(2×1) along Γ X (parallel to thesurface Bi chains) measured with (a) p-polarized and (b) s-polarized photons at 45 K.

(a) Sy (p-pol.)(b) Sy (s-pol.)(c) Sx (p-pol.)Figure 2. Spin-resolved energy distribution curves of Bi/InAs(110)-(2×1)
along Γ X (parallel to the surface Bi chains). The solid (open) triangles
correspond to spin polarization parallel (antiparallel) to (a-b) [001] and (c)[1̅10] direction. θe = 10° corresponds to k// = 0.14 Å-1.
Figure 2 (a) and (b) show the spin-resolved energy distribution curves (EDCs) taken with pand s-polarized lights, respectively. The peak positions of the EDCs agree well with the surface
band dispersions shown in Fig. 1. The spin polarizations of the peaks in Figs. 2 (a) and (b)invert together with the sign inversion of the emission angles (θe), indicating that these spinpolarizations obey time-inversion symmetry. The polarization orientations in Fig. 2 (b) are thesame for one side of the emission angles. These spin-polarization is consistent with a typical,
helical spin texture expected from the Rashba-type SOI. In contrast, the polarizations in Fig. 2(a) inverts within the same parabola (e.g. see -5.4° and -11.4°). Based on our density-functionaltheory (DFT) calculation, it would be because the fractional ratio of the orbital componentschanges depending on the k// positions; the pz orbital of surface Bi is entangled with the opposite
spin polarizations to the other orbitals. Figure 2 (c) shows spin-resolved EDCs for spinpolarization along [1̅10], parallel to k//. One can find clear polarizations whose sign do notchange with the sign of emission angle. Apparently, such behaviour breaks the time-inversion
symmetry. Since this sample has no magnetic order, this spin polarization should not from thespin polarization of the initial states but from photoexcitation process, so called final-state effect.
In summary, we have found many unconventional spin textures on Bi/InAs(110), whichcannot be understood by a simple helical spin texture expected for Rashba effect. Detailed
analysis including DFT calculations and spin-dependent photoexcitation processes are ongoing.
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 (2017) 14588.
[5] R. Noguchi et al., Phys. Rev. B 95, 041111(R).
[6] K. Kuroda et al., Phys. Rev. B 94 (2016) 165162.
[7] T. Nakamura et al., Phys. Rev. B 98, 075431 (2018).

SPIN-POLARIZED QUASI-ONE-DIMENSIONAL METALLIC
SURFACE ELECTRONIC STATE OF BI/GASB(110)-(2×1)
Takuto Nakamura1, Yoshiyuki Ohtsubo2,1, Kenta Kuroda3, 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

Spin-polarized low-dimensional electronic structures due to Rashba effect and topologicalorder at surfaces and interfaces are extensively studied for a fascinating playground for various
spintronic phenomena [1-3]. Among them, one-dimensional (1D) or quasi-1D (Q1D) systems
are expected to exhibit the interesting electronic behaviours such as the spin-dependent densitywave formation [4] and Majorana bound states at the junction with superconductors [5].
Recently, we have reported a semiconducting spin-polarized Q1D surface electronic structureof Bi/InAs(110)-(2×1) with a small bulk bandgap (~0.3 eV) [6]. As a next step for spintronic
application and observation of spin-dependent electronic phenomena, to make this Q1D state
metallic is desirable. In this work, we have tried to tune the Fermi-level (EF) of the similar spinpolarized Q1D states on Bi/GaSb(110)-(2×1). The surface electronic structure of
Bi/GaSb(110)-(2×1) was observed by laser-based angle-resolved photoelectron spectroscopy(ARPES) and spin-resolved ARPES (SARPES) at ISSP.
The GaSb(110) clean substrates were prepared by the repeated cycles of Ar sputtering (0.5 1 keV) and annealing up to 450 ℃. After substrates cleaning, a few monolayers of Bi was
evaporated. Subsequent annealing at 280 ℃ resulted in the sharp and low-background (2×1)
low-energy electron diffraction patterns, suggesting the well-ordered surface atomic structure.
Figure 1 (a) is a Fermi contour of the Bi/GaSb(110)-(2×1) with the binding energy windowof 0 ± 20 meV (EF). As shown there, the metallic surface state is observed, forming a butterflylike Fermi surface around the Y point. Figure 1 (b) shows an ARPES intensity plot along YM. The hole-bands with Rashba-type paired parabolic dispersions were observed. The obtained
band dispersion is quantitatively the same as the Q1D surface states on Bi/InAs(110)-(2×1) [5].
(a)

(b)

(c)

ഥ
Y

Figure 1. Electronic structure of Bi/GaSb(110)-(2×1) surface measured by ARPES at 14 K. (a) A
constant-energy contour at a binding energy of 0 ± 20 meV. (b) Band dispersion along Y-M (ky =0.26 Å-1). (c) the surface Brillouin zone of Bi/GaSb(110)-(2×1).

Figure 2. Spin- resolved energy distribution curves along Y-M at 14 K. (a) +kF (kx = 0.10 Å-1).
(b) - kF (kx = -0.10 Å-1). The filled (open) triangles correspond to spin polarization parallel (antiparallel) to [001].

It would be due to the qualitatively common surface atomic structures of Bi/GaSb(110)-(2×1)
and Bi/InAs(110)-(2×1), the “missing – row” model with the zig-zag Bi chains [7, 8].
Figures 2 shows the spin-resoled energy distribution curves (EDCs) at the Fermi wavevector(kF) along Y-M taken by SARPES. The spin-polarized peaks in Fig. 2 were clearly observed
around EF. These peak positions are consistent with the band dispersions in Fig. 1 (b). The spinpolarization of these peaks inverts together with the signs of the wavenumbers, indicating thetime-reversal symmetry, consistent with the Rashba-type spin-splitting.
These results suggest that Bi/GaSb(110)-(2×1) surface is the good Q1D candidate forfollowing researches of spin-dependent phenomena, hosting a metallic and widely spin split
surface states in Q1D.
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] B. Braunecker et al., Phys. Rev. B 82, 045127 (2010).
[5] V. Mourik et al., Science 336, 1003 (2012).
[6] T. Nakamura et al., Phys. Rev. B 98, 075431 (2018).
[7] M. G. Betti et al., Phys. Rev. B 59, 15760 (1999).
[8] S. C. A. Gay and G. P. Srivastava, Phys. Rev. B 61, 2688 (2000)

MODIFICATION OF SURFACE STATE OF TOPOLOGICALINSULATOR 𝐁𝐢𝟐 𝐒𝐞𝟑 BY ADSORPTION OF ORGANIC DONOR
MOLECULE TTN
T. Kitazawa, K. Shimozawa, and Shin groups
Department of Physics, Faculty of Science and Technology, Tokyo University of Science
Synchrotron Radiation Laboratory, The Institute for Solid State Physics, The University of Tokyo

Topological Insulators (TI), whose surface states provide various exotic quantum phenomena,
attract much attention in quantum matter physics. TI surface states (TSS) have a helical-spin
texture and in which spin-locked electrons are topologically protected from back scattering due
to the time reversal symmetry attributed by strong spin-orbital coupling. The spintronic devices
taking advantage of TI features are now expected to be realized. Thus, controlling the TSS bycreating interfaces between TI and other materials is an important problem for applying TIs for
spintronics devices. Recently, many researchers have worked on the TI interface with anothermaterial [1]. However, few previous researches related to the TI interface with organic materials
have been reported [2].
In our research, we aimed to control the TSS of Bi) Se, (111) surface, where a single Dirac
Cone is located at the Γ. point in SBZ, by adsorption of organic donor molecule TTN(Tetrathianaphthacene) onto the TI surface. To this end, we performed ARPES measurement
by using angle-resolved photoelectron spectroscopy (ARPES) machine at the Laser andSynchrotron Research Center in the ISSP and studied the thickness dependence of TSS. The
experiment was carried out at 140K using s-polarized VUV laser (ℎ𝜈 = 6.994 eV) underultrahigh vacuum conditions (~1 × 10<= Pa). The TTN molecule was deposited using Knudsen
cell evaporator. The thickness of the molecule layer was estimated by using a quartz-crystal
microbalance. The deposition process was performed in situ and at room temperature.
The experimental results are shown in Figure 1. The Bi) Se, single crystal synthesized by
Yaguchi group in Tokyo University of Science shows clear Dirac Cone dispersion on its pristine(111) surface, where DP is located at 𝐸 − 𝐸B = −0.27 eV. After TTN deposition of ~5Å, theTSS electronic structure changes dramatically. The DP shifts to higher binding energy (𝐸 −𝐸B = −0.39 eV) and the Fermi surface area increases as the circular contour shape turns into
the hexagonal warped one [3,4]. At the same time, the bulk conduction band (BCB) state
appears inside the TSS with the clear band bottom edge. It appears that n-type doping due tothe adsorption of TTN increases the density of surface electrons and consequently leads fermi
level to move into the BCB (away from the DP). Moreover, it is speculated that the surface
electrostatic potential is perturbed by n-type doping, which causes triangle-like downward
band-bending of the bulk electronic bands close to the surface. We interpret the clear BCB
bottom edge as an intensity of the electrons weakly localized near the surface.
At further TTN deposition of 10Å , the DP shifts to higher binding energy ( 𝐸 − 𝐸B ≈−0.55 eV) and Femi surface area gets larger, which means the TSS is further n-doped by TTN
deposition. Besides, we observe the parabolic dispersion attributed to the BCB inside the TSS.
This indicates that the relatively strong downward band-bending occurs at the TTN/Bi) Se,
interface where the electrons are accumulated in the potential well forming a two-dimensional
electron gas (2DEG) [5,6]. The flat feature lying in the lower energy region is attributed to the
HOMO state of TTN. For the TTN deposition of 20Å, the electronic structure is virtuallyunchanged compared to 10Å deposition. It is assumed that the TTN molecular layer achieved
1ML under the 10Å thickness.
We need to perform other experiments to verify whether the charge transfer from TTN toBi) Se, surface occurs or not and establish the adsorption structure.

Figure 1. Measured ARPES spectra and Fermi surface maps for (a and e) pristine Bi) Se, , (b and f)
TTN(5Å)/Bi) Se, , (c and g) TTN(10Å)/Bi) Se, , (d and h) TTN(20Å)/Bi) Se, at 140K.

REFERENCES[1] L. Wray et al, Nature Physics, 7, 32-37(2011).
[2] J. Wang et al, Journal of Physical Chemistry C, 118, 14860-14865(2014).
[3] L. Fu et al, Phys. Rev. Lett., 103, 266801(2009)[4] K. Kuroda et al, Phys. Rev. Lett., 105, 076802(2010)[5] M.S. Bahramy et al, Nat. Commun., 3, 1159(2012)[6] M. Bianchi et al, Nat. Commun., 1, 128(2010)
AUTHORST. Kitazawaa, K. Shimozawaa, K. Yajib, Y. Ishidab, K. Kurodab, A. Harasawab, H. Kondoa, T.
Yamanakaa, H. Yaguchia, K. Masec, S. Shinb, K. Akaiked, K. KanaiaaTokyo University of Science
ISSP
c
KEK
d
AIST
b

INVESTIGATION OF TOPOLOGICAL PROPERTIES IN
SUPERCONDUCTING TASE3
Chun Lin1, Ryo Noguchi1, Kenta Kuroda1, Peng Zhang1, Shunsuke Sakuragi1, Jiadi Xu1,
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

Department of Physics, Tokyo University of Science3

Department of Applied Physics, Hokkaido University
4

Department. of Physics, Gakushuin University

Topological insulators (TIs), which are characterized by the fully gapped bulk states but
gapless surface states [1], have attracted a lot of attentions in recent years. Moreover, theproposed topological superconductors [2], in which the Majorana states emerge within the
superconducting gap, further boost the progress in this area owing to the potential applications
in topological quantum computation. The attractive charge density waves (CDW) in
transition-metal trichalcogenides MX3 (M: Nb, Ta; X: S, Se) with layered quasi-1D chain-likestructures [Fig. 1 (a)] have been extensively discussed in the early ages [3]. As one exception,
TaSe3 is appealing in that superconductivity emerges at ultra-low temperatures [4], differently
from other analogues which undergo multiple CDW transitions [3]. It is considered that theanomaly stems from the poor nesting conditions in TaSe3 due to more 3D electronic structuresand competing CDW can be induced by Cu doping or in mesowires which reduce thedimensionality, as shown in Fig. 1 (b) [4-6]. Nevertheless, direct demonstrations of theelectronic structures are still deficient. More interestingly, the recent first-principle
calculations indicate that TaSe3 actually belongs to a strong TI phase and moreover predict
intriguing topological phase transitions among weak TI, strong TI, Dirac semimetal, andnormal insulator phases through uniaxial strains, as shown in Fig. 1 (c) [7]. TaSe3 thus not
only offers a simple system to study the interplay between superconductivity and topologicalsurface states (TSSs), but also provides an ideal platform for investigating topological phasetransitions. A direct probing of the electronic structures via angle-resolved photoemission
spectroscopy (ARPES) is consequently needed to elucidate the novel topological properties.
So far, to our knowledge, there is only one direct measurement of the electronic structure of
TaSe3 by ARPES presenting blurred and inconclusive dispersions due to weak intensity andmodest energy resolution [8].
In order to clearly reveal the electronic structure of TaSe3 and confirm its topologicalproperties [7], we have performed high-resolution laser spin-resolved ARPES measurements
at the Institute for Solid State Physics, the University of Tokyo [9].

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 (right)[6]. (c) Strain-induced topological phase diagram of TaSe3 including strong topological insulator (TI), weak TI,
Dirac semimetal, and normal insulator phases [7].

For comparison with the calculation [7], Figure 2 (a) shows the 3D Brillouin zone andtheoretical Fermi surface of TaSe3, followed by the calculated TSSs at Γ and X points on(-101) surface in Fig. 2 (b). Figure (c) is schematic of the experimental ARPES setup [9] andtypical sample size. In Fig. 2 (d) – (f), we show the observed Fermi surface and dispersions atΓ and X points on (-101) surface obtained using 7 eV laser at 10 K. Obviously, the observed
band structure is in good agreement with the calculations shown in Fig. 2 (b) [7]. Figure 2 (g)shows one typical spin-revolved result at the cut on the sharp low-energy excitations indicatedin Fig. 2 (f) by the dash line. One could clearly resolve the distinct signals from spin up anddown states. Spin polarization [9] up to 30% has been detected in the low-energy excitations,which are possibly the TSSs.
In conclusion, we have performed laser spin-ARPES measurements on TaSe3, the clearly
resolved low-energy electronic structure qualitatively agrees with the calculated TSSs. The
observation of spin polarization further implies that the sharp low-energy excitations areprobably TSSs.
Fig. 2 (a) 3D Brillouin zone andcalculated 3D Fermi surface of
TaSe3 [7]. (b) Calculated topologicalsurface states at Γ and X points on(-101) surface [7]. Momentum cut
positions are indicated in (a) withblue and red arrows, respectively. (c)ARPES setup. Typical samples sizeis ~ 1×0.1×0.01 mm. (d) Observed
Fermi surface on (-101) surface at
10 K. Γ and X points are indicatedby blue and red arrows, respectively.
(e) Corresponding dispersion at Γ
point. (f) Dispersion at X point andits curvature plot enhancing intensity
maxima [10]. (g) Spin polarization
along X direction at the cut indicatedin (f) by a dash line. Red and blue
curves are EDCs of spin up anddown states, respectively. Up to
30 % spin polarization is detected

REFERENCES[1] X.-L. Qi, S.-C. Zhang, Rev. Mod. Phys. 83, 1057–1110 (2011).
[2] L. Fu and C. L. Kane, Phys. Rev. Lett. 100, 096407 (2008).
[3] P. Monceau, Adv. Phys. 61, 325–581 (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 Cond-Mat (2019).
[7] S. Nie et al., Phys. Rev. B. 98, 125143 (2018).
[8] A. Perucchi et al., Eur. Phys. J. B. 39, 433–440 (2004).
[9] K. Yaji et al., Rev. Sci. Instrum. 87, 053111 (2016).
[10] P. Zhang et al., Rev. Sci. Instrum. 82, 043712 (2011).

Observation of the interface state for Bi layer deposited on a MnGa thin film
M. Kobayashi1,2, T. Takeda2, N. H. D. Khang3, M. Suzuki4, A. Fujimori4,
K. Kuroda5, K. Yaji5, S. Shin5, M. Tanaka1,2 and P. N. Hai3
1

Center for Spintronic Research Network, The University of Tokyo,
Department of Electrical Engineering and Information Systems, The University of Tokyo
3
Department of Electrical and Electronic Engineering, Tokyo Institute of Technology
4
Department of Physics, The University of Tokyo
5
Laser and Synchrotron Research Center, The Institute for Solid State Physics, The University of Tokyo
2

Spin-orbit torque switching using the spin Hall effect has attracted much attention due to its
potential application for ultra-high speed magnetoresistive random-access memories. Recently,
spin-orbit torque switching using topological insulator Bi1-xSbx (BiSb) (x = 0.1) has beensuccessfully developed [1]. Here, the BiSb(012) layer as a spin current source is grown on
perpendicular ferromagnet Mn0.6Ga0.4 (MnGa) by molecular beam epitaxy (MBE) method.
Since the topological state on the BiSb(012) surface has colossal spin Hall angle and highelectric conductivity [1], the spin Hall conductivity of BiSb(012) is higher than that ofconventional heavy metals [2-4] and other topological insulators [5]. Actually, the BiSb/MnGaheterostructure has demonstrated magnetization reversal via spin-orbit torque with extra-low
current density that is one to two orders of magnitude lower than the current for conventional
magnetization reversal. From comparison of spin Hall angles between the BiSb(001) and (012)surfaces, the number of Dirac cones (DCs) crossing the Fermi level (EF), which is considered
to contribute to the large spin Hall effect, in the (012) surface is larger than that in the (001)surface [6].
To understand the origin of the colossal spin Hall angle in topological insulators andferromagnets, the band structure including the topological surface state of the BiSb/MnGaheterostructure has to be investigated. Therefore, we have performed spin- and angle-resolved
photoemission spectroscopy (SARPES) measurements on a Bi/MnGa heterostructure.
A MnGa thin film was grown on a GaAs(001) substrate by the MBE method. The film wascapped by amorphous Se layer to protect the surface from oxidization. Details of the film
growth are described elsewhere [1]. The film was annealed at 550 oC under ultra-high vacuum
better than 10-7 Pa to remove the Se capping layer. Bi layer was deposited on the clean MnGa
surface at 150 oC with the thickness of about 1 bilayer. Clean surfaces of the thin films before
and after Bi deposition were confirmed by low-energy electron-diffraction (LEED)
measurements. ARPES with He-I (21.2 eV) and He-II (40.8 eV) light sources and 7eV-laserbased spin- and angle-resolved photoemission spectroscopy (Laser-SARPES) measurements
were conducted with the ARPES instrument using DA-30 photoemission analyzer equippedwith VLEED spin detector (Scienta Omicron, Inc.) at ISSP, University of Tokyo [7]. The
measurements were performed at 30 K in a base pressure below 10-9 Pa. The polarization of thelaser was linear-horizontal.
Figure 1(a) shows an angle-integrated photoemission spectrum of the Bi/MnGa thin film
measured with the He-II source. The core-level peaks of Bi 5d (binding energy EB ~ 25 eV)from the deposited Bi layer and Ga 3d (EB ~18 eV) from the MnGa underlayer are observed.
Since the thickness of the Bi layer is not large, photoemission signals from the MnGa layer are
also detected. Figure 1(b) shows ARPES spectra along the G-X symmetry line of theBi(012)/MnGa thin film taken with the He-I light source. The concave band around EB = 0.1~
0.5 eV centered at the G point originates from the MnGa underlayer. Since the hole-like bandcrossing EF near the G point appears after the Bi deposition, this band near the G point comesfrom the Bi layer.
The hole-like band near the G point has been investigated using Laser-SARPES. Figure 2(a)shows the ARPES spectrum along the G-X line taken with the 7-eV laser. While the banddispersion is unclear with the He-I source, the sharp dispersion can be seen near the G point in
the Laser-ARPES spectrum due to the higher energy and momentum resolution. The dispersionof this band looks linear, as shown in Fig. 2(b). The SARPES measurements have revealed thatthe sharp band is spin-polarized along the direction perpendicular to the G-X line and the spinpolarization is flipped with the momentum direction, as shown in Fig. 2(c). Based on theobservations, the band near the G point is expected to be the DC state of the deposited Bi layer.
Therefore, we have succeeded in observing the topological interface state between the Bi and
MnGa layers using Laser-SARPES. Further electronic structure study such as the observationsof the entire band structure and DC states near the Brillouin zone boundary will shed light on
the origin of the large spin Hall angle of the interface between the topologicalinsulator/ferromagnet heterostructures.
References

(a)

Bi 5d

(c)

G

G

0

G

Ga 3d

30

25

20

15

10

5

0

Binding Energy (eV)

G

(b)

X

0.1

0.1

up

0

0.2

0.2

y spinBinding Energy (eV)

(b)

(a)

Bi/MnGaHe II

Binding Energy (eV)Intensity (arb. units)[1] N. H. D. Khang et al., Nat. Mater. 17, 808 (2018).
[2] L. Liu et al., Science 336, 555 (2012).
[3] C.-F. Pai et al., Appl. Phys. Lett. 101, 122404 (2012).
[4] L. Liu et al., Phys. Rev. Lett. 106, 036601 (2011).
[5] A. R. Melnik et al., Nature 511, 449 (2014).
[6] X.-G. Zhu et al., New J. Phys. 15, 103011 (2013).
[7] K. Yaji et al., Rev. Sci. Instrum. 87, 053111 (2016).

0.3

0.3
-0.20

-0.10

0

0.10-0.20

Momentum (1/Å)
0.5

-0.4

-0.2

0

0.2

-0.10

0

0.10

Momentum (1/Å)

-0.05

0

Momentum (1/Å)

0.05

down

0.4

0.4

Momentum (1/Å)

Fig.1. Photoemission spectra of
Bi/MnGa thin film. (a) Angleintegrated spectrum taken with the
He-II source. (b) ARPES spectrum
along the G-X line taken with the HeI source.

Fig.2. Laser-SARPES spectra of Bi/MnGa thin film. (a) ARPES
spectrum along the G-X line taken with the 7-eV laser. (b) 2nd derivative
ARPES spectrum. The derivation has been performed along the
momentum direction. (c) SARPES spectrum. The spin direction isperpendicular to the G-X direction (y spin).

SWITCHABLE WEAK TOPOLOGICAL INSULATOR STATE INQUASI-ONE-DIMENSIONAL BISMUTH IODIDE
R. Noguchi,1 T. Takahashi,2 K. Kuroda,1 K. Yaji,1 A. Harasawa,1
S. Shin,1 T. Sasagawa,2 T. Kondo1
1
ISSP, University of Tokyo，2MSL, Tokyo Institute of Technology
A topological insulator (TI) is characterized by an insulating bulk with conductive edges
determined by Z2 topological invariants. In three dimensions, a TI is classified as either a strong
TI (STI) or a weak TI (WTI) [1], and the STI phase was experimentally confirmed rapidly after
the theoretical predictions through the observation of topological surface states (TSS) by angleresolved photoemission spectroscopy (ARPES) [2]. In contrast, the WTI phase has so far eluded
experimental verification, since TSSs emerge only on side surfaces, which are typically notcleavable and undetectable in real 3D crystals. In this work, we have investigated the WTI statein β-Bi4I4 with naturally cleavable top and side planes, which is necessary for the experimental
confirmation of the WTI state.
β-Bi4I4 is predicted to be in proximity of three different topological phase by theoretical
calculations: a STI (1;110) a WTI (0;001) or a normal insulator (0;000) phase is realized
sensitively depending on the calculation parameters [3, 4]. By using laser-SARPES, we havedemonstrated that a spin-polarized quasi-1D TSS exist in the bulk band gap of β-Bi4I4,
indicating the WTI phase in the real crystal. In contrast, α-Bi4I4 is predicted to be topologicallytrivial and no TSS has been detected by ARPES in this compound. From these results, we expectthat the phase transition between the WTI phase and the normal insulator phase occurs
following the structural transition between the β-phase and the α-phase. However, one may
wonder (i) why the observed Fermi surface shown in fig. 1(b) is quite anisotropic along ky and(ii) whether the topological phase transition really happens in single crystal piece. Here, we
discuss these points by using laser-(S)ARPES [5].
First, we investigated the photoemission intensity distribution of the topological surface stateson the side surface by conducting light-polarization dependent ARPES. The experimental
geometry for the ARPES measurements is shown in Fig. 1(a), where θ is the measurement angle
for each ARPES image. With p-polarized light, the photoemission intensity distribution isclearly anisotropic both at the Γ" point and the Z" point of the (100) surface Brillouin zone asshown in Fig. 1(c) and 1(e), where one branch of the Dirac dispersion (-kF) is clearly observed
while the photoemission intensity of the other part (+kF) is almost vanishing. In contrast, withs-polarized light, the photoemission intensities from both branches are almost the same asshown in Fig. 1(d) and 1(f). These results can be explained by the interference of photoemission
p-polarization s-polarization
x (z)

(a)

G

(b)

0 (c)

Z

49

s-pol
p-pol

b (ky)

eanalyzer

q

36

y

Chain direction

q (deg)50°hn

E-EF (eV)

72

G

(d)

G

q =72°
High

-0.2
-0.4

28

-0.1

0

0.1

-0.1

0

0.1

Low

-1

20

ky (Å )0 (e)p-pol.

E-EF (eV)100q =72°

Zq =49°(f)Z

q =49°-0.2-0.4

chain (ky)
-0.1

0

0.1

-0.1

0

0.1

-1

ky (Å )

Fig. 1 (a) Experimental geometry of laser-ARPES. (b) The Fermi surface mapping obtained with p-polarized light.
(c, d) ARPES band maps across the 𝚪" point of the (100) surface taken with p-polarized light and s-polarized light,
respectively. (e, f) Same as (c) and (d), but for the momentum cut across 𝐙" point of the (100) surface.

processes. Since the p-pol. light has both of in-plane and out-of-plane components [Fig. 1(a)],it excites electrons with different orbitals at the same time if the TSS is composed of multiple
orbitals. Generally, TSS consists in Bi-based topological insulators of Bi 6px,y,z orbitals due tothe strong spin-orbit coupling of bismuth [6], and hence the observed photoemission intensity
anisotropy is attributed to the result from the interference of photoelectrons from Bi 6py (inplane) and 6px (out-of-plane) orbitals excited by p-pol. light. In the case of ARPES with s-pol.
light, the interference effect becomes smaller because s-pol. light has smaller out-of-planecomponent and excites only Bi 6px orbital.
Second, we show that the topological phase transition between the WTI phase and normal
insulator phase follows the structural phase transition between β- and α-Bi4I4, and thus the
topological phase is switchable in this material. The procedure for demonstrating the phasetransition in single piece of Bi4I4 crystal is shown in Fig. 2(a) and 2(d). To unambiguously
investigate the effect of the structural transition, the same crystal pieces have been cleaved twice
after mild cooling and after rapid cooling. As shown in the ARPES images, quasi-1D TSS
observed after rapid cooling [Fig. 2(b)] disappeared after mild cooling [Fig. 2(c)], showing thephase transition from the WTI phase to the normal insulator phase. In addition, the band gap
observed after mild cooling [Fig. 2(e)] was gapped with the appearance of TSS after quenching[Fig. 2(f)], reflecting the phase transition from the normal insulator phase to the WTI phase.
From these results, it is concluded that the topological phase transition in fact follows the
structural phase transition and hence the topological phase in Bi4I4 can be controlled bytemperature.
In conclusion, these results support the switchable WTI phase in Bi4I4 observed in ourprevious measurements. Since the bulk of Bi4I4 is insulating, the TSSs can be probed not only
by ARPES but also by transport measurements. Importantly, the capability of controlling the
topological phase using structural phase transition is beneficial to making devices such as spintransistor. Our work will thus stimulate further in-depth study on exotic properties of the WTIstate and put the foundation for future WTI-based spintronic applications.

Fig. 2 (a) Experimental sequence to demonstrate the topological phase transition following the structural phasetransition from the β-phase to the α-phase using the same crystal piece. (b, c) ARPES images at θ=0° after
quenching (b) and after mild cooling (c) for the same crystal piece. (d) Same as (a), but from the α-phase to the βphase. (e, f) ARPES images at θ=0° after mild cooling (e) and after quenching (f) for the same crystal piece.

REFERENCES[1] L. Fu, C. L. Kane, and E. J. Mele, Phys. Rev. Lett., 98, s106803 (2007).
[2] Y. Xia et al., Nat. Phys. 5, 398 (2009).
[3] G. Autès et al., Nat. Mater. 15, 154 (2015).
[4] C.-C. Liu et al., Phys. Rev. Lett. 116, 66801 (2016).
[5] R. Noguchi et al., Nature 566, 518 (2019).
[6] H. Zhang, C.-X. Liu, and S.-C. Zhang, Phys. Rev. Lett. 111, 066801 (2013).

Electronic and spin structure of heavy element
thallium single crystal thin films
Yoshitaka Nakata1, Takahiro Kobayashi1, Koichiro Yaji2, Kazuyuki Sakamoto1,3,4
1

2

Department of Materials Science, Chiba University
Institute for Solid State Physics, the University of Tokyo, Kashiwa
3
Molecular Chirality Research Center, Chiba University
4
Department of Material and Life Science, Osaka University

Introduction
In solid bulk, the two bands with opposite spins are usually degenerated due to the presenceof both the time-reversal and space-inversion symmetries. However, in case of surface orinterface, where the space-inversion symmetry is broken, the spin degeneracy is lifted by the
spin-orbit coupling (SOC). This is called the Bychkov-Rashba effect or simply the Rashba effect[1]. In the early studies, the observation of giant Rashba effects on surface alloys formed by theadsorption of 1/3 ML of high-Z elements Bi or Pb on the Ag(111) surface [2,3] has attracted a
lot of interest. The spin-split was discussed based on a simple Rashba effect in these studies.
More recently, several peculiar Rashba spins have been observed, and their origins werediscussed based on e.g., the symmetry of the surface [4]. Tl atomic layers formed on Si surfaces
are one of such exotic systems. Together with this interesting spin-related phenomenon, Tl isalso known to exhibit superconductivity at below 2.4 K in crystal phase [5]. However, due tothe difficulty to create a single Tl crystal that is large enough to perform photoemission
measurements, there is no experimental report on its band structure so far. Moreover, since a Tl
single crystal film would show spin-split bands and thus has the possibility of becoming a
topological superconductor [6], we planned to create a Tl single crystal film and measure its
band structure.
Experimental details
The Ag(111) surface was first sputtered with Ar+ with an accelerating voltage of 500 V inhigh vacuum (5x10-3 Pa) to obtain a clean surface. The quality of the Ag(111) surface wasconfirmed by the observation of sharp LEED spots and a clear surface state. The Tl single
crystal film was then prepared by the deposition of 2 ML of Tl on the Ag(111) clean surface.
The LEED pattern of the Tl/Ag(111) surface is shown in Fig. 1. Clear and sharp spots
originating from the Tl double-layer single crystal are observed. ARPES and SARPES were
performed using the laser-SARPES machine developed at ISSP [7]. All PES measurement was
carried out at a sample temperature of 35 K under ultra-high vacuum (ca.1×10 -8 Pa).
Results and discussion

As shown in Fig. 1, Tl double-layer single crystal thin film has two domains that are rotated± 4° from the symmetric axis of Ag(111). This make the fermi surface and band structurecomplicated as shown in Figs. 2 (a) and (b). By considering the number of surface states�Ag direction, we found that the Tl single
observed in the band dispersion along the 𝛤𝛤� − 𝐾𝐾

crystal film has two metallic bands (as shown in Fig. 2 (a) the bands of Tl are degenerated in�Ag direction). Of the two metallic bands, the one with lower binding energy at thethe 𝛤𝛤� − 𝐾𝐾
� point of the Tl Brillouin zone shows a clear split in the energy direction. In order to
𝐾𝐾

investigate the origin of this splitting, we performed SARPES measurement, and found that the� point is a spin split, and the spin-polarization vector point along the wave vector
split at the 𝐾𝐾

directions. This means that the origin of this spin-split is not an ordinary Rashba effect, and weare now tackling this problem by considering other possible origins.
(a)Fig. 1 LEED pattern of
Tl/Ag(111) obtained with
an electron energy of 85 eV.

Summary(b)

� point of the Brillouin zone ofFig. 2 (a) Fermi surface around the 𝐾𝐾
Tl. Green lines indicate the Brillouin zones of Tl from the twodomains. (b) Band structure cut at the white dashed line in (a).

Summary
We obtained a Tl double-layer single crystal film on a Ag(111) surface that is large enough tomeasure its band structure. The presence of two metallic bands and a split of one of them at the� point of the Tl Brillouin zone was confirmed by ARPES. This split is a spin-split, and the
𝐾𝐾
spin-polarization vector point along the wave vector directions.
References[1] Y. A. Bychkov and E. I. Rashba, JETP Lett. 39, 78 (1984).
[2] C. R. Ast et al., Phys. Rev. Lett. 98, 186807 (2007).
[3] F. Meier, H. Dil, J. Lobo-Checa, L. Patthey, and J. Osterwalder, Phys. Rev. B 77, 165431 (2008).
[4] for example, E. Annese et al., Phys. Rev. Lett. 117, 016803 (2016).
[5] T. Imamura, K. Okamoto, M. Saito, and M. Ohtsuka. J. Phys. Soc. Jpn, 40, 1256 (1976).
[6] J. D. Sau et al., Phys. Rev. Lett. 104, 040502 (2010).
[7] K. Yaji et al., Rev. Sci. Instrum., 87, 053111 (2016).

SPIN-POLARIZED BANDS OF AN IN BYLAYER FILM FORMED ON
Si(111)
Takahiro Kobayashi1, Yoshitaka Nakata1, Koichiro Yaji2, Shunsuke Yoshizawa4, Fumio
Komori2, Shik Shin2, Takashi Uchihashi3, Kazuyuki Sakamoto1,4,5
1Department of Materials Science, Chiba University
The Institute for Solid State Physics, the University of Tokyo
3
National Insutitute for Materials Science4
Molecular Chirality Research Center, Chiba University
5
Department of Material and Life Science, Osaka University

2

Introduction
In two-dimensional (2D) systems such as surfaces or heterointerfaces, the electron bands are
split due to spin-orbit coupling and the space inversion asymmetry along the direction
perpendicular to the plane. This effect is called the Rashba effect [1], which leads the spinpolarization vectors to be parallel to the 2D plane and perpendicular to the wave vector in an
ideal 2D system. This Rashba effect was observed on the surface of heavy elements [2], heavy
element atomic layers formed on solid surfaces [3] and so on. One of the main motivations ofthese early studies was to create a system with large Rashba splitting that can be used as amaterial for spintronics devices. In this project, motivated by the theoretical work reported by
Sau et al. [4], we focus on the spin-polarized band induced in such atomic layers that has
potential to become topological superconductor. The system that we studied is the In/Si (111)(√7 × √3) surface, a system that is formed by the adsorption of 2.4 ML of In ontop of Si(111)[5] and shows superconductivity below ca. 3 K [6].
Experimental details
Angle-resolved photoelectron spectroscopy (ARPES) and spin-resolved ARPES (SARPES)
measurements were performed with the laser-SARPES machine developed at ISSP [7]. A laser
light source with a photon energy (hν) of 6.994 eV was used as the excitation light, and a Scienta
Omicron DA30-L analyzer equipped with a VLEED spin detector was used for PES
measurements. An n-type doped Si(111) wafer with a resistivity of < 0.01Ω•cm was firstannealed at 1520 K to obtain a clean Si(111)-( 7 × 7 ) surface. Following the adsorption ofslightly more than 2.4 ML of In, the sample was annealed at 570 K to obtain the In/Si (111)(√7 × √3) surface. The quality of the sample and the presence of a single (√7 × √3) domain was
confirmed by low-energy electron diffraction (LEED) (Fig. 1) and the observation of clear
surface states. All PES measurement was carried out at a sample temperature of 35 K underultra-high vacuum (ca.1×10 -8 Pa).

Results and discussion
By using ARPES, we observed a circular Fermi surface together with a butterfly shape one.
Although these Fermi surfaces shows good agreement with those reported previously on In/Si(111)-(√7 × √3) [8,9], thanks to the high k-resolution of the present setup (∆𝑘𝑘 = 1.4 × 10−3 Å−1),
we could observed a clear split as shown in Fig. 2 (note that not only the circular Fermi surface
shown in Fig. 2 but also the butterfly shape one shows a clear split). Since this split might
originate from the Rashba effect, we have performed SARPES measurements to obtain
information about the spin polarization of the split Fermi surfaces. Of the two Fermi surfaces,
the spin-polarization vector of the circular one was parallel to the surface and perpendicular to
the wave vector. This result indicates that the spin-split of the circular Fermi surface might
originate from the Rashba effect. In contrast to the circular Fermi surface, the spin-polarization

vector of the butterfly shape Fermi surface was completely different, and cannot be explainedby the Rashba effect. The spin-polarization vector of the butterfly shape Fermi surface wasparallel to the surface, and also parallel to the wave vector. Since the peculiar spin-polarization
vector of the butterfly shape Fermi surface was observed at k point without any symmetry (a kpoint with symmetry means a point like the Γ point of the Brillouin zone), this cannot be apeculiar spin originated from the symmetry of the surface like those observed in our former
studies [10] as well. We are now tackling this problem by considering other possible origins,and hope to get the answer soon.
Summary
By using a high-energy and high-k resolution SARPES system, we succeeded to observe thespin splits Fermi surfaces of In/Si (111)-(√7 × √3), a system that becomes superconductor at
below 3 K. The spin-polarization vector of one of the observed Fermi surfaces cannot be explainby neither a simple Rashba effect nor a symmetry induced peculiar Rashba effect.

Fig. 1. LEED image of the In/Si (111)-(√7 × √3) surface
obtained with an electron energy of 85 eV.

Fig. 2. Fermi surface of In/Si (111)-( √7 × √3 ). The
circular Fermi surface shows a clear split.

REFERENCES[1] Y. A. Bychkov and E. I. Rashba, JETP Lett. 39, 78 (1984).
[2] for example, G. Nicolay, F. Reinert, S. Hüfner, and P. Blaha, Phys. Rev. B 65, 033407 (2001).
[3] for example, C. R. Ast et al., Phys. Rev. Lett. 98, 186807 (2007).
[4] J. D. Sau et al., Phys. Rev. Lett. 104, 040502 (2010).
[5] K. Uchida et al., Phys. Rev. B 87, 165433 (2013).
[6]T. Uchihashi et al., Phys. Rev. Lett.. 107, 207001 (2011).
[7] K. Yaji et al., Rev. Sci. Instrum., 87, 053111 (2016).
[8] E. Rotenberg et al., Phys. Rev. Lett.. 91, 246404 (2003).
[9] S. Yoshizawa et al., Nano Lett. 17, 2287 (2017).
[10] for example, K. Sakamoto et al., Phys. Rev. Lett. 102, 096805 (2009), 117, 106803 (2016).

MULTIPLE TOPOLOGICAL STATES IN IRON-BASEDSUPERCONDUCTORS
Peng Zhang1, Zhijun Wang2, Xianxin Wu3, Koichiro Yaji1, Yukiaki Ishida1, Yoshimitsu
Kohama1, Guangyang Dai4, Yue Sun1, Cedric Bareille1, Kenta Kuroda1, Takeshi Kondo1, Kozo
Okazaki1, Koichi Kindo1, Xiancheng Wang4, Changqing Jin4, Jiangping Hu4, Ronny Thomale3,
Kazuki Sumida6, Shilong Wu6, Koji Miyamoto6, Taichi Okuda6, Hong Ding4, G. D. Gu7,
Tsuyoshi Tamegai5, Takuto Kawakami8, Masatoshi Sato8 and Shik Shin1
1

Institute for Solid State Physics, University of Tokyo, Kashiwa, Japan. 2Department of Physics,
Princeton University, Princeton, NJ, USA. 3Theoretical Physics, University of Wurzburg Am Hubland,
Wurzburg Germany 4Institute of Physics, Chinese Academy of Sciences, Beijing, China. 5Department
of Applied Physics, University of Tokyo, Tokyo, Japan. 6Hiroshima Synchrotron Radiation Center,
Hiroshima University, Higashi-Hiroshima, Japan. 7Brookhaven National Laboratory, Upton, NY,
USA. 8Yukawa Institute for Theoretical Physics, Kyoto University, Kyoto, Japan.

Topological materials and unconventional iron-based superconductors are both important areasof study but, to date, relatively little overlap has been identified between these two fields.
However, the combination of topological bands and superconductivity promises the
manifestation of exotic superconducting states, including Majorana fermions, the centralcomponent of topological quantum computation. Here, using high-resolution spin-resolved
ARPES, we have identified both topological insulator (TI) and Dirac semimetal (TDS) statesnear the Fermi energy in different iron-based superconducting compounds, potentially allowing
access to several different superconducting topological states in the same material. These results
reveal the generic coexistence of superconductivity and multiple topological states in ironbased superconductors, indicating that this broad class of materials is a promising platform forhigh-temperature topological superconductivity.
The topological insulator and Dirac semimetal states in Li(Fe,Co)As.

Figure 1 Spin polarization of the Diraccones from TI and TDS states inLi(Fe1−xCox)As. (a) Intensity plot along kx
for x = 3% sample with a 7-eV laser. (b)Sketch of the spin-polarized surface bands.
The blue and red lines stand for opposite
spin polarizations, as confirmed by the datain (c). (c) Spin polarization along x
direction at Cut 1 and Cut 2, which areindicated by two vertical lines in (a) and(b). (d–f) Same as (a–c), but for samples
with x = 9%. (g-i) same as (a–c), but for
samples with x = 12%.

We studied a series of Li(Fe,Co)As samples with different Co content by using laser-based
ARPES. On the x = 3% samples, only one Dirac band was observed below Fermi level (Fig.
1(a)), similar to the case of Fe(Te,Se) [1,2]. By doping more electrons, we further observed
another Dirac cone in the x = 9% and 12% samples, as shown in Fig. 1(d,g), which is above
Fermi level in the parent compound. The two Dirac cones are consistent with the DFT

calculations: the upper cone comes from the the surface component of the TDS states, while
the lower cone comes from the TI surface states. Because of their surface nature, both Diraccones should be spin-polarized, and the calculated spin patterns are sketched in Fig. 1(b,e,h).
We further carried out spin-resolved measurements to check their surface nature. A pair of spinpolarized energy distribution curves along ky was measured, as indicated in Fig. 1(b,e,h). For
both x = 3% and 9% samples, the spin polarization of the TI Dirac cone is resolved (Fig. 1(c,f)),and consistent with theoretical calculations. For x = 9% and 12% samples, the spin polarizationof the TDS surface cone is observed, as shown in Fig. 1(f,i). Due to the the weak intensity of
the upper part of the TDS cone, only the spin polarization of the lower part is resolved.
The topological Dirac semimetal states in Fe(Te,Se).
The TI surface states in Fe(Te,Se) was observed previously in Ref. [2]. The TDS cone in
Fe(Te,Se) cannot be resolved directly, because of the difficulty to dope this sample. However,
we may obtain some evidences of the TDS bands in Fe(Te,Se) by measuring the associatedsurface states, as shown in Fig. 2(a). The spin-resolved intensity plots of Fe(Te,Se) are shownin Fig. 2(b). We measured four cuts with high resolution at high symmetry lines, as indicatedin Fig. 2(c). Fig. 2(d-g) display the spin polarization curves for the four cuts. The magnitude ofspin polarization of the band is about 50%, indicating coexistence of unpolarized bulk andpolarized surface states. The results are consistent with the spin-resolved spectrum from
calculations in Fig. 2(a), thus support the existence of the TDS states above EF in Fe(Te,Se).
Figure 2 Spin polarization of the band at EF
and its Dirac semimetal origin in Fe(Te,Se).
(a) Spin-resolved (001) surface spectrum
from a k·p model calculation. (b) Spin-resolved
intensity plot at Γ with a 7-eV laser. (c) Sketchof the spin-orbit pattern from model
calculations. (d-g) Spin-resolved energy
distribution curves at Cuts 1–4. Because of thematrix element effect, Cuts 1,3 and 2,4 aretaken with s-polarized and p-polarized
photons, respectively.

Our findings of the TI and TDS states in Li(Fe,Co)As and Fe(Te,Se) prove the generic existenceof different types of topological states in iron-based superconductors. Their simple structures,
multiple topological states, and a tunable Fermi level make iron-based superconductors ideal
platforms for the study of topological superconductivity, Majorana bound states, and as such,potentially, topological quantum computation.
References:[1] Z. Wang et al., Phys. Rev. B 92, 115119 (2015).
[2] P. Zhang et al., Science 360, 182 (2018).
[3] P. Zhang et al., Nature Phys. 15, 41 (2019).

ELECTRONIC BAND STRUCTURE OF TRANSFER GRAPHENE
ON SI(111) SUBSTRATE
Koichiro Yaji, Fumio Komori and Shik Shin
Synchrotron Radiation Laboratory, The Institute for Solid State Physics, The University of Tokyo

Electronic band structure of graphene, which has a linear dispersion around a K point, has
attracted much attention not only because of fundamental interest but also possible applicationsof their electronic conduction. The epitaxial graphene has been experimentally made on
substrates by several methods so far. For example, the graphene grown on Si-terminated surfaceof SiC(0001) has been well known and established. In contrast to the former studies, in thepresent study, we have fabricated the single-layer graphene on a Si(111) substrate by transfer
method and have investigated the electronic band structure of the transfer graphene by angleresolved photoemission spectroscopy (ARPES).
We used monolayer graphene film made for transfer that was commercially available. The
Si(111) substrate was cut from a wafer. The transfer procedure was as follows: The sacrificiallayer/graphene was put in deionized water. The Si(111) substrate was introduced into thedeionized water and scoop the sacrificial layer/graphene from below. After that the sacrificiallayer/graphene was deposited on the Si(111) substrate [Fig. 1(a)]. Then, the sacrificial
layer/graphene/substrate was taken out from the water and let it dry 30 minutes in air.
Subsequently, the sample is heated in an oven at 150°C for 1.5h. And then, the sample is stored
in a vacuum desiccator for 24h to avoid detachment of the graphene from the substrate. The
sacrificial layer is removed in acetone for 1h, and then the sample is put into ethanol for 1h.
After this procedure, the sample was introduced into the UHV chamber. And then, the sample
were annealed at 400C for 30 min. The surface quality was checked by low energy electron
diffraction, where we observed a ring pattern due to the multi domain structure of the transfer
graphene. Here, the average size of one domain is approximately several micrometers.
ARPES measurements were performed at the Institute for Solid State Physics, The Universityof Tokyo [1]. The photoelectrons were excited by 21.2-eV photons from a He lamp and wereanalyzed with a ScientaOmicron DA30-L analyzer. The sample temperature was set to 40K
during the measurements.
Figure 1(b) shows the ARPES image of thetransfer graphene on Si(111) around the K point.
We find a Dirac-cone like band for which thespectral width is broadened due to the multi domain
structure of the surface. The Dirac point is locatedat the binding energy of 0.2-0.3 eV. The doping
level of the p-band is smaller than that of thegraphene on SiC(0001) substrate [2]. In the present
ARPES measurements, the spot size of our He lamp Fig. 1(a) Photo of the sacrificialis 0.5 mm on the sample. We could observe a clear layer/graphene scooping on the Si(111)band image if we would use the excitation light substrate. (b) ARPES image of thetransfer graphene on Si(111).
with the spot size on the order of micrometers.
REFERENCES[1] K. Yaji et al., Rev. Sci. Instrum. 87, 053111 (2016).
[2] K. Nakatsuji et al., Phys. Rev. B 82, 045428 (2010).

STUDY ON ELECTRONIC STRUCTURES IN HALF-METALLICFERROMAGNET CoS2 BY HIGH-RESOLUTION SPIN- AND
ANGLE-RESOLVED PHOTOEMISSION SPECTROSCOPY
Hirokazu Fujiwara1, K. Terashima2, T. Wakita2, Y. Yano1, W. Hosoda1, N. Kataoka1, A.
Teruya3, M. Kakihana3, M. Hedo4, T. Nakama4, Y. Ōnuki4, K. Yaji5, A. Harasawa5, K. Kuroda5,
S. Shin5, Y. Muraoka1,2, and T. Yokoya1,21Graduate School of Natural Science and Technology, Okayama University, Okayama 700-8530, Japan
Research Institute for Interdisciplinary Science, Okayama University, Okayama 700-8530, Japan
3
Graduate School of Engineering and Science, University of the Ryukyus, Nishihara, Okinawa 903-0213, Japan
4
Faculty of Science, University of the Ryukyus, Nishihara, Okinawa 903-0213, Japan
5
Institute for Solid State Physics, The University of Tokyo, Kashiwa, Chiba 277-8581, Japan2

CoS2 is a ferromagnetic metal whose ordered moment is reported to be approximately 0.9µB/Co with an easy-axis along the [111] direction [1–3]. The ordered moment is close to 1
µB/Co in a low-spin state of Co2+ (configuration: t2g6eg1), suggesting that CoS2 has a
half-metallic electronic structure including completely spin polarized eg bands crossing the
Fermi level (EF). The half-metallicity has been also supported by theoretical band structure
calculated within generalized gradient approximation (GGA) and experimental studies using
de Haas–van Alphen (dHvA) effect [4,5]. On the other hand, the results of band calculation
within local spin-density approximation (LSDA) and photoemission spectroscopy
measurements indicated that CoS2 has a “nearly” half-metallic electronic structure in which abottom of the minority spin eg band touches or slightly crosses EF near the R point [6]. So far,comprehensive understanding of half-metallicity of CoS2 has not been obtained. In this work,
we performed high-resolution spin- and angle-resolved photoemission spectroscopy(SARPES) measurements on a high-quality CoS2 single crystal in order to reveal thehalf-metallicity.
Single crystals of CoS2 were synthesized by the vapor transport method,[3] and were used
for SARPES measurements. Laser-based SARPES experiments were performed at theInstitute for Solid State Physics, The University of Tokyo [7]. The p-polarized light with hν =6.994 eV was used to excite the photoelectrons. Photoelectrons were analyzed with a
combinationofaScientaOmicron
DA30L
analyzerandavery-low-energy-electron-diffraction (VLEED) type spin detector. The energy and angularresolutions were set to 6 meV and 0.3˚ (corresponding to ~0.005 Å) for spin-integrated
ARPES and 10 meV and 1˚ (corresponding to ~0.02 Å) for SARPES, respectively.
Calibration of EF for the samples was achieved using a gold reference. The data were taken at
T = 40 K. Clean surfaces for all measurements were obtained by in situ cleaving of thesamples.
Our high-resolution SARPES results are summarized in Fig. 1. By the 7-eV laser, weobserved bands located at –1 eV, –0.5 eV, –50 meV, near EF, and a dispersive hole band. The
bands located around –1 eV are assigned to t2g bands based on a photoemission spectroscopy
study [6].
For the eg bands located between –0.5 eV and EF, we obtained the spin-polarized band
structure clearly. We named the eg bands as shown in Figs. 1 (a) and (d). At –0.5 eV, the
SARPES intensity map for the majority spin bands has a clear structure corresponding to theα band, although that for the minority spin bands has no structure at –0.5 eV. In the same way,
it is identified that the β band has the majority spin character. The result that the majority spincharacter of the α and β bands is consistent with the theoretical band structures.[4] On theother hand, the SARPES intensity map for the minority spin bands has clear band dispersion
corresponding to the γ and δ bands.

Here, we discuss the half-metallicity of CoS2. We observed both of the majority andminority spin bands crossing EF, as shown in Fig. 1, providing spectroscopic evidence for‘normal’ ferromagnetic metallicity, not half-metallicity, of CoS2. This result is consistent withclaims by a photoemission studies.[6] The non-half-metallicity is also supported by the factthat the ordered moment µs ~ 0.9 µB/Co at 2 K[3] is not exactly µs = 1 µB/Co which is the
expected ordered moment when the eg electrons are completely spin-polarized.
In summary, we performed high-resolution SARPES in order to reveal the electronic
structure of ferromagnetic CoS2. We observed t2g bands and highly spin-polarized eg bands.
We observed a minority spin band crossing EF, demonstrating that CoS2 is not a half-metallic
ferromagnet at least above 40 K. This report demonstrates that the high-resolution SARPES isa powerful tool to identify half-metallicity of theoretically proposed borderline half-metallic
ferromagnets.

Fig. 1. (a)–(c) ARPES (a) and SARPES ((b) Majority spin bands; (c) Minority spin bands) intensity maps asa function of energy to EF and kx. (d)–(f) Same as (a)–(c) but measured in the vicinity of EF.

REFERENCES[1] H. S. Jarrett et al., Phys. Rev. Lett. 21, 617 (1968).
[2] H. Hiraka et al., J. Phys. Soc. Jpn. 63, 4573 (1994).
[3] A. Teruya et al., J. Phys. Soc. Jpn. 85, 064716 (2016).
[4] T. Shishidou et al., Phys. Rev. B 64, 180401(R) (2001).
[5] R. Yamamoto et al., Phys. Rev. B 59, 7793(R) (1999).
[6] Takahashi et al., Phys. Rev. B 63, 094415 (2001).
[7] K. Yaji et al., Rev. Sci. Instrum. 87, 053111 (2016).

ANGLE-RESOLVED PHOTOEMISSION SPECROSCOPY STUDY OF
TYPE-II WEYL SEMIMETAL WTe2
Yuxuan Wan1,2, Lihai Wang3, Keisuke Koshiishi1, Masahiro Suzuki1, Jaewook Kim4, Kenta
Kuroda2, Koichiro Yaji2, Ryo Noguchi2, Ayumi Harasawa2, Shik Shin2, Sang-Wook Cheong3,
Takeshi Kondo2, Atsushi Fujimori1
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 a Weyl fermion has been observed in particle physics research. But
some topological materials can host electrons behaving as Weyl fermions, which are called
Weyl semimetals. In Weyl semimetals, there are bands crossing linearly near Weyl points,
which means that electrons near Weyl points have zero-mass [1-3].
In some Weyl semimetals, the hole- and electron-like pockets touch each other at several
Weyl points, and these kinds of Weyl semimetals are called type-II Weyl semimetals. Weyl
points are topologically protected, and topological surface states (Fermi arcs) connected a pairof Weyl points with opposite chirality. WTe2 is a kind of quasi-2D type-II Weyl semimetal. The
space inversion symmetry along c-axis is broken, and the top and bottom surfaces are different.
According to theoretical calculation, the surface electronic states of the two surfaces aredifferent [4]. However, so far there is no experimental research showing the differences between
the two surface states. On the other hand, some of Weyl points in WTe2 locate above the Fermi
level (EF), which cannot be observed by angle-resolved photoemission spectroscopy directly,
and neither the Fermi arcs above EF have been observed. On the other hand, it is difficult to say
whether the band structure is topological surface state or not by ARPES. Spin-ARPES canprovide more evidence.
Experiment
The single crystals of WTe2 with large domains were grown via the iodine vapor transport
method [4]. We cut one piece of WTe2 sample into two halves and turn one of them upside down.
We cleaved them in situ.
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 ofsurfaces of WTe2: surface A
and B. We used the 7 eV
laser ARPES to map theenergycontours
andmeasuredtheband
structures. To observe thetopological Fermi arcs are
Fig.1 Band dispersion on surface A and B of WTe2 measured by laser
surfaces
stateswhich ARPES. The surface states connect the bulk electron- and hole-like
connect hole and electron pocket.

pockets, and parts of them are above the EF, we measured the samples at a high temperature(over 150 K) which unoccupied states about 30 meV above EF could be seen. SARPES
measurements on candidate topological surface states were taken after the ARPES
measurements.
Results
We measured two kinds of surfaces of WTe2: surfaces A and B with 7 eV laser-ARPES. In
Fig. 1, we can see the band structures where the surface state connected the hole- and electronlike bulk states on both surface A and B. The surface states of the opposite surfaces are supposed
to be different.
Figure 2 and 3 show the SARPES results of the candidate topological surface states. For
surface A (Fig. 2), clear spin reversal could be observed for the surface states for both x- and ydirection. The surface state for surface A is spin-polarized. For surface B (Fig.3), the spinpolarization along x-direction could be observed, but no clear evidence showing that the surface
state on surface B is spin-polarized along y-direction.
We observed two kinds of surfaces 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 isimportant evidence for that the observed surface states are topological Fermi arcs.

Fig.2 (Left)The spin-polarization along x- andy-direction on surface A of WTe2.

Fig.3 (Right) The spin-polarization along xdirection on surface B of WTe2.

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)

６．Staff
Director:

SHIN Shik, Professor
KOMORI Fumio, Professor
HARADA Yoshihisa, Professor
MATSUDA Iwao, Associate Professor
WADATI Hiroki, Associate Professor
YAMAMOTO Susumu, Research Associate
YAJI Koichiro, Research Associate
MIYAWAKI Jun, Research Associate
HIRATA Yasuyuki, Research Associate
FUKUSHIMA Akiko, Technical Associate
HARASAWA Ayumi, Technical Associate
SHIBUYA Takashi, Technical Associate
KUDO Hirofumi, Technical Associate

Technical Assistant:

FUJISAWA Masami
KOSEGAWA Yuka
ARAKI Mihoko

Secretary:

AIHARA Yumiko
HARADA Misa
YOSHIZAWA Motoko
TSUTSUMI Yumiko
IKEDA Kuniko
HIROMOTO Chiho

Graduate Student:

ITO Suguru
YAMAMOTO Kohei
IKEDA Keisuke
TSUKADA Tomoyuki
WATANABE Ryusuke
UGALINO Ralph John

Contract Researcher:

OSHIMA Masaharu
TAKUBO Kou
CUI Yitao
AKADA Keishi
YAMAZOE Kosuke
YAMAGAMI Kohei
ZHANG Yujun
OHDAIRA Takeshi (2018.5~)
EL Moussaoui Souliman (2018.9~)

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JournalVol
Keishi Akada, Takaaki Sudayama,
Operando measurement of single crystalline
J.Electron
Daisuke Asakura, Hirokazu Kitaura,
Li4Ti5O12 with octahedral-like morphology
233
Naoka Nagamura, Koji Horiba, Masaharu Spectrosc.Relat.
by microscopic X-ray photoelectron
Phenom.
Oshima, Eiji Hosono, and Yoshihisa
spectroscopy
HaradaKeishi Akada, Takaaki Sudayama,
Operando measurement of single crystalline
J.Electron
Daisuke Asakura, Hirokazu Kitaura,
Li4Ti5O12 with octahedral-like morphology
233
Naoka Nagamura, Koji Horiba, Masaharu Spectrosc.Relat.
by microscopic X-ray photoelectron
Phenom.
Oshima, Eiji Hosono, and Yoshihisa
spectroscopy
HaradaI. Tateishi, N. T. Cuong, C. A. S. Moura,
M. Cameau, R. Ishibiki, A. Fujino, S.
Semimetallicity of free-standing
Okada, A. Yamamoto, M. Araki, S. Ito, S. Phys. Rev.
3hydrogenated monolayer boron from MgB2 Yamamoto, M. Niibe, T. Tokushima, D. E. Materials
Weibel, T. Kondo, M. Ogata, and I.
MatsudaMass transport in the PdCu phase
Jiayi Tang, Susumu Yamamoto, Takanori
structures during hydrogen adsorption
Koitaya, Akitaka Yoshigoe, Takuma
Appl. Surf. Sci.
480
andabsorption studied by XPS underTokunaga,Kozo Mukai, Iwao Matsuda,hydrogen atmosphere
Jun Yoshinobu
Enhanced Photoresponsivity of Fullerene in
Kenichi Ozawa, Susumu Yamamoto, Marie
the Presence of Phthalocyanine: A TimeD’angelo, Yuto Natsui, Naoya Terashima, J. Phys. Chem. C
123
Resolved X-ray Photoelectron Spectroscopy
Kazuhiko Ma, and Iwao Matsuda
Study of Phthalocyanine/C60/TiO2(110)
Hydrogen adsorption and absorption on a
J. Tang, S. Yamamoto, T. Koitaya, Y.
Pd-Ag alloy surface studied using in-situ XYoshikura, K. Mukai, S. Yoshimoto, I.
Appl. Surf. Sci.
463
ray photoelectron spectroscopy underMatsuda, J. Yoshinobu
ultrahigh vacuum and ambient pressure
Yusuke Hashimoto Munetaka Taguchi
Site‐sensitive X‐ray photoelectron
Shun Fukami Hiroki Momono Tomohiro Surface andspectroscopy of Fe3O4 by photoelectron
51
Matsushita Hiroyuki Matsuda Fumihiko Interface Analysis
diffraction
Matsui Hiroshi Daimon
Keiichi Omika, Yasunori Tateno, Tsuyoshi
Kouchi, Tsutomu Komatani, Seiji
Operation Mechanism of GaN-based
Yaegassi, Keiichi Yui, Ken Nakata, Naoka
Transistors Elucidated by Element-Specific
Scientific Reports
8
Nagamura, Masato Kotsugi, Koji Horiba,
X-ray Nanospectroscopy
Masaharu Oshima, Maki Suemitsu &Hirokazu Fukidome
Monatomic Two-Dimensional Layers:
Modern Experimental Approaches for
Iwao Matsuda
Elsevier
Structure, Properties, and Industrial Use(Micro and Nano Technologies)
Sh. Yamamoto, Y. Kubota, K. Yamamoto,Y. Takahashi, K. Maruyama, Y. Suzuki, R.
Hobara, M. Fujisawa, D. Oshima, S.
Owada, T. Togashi, K. Tono, M. Yabashi,
Y. Hirata, S. Yamamoto, M. Kotsugi, H.
Femtosecond resonant magneto-optical Kerr
Wadati, T. Kato, S. Iwata, S. Shin, I.
effect measurement on an ultrathin
Matsuda1. Sh. Yamamoto, Y. Kubota, K. JJAP
57
magnetic film in a soft X-ray free electron
Yamamoto, Y. Takahashi, K. Maruyama,laser
Y. Suzuki, R. Hobara, M. Fujisawa, D.
Oshima, S. Owada, T. Togashi, K. Tono,
M. Yabashi, Y. Hirata, S. Yamamoto, M.
Kotsugi, H. Wadati, T. Kato, S. Iwata, S.
Shin, I. Matsuda
Interfacial carrier dynamics of graphene on
T. Someya, H. Fukidome, N. Endo, K.
SiC, traced by the full-range time-resolved
Appl. Phys. Lett.
113
Takahashi, S. Yamamoto, I. Matsuda
core-level photoemission spectroscopy
Probing the Valence Electronic Structure of A. W. Hahn, B. E. Van Kuiken, V. G.
Low-Spin Ferrous and Ferric Complexes
Chilkuri, N. Levin, E. Bill, T. Weyhermü
Inorg. Chem.
57
Using 2p3d Resonant Inelastic X-rayller, A. Nicolaou, J. Miyawaki, Y. Harada
Scattering (RIXS).
and S. DeBeer
Electronic States of Quinones for Organic
Naoka Nagamura, Ryosuke Taniki, Yuta
ACS Applied
Energy Devices: The Effect of Molecular
Kitada, Asuna Masuda, Hiroaki
1
Energy Materials
Structure on Electrochemical
Kobayashi, Nobuto Oka, and Itaru Honma
Characteristics
Effect of amino group protonation on the
Y. Horikawa, T. Tokushima, O.
Phys. Chem.
carboxyl group in aqueous glycine observed Takahashi, Y. Harada, A. Hiraya and20
Chem. Phys
by O 1s X-ray emission spectroscopy.
S.Shin

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S. Yamamoto, K. Takeuchi, Y. Hamamoto,
Enhancement of CO2 adsorption on oxygen- R.‐Y.Liu, Y. Shiozawa, T. Koitaya, T.
Phys. Chem.
functionalized epitaxial graphene surface
Someya, K. Tashima, H. Fukidome, K.
Chem. Phys
under near-ambient conditions
Mukai, S. Yoshimoto, M.Suemitsu, Y.
Morikawa, J. Yoshinobu and I.Matsuda
Electronic Structure of Ce-Doped and M. Horio, Y. Krockenberger, K. Yamamoto,
Undoped Nd2CuO4 Superconducting Thin Y. Yokoyama, K. Takubo, Y.
Films Studied by Hard X-Ray
Hirata, S. Sakamoto, K. Koshiishi, A.
Phys. Rev. Lett.
Photoemission and Soft X-Ray Absorption
Yasui, E. Ikenaga, S. Shin, H.
Spectroscopy
Yamamoto, H. Wadati, and A. Fujimori
Electronic States of Quinones for Organic
Naoka Nagamura, Ryosuke Tanik, Yuta
ACS Appl. Energy
Energy Devices: The Effect of Molecular
Kitad, Asuna Masuda, Hiroaki Kobayash,
Mater
Structure on Electrochemical
Nobuto Oka, and Itaru Honma
Characteristics
Masaharu Oshima, Daisuke Mori, Aki
Photoelectron Nano-spectroscopy of
e-Journal of
Takigawa, Akihiko Otsuki, Naoka
Reactive Ion Etching-Induced Damages to
Surface Science
Nagamura, Shun Konno, Yoshinobu
the Trench Sidewalls and Bottoms of 4Hand
Takahashi, Masato Kotsugi, Hiroshi
SiC Trench-MOSFETs
Nanotechnology
NohiraSh. Yamamoto, T. Omi, H. Akai, Y.
Kubota, Y. Takahashi, Y. Suzuki, Y.
Hirata, K, Yamamoto, R. Yukawa, K.
Element Selectivity in Second-Harmonic
Horiba, H. Yumoto, T. Koyama, H. Ohashi,
Generation of GaFeO3 by a Soft-X-Ray
Phys. Rev. Lett.
S. Owada, K. Tono, M. Yabashi, E.
Free-Electron Laser
Shigemasa, S. Yamamoto, M. Kosugi, H.
Wadati, H. Kumigashira, T. Arima, S.
Shin, I. Matsuda
Benjamin E. Van Kuiken, Anselm W.
Hahn, Brahamjot Nayyar, Christine E.
Electronic Spectra of Iron-Sulfur Complexes Schiewer, Sonny C. Lee, Franc Meyer,
Inorg. Chem.
Measured by 2p3d RIXS Spectroscopy
Thomas Weyhermüller, Alessandro
Nicolaou, Yi-Tao Cui, Jun Miyawaki,
Yoshihisa Harada, and Serena DeBeer
Y. Yokoyama, Y. Yamasaki, M. Taguchi, Y.
Hirata, K. Takubo, J. Miyawaki, Y.
Tensile-Strain Dependent Spin States inHarada, D. Asakura, J. Fujioka, M.
Phys. Rev. Lett.
Epitaxial LaCoO3 Thin Films
Nakamura, H. Daimon, M. Kawasaki, Y.
Tokura and H. Wadati
Alkali-metal induced band structure
S. Ito, B. Feng, M. Arita, T. Someya, W.-C.
deformation investigated by angle-resolved Chen, A. Takayama, T. Iimori, H.
Phys. Rev. B
photoemission spectroscopy and firstNamatame, M. Taniguchi, C.-M. Cheng,
principles calculations
S.-J. Tang, F. Komori, and I. Matsuda
Benoit Mortemard de Boisse Shin‐ichi
Highly Reversible Oxygen‐Redox Chemistry Nishimura Eriko Watanabe Laura
Advanced Energy
at 4.1 V in Na4/7−x[□1/7Mn6/7]O2 (□: Mn Lander Akihisa Tsuchimoto Jun Kikkawa
Materials
Vacancy)
Eiichi Kobayashi Daisuke Asakura
Masashi Okubo Atsuo Yamada
Correlation between Photocatalytic Activity
K. Ozawa, S. Yamamoto, R. Yukawa, R.-Y.
and Carrier Lifetime: Acetic Acid on SingleLiu, N. Terashima, Y. Natsui, H. Kato, K. J. Phys. Chem C
Crystal Surfaces of Anatase and Rutile
Mase, I. Matsuda
TiO2
Encyclopedia ofInterfacial
Time-Resolved Photoelectron Spectroscopy Iwao Matsuda
Chemistry: Surface
Science andElectrochemistry
Myrtille O.J.Y. Hunault, Yoshihisa
Direct observation of Cr3+ 3d states inHarada, Jun Miyawaki, Jian Wang,
ruby: towards experimental mechanistic
J. Phys. Chem. A
Andries Meijerink, Frank M.F de Groot,
evidence of metal chemistry
and Matti M. van Schooneveld
Resonant Inelastic X-ray Scattering
Yoshihisa HaradaSync. Rad. NewsK. Takubo, K. Yamamoto, Y. Hirata, H.
Commensurate versus incommensurate
Wadati, T. Mizokawa, R. Sutarto, F. He,
charge ordering near the superconducting
K. Ishii, Y. Yamasaki, H. Nakao, Y.
Phys. Rev. B
dome in Ir1−xPtxTe2 revealed by resonant
Murakami, G. Matsuo, H. Ishii, M.
x-ray scattering
Kobayashi, K. Kudo, and M. Nohara
R.-Y. Liu, K. Ozawa, N. Terashima, Y.
Controlling the surface photovoltage on
Natsui, B. Feng, S. Ito, W.-C. Chen, C.-M.
Appl. Phys. Lett.
WSe2 by surface chemical modification
Cheng, S. Yamamoto, H. Kato, T.-C.
Chiang, I. Matsuda
Compendium ofResonant Inelastic X-Ray Scattering
Yoshihisa Harada
Surface andInterface Analysis
Compendium of
Time-Resolved Photoelectron Spectroscopy Iwao Matsuda
Surface andInterface Analysis

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K. Yamamoto, Y. Hirata, M. Horio, Y.
Yokoyama, K. Takubo, M. Minohara, H.
Phys. Rev. B
Kumigashira, Y. Yamasaki, H. Nakao, Y.
Murakami, A. Fujimori, H. Wadati
C.H.Lin, A.Huang, W.W.Pai, W.C.Chen,
Single-Layer Dual Germanene Phases on
T.R.Chang, R.Yukawa, C.M.Cheng,
Phys. Rev.
Ag (111)
C.Y.Mou, I.Matsuda, T.C.Chiang,
Materials
H.T.Jeng, S.J.Tang
Chi-Cheng Lee, Baojie Feng, Marie
Peculiar bonding associated with atomic
D'angelo, Ryu Yukawa, Ro-Ya Liu,
doping and hidden honeycombs inPhys. Rev. B
Takahiro Kondo, Hiroshi Kumigashira,borophene
Iwao Matsuda, and Taisuke Ozaki,
A table-top formation of bilayer quasi-free- K.S.Kim, G.H.Park, H.Fukidome,
standing epitaxial-graphene on SiC(0001)
T.Someya, T.Iimori, F.Komori, I.Matsuda, Carbon
by microwave annealing in air
M.Suemitsu
Strong Hydrogen Bonds at the Interface
H.S.Kato, S.Yoshimoto, A.Ueda,
between Proton-Donating and -Accepting
S.Yamamoto, Y.Kanemitsu, M.Tachikawa, Langmuir
Self-Assembled Monolayers on Au(111)
H.Mori, J.Yoshinobu, I.Matsuda
K. Nagai, H. Fujiwara, H. Aratani, S.
Electronic Structure and Magnetic
Fujioka, H. Yomosa, Y. Nakatani, T. Kiss,
Properties of the Half-Metallic Ferrimagnet A. Sekiyama, F. Kuroda, H. Fujii, T.
Phys. Rev. B
Mn2VAl Probed by Soft X-ray
Oguchi, A. Tanaka, J. Miyawaki, Y.
Spectroscopies
Harada, Y. Takeda, Y. Saitoh, S. Suga,
and R. Y. Umetsu
F. Pfaff, H. Fujiwara, G. Berner, A.
Raman and fuorescence-like components in Yamasaki, H. Niwa, A. Gloskovskii, W.
resonant inelastic x-ray scattering on
Phys. Rev. B
Drube, O. Kirilmaz, A. Sekiyama, J.
LaAlO3/SrTiO3 heterostructures
Miyawaki, Y. Harada, S. Suga, M. Sing,and R. Claessen

Thickness dependence and dimensionality
32 effects of charge and magnetic orderings in
La1/3Sr2/3FeO3 thin films

97

075134
(1-6)

2017

33

2

024003
(1-8)

2018

97

075430
(1-5)

2018

130

792-7982018

34

2189-2197 2018

34

35

36

37

38

97

35143

2018

97

035110

2018

Vol.

Page

Year

30

075001

2018

Journal of Japan
Laser Processing
Society

25

39

2018

Science

360

182

2018

Japanese Journalof Optics

47

142

2018

J. Vis. Exp.

136

e570902018

Phys. Rev. B

98

Phys. Rev. B

98075431

2018

Phys. Rev. Lett.

121

25722012018---E-Labo--Paper
No Title

1

Surface electronic states of Au-induced
nanowires on Ge(001)2

Optical spin control of photoexcitedelectrons using a polarization-controllable
laser (in Japanese)

3

4

5

6

7

8

JournalAuthors
Koichiro Yaji, Ryu Yukawa, Sunghun Kim,
Yoshiyuki Ohtsubo, Patrick Le Fèvre,J. Phys.: Condens.
François Bertran, Amina Taleb-Ibrahimi,
Matter
Iwao Matsuda, Kan Nakatsuji, Shik Shinand Fumio Komori
Koichiro Yaji, Kenta Kuroda, Fumio
Komori, Shik Shin

Peng Zhang, Koichiro Yaji, Takahiro
Observation of topological superconductivity Hashimoto, Yuichi Ota, Takeshi Kondo,
on the surface of iron-based superconductor Kozo Okazaki, Zhijun Wang, JinshengWen, G. D. Gu, Hong Ding, and Shik Shin
Optical spin control in photoexcitation
process revealed by laser-based spin- andKoichiro Yaji, Kenta Kuroda, Fumio
angle-resolved photoelectron spectroscopy
Komori, Shik Shin(in Japanese)
Experimental Methods for Spin- and AngleKenta Kuroda, Koichiro Yaji, Ayumi
Resolved Photoemission Spectroscopy
Harasawa, Ryo Noguchi, Takeshi Kondo,
Combined with Polarization-Variable Laser
Fumio Komori, Shik Shin
(Video Jounal)
Koichiro Yaji, Ayumi Harasawa, Kenta
Rashba spin splitting of L-gap surface
Kuroda, Ronghan Li, Binghai Yan, Fumio
states on Ag(111) and Cu(111)
Komori, and Shik Shin
Takuto Nakamura, Yoshiyuki Ohtsubo,
Yuki Yamashita, Shin-ichiro Ideta,
Giant Rashba splitting of quasi-oneKiyohisa Tanaka, Koichiro Yaji, Ayumi
dimensional surface states on Bi/InAs(110)Harasawa, Shik Shin, Fumio Komori, Ryu(2×1)
Yukawa, Koji Horiba, Hiroshi
Kumigashira, and Shin-ichi Kimura
Hirokazu Fujiwara, Kensei Terashima,
Masanori Sunagawa, Yuko Yano,
Takanobu Nagayama, Tetsushi Fukura,
Origins of Thermal Spin Depolarization in Fumiya Yoshii, Yuka Matsuura, Makoto
Half-Metallic Ferromagnet CrO2
Ogata, Takanori Wakita, Koichiro Yaji,
Ayumi Harasawa, Kenta Kuroda, Shik
Shin, Koji Horiba, Hiroshi Kumigashira,
Yuji Muraoka, and Takayoshi Yokoya

041404（R） 2018