Research Activities– Synchrotron Radiation Experiments –16BG015

BL-1

Electrical Tuning of the Excitonic Insulator
Ground State of Ta2NiSe5
Keisuke Fukutania, Roland Staniaa, Jiwon Junga,b, Eike F. Schwierc,
Kenya Shimadac, Chang Il Kwona,b, Jun Sung Kima,b, Han Woong Yeoma,baCenter for Artificial Low Dimensional Electronic Systems, Institute for Basic Science (IBS),
77 Cheongam-ro,Nam-gu, Pohang 37673, Republic of Koreab
Department of Physics, Pohang University of Science and Technology (POSTECH),
77 Cheongam-ro,Nam-gu, Pohang 37673 Republic of Koreac
Hiroshima Synchrotron Radiation Center, Hiroshima University, 2-313 Kagamiyama, HigashiHiroshima, Hiroshima, Japan

In the recent years, as the experimental identifications of so-called excitonic insulator
become realistic after more than half a century since their predictions [1], it has garneredconsiderable attentions as a newly accessible state of matter. Numerous researches have been
devoted to understanding the properties of this exotic state of matter over the last few years[2]. On the other hand, how such ground state properties can be modified or manipulated areless well understood. Therefore, revealing the mechanisms to control the excitonic insulator
phase not only extends the spectrum of fundamental physics to explore, but is also ofparamount importance for realizing the device applications of excitonic insulators.
In this study, we have performed high-resolution angle-resolved photoemission
spectroscopy (ARPES) on the excitonic insulator Ta2NiSe5 and demonstrate that its excitonic
insulator ground state can be electrically controlled by electropositive surface adsorbates. The
direct observations of the band structures for various adsorbate coverages (as shown in Figure1) as well as the theoretical model analysis evidence that the excitonic insulator phase can be
modified by the surface dipole layers.
FIGURE 1. The evolution of the valenceband maximum in Ta2NiSe5 as a function of
K coverage, revealed by ARPES intensity
maps [(a), (b) and (c)] and theircorresponding second derivatives [(d), (e)and (f)]. The filled circles in (d)-(f) indicate
the peak positions of the energy distribution
curves (EDCs) as extracted from the ARPES
spectra. The ARPES measurements were
performed at BL-1 at HiSOR.

REFERENCES
1. W. Kohn, Phys. Rev. Lett. 19, 439 (1967); D. Jerome et al., Phys. Rev. 158, 462 (1967).
2. Y. Wakisaka et al., Phys. Rev. Lett. 103, 026402 (2009); Y. F. Lu et al., Nat. Commun. 8, 14408(2017); S. Mor et al., Phys. Rev. Lett. 119, 086401 (2017).

- 11 -

BL-1 and μ-Laser

17BG023

Electronic Structure and H-T Phase Diagram ofEu(Fe1-xRhx)2As2
ShaozhuXiaoa,Darren C. Peetsa, YaFenga, Wen-He Jiaob,Eike F. Schwierc,
Kenya Shimadac and ShaolongHea,a

Ningbo Institute of Materials Technology and Engineering,Chinese Academy of Sciences, Ningbo,
Zhejiang 315201, Chinab
Department of Physics, Zhejiang University of Science and Technology, Hangzhou, Zhejiang 310023,
China
cHiroshima Synchrotron Radiation Center, Hiroshima University, Higashi-Hiroshima 739-0046, Japan
Keywords:ARPES, EuFe2As2, iron-based superconductor.

Superconductivity and magnetic order are normallyantagonistic and often vie for the same unpaired
electrons. More generally, the zero-field ground state of a superconductor withsinglet pairing is perfectly
diamagnetic, and magnetismwithin the material reduces the energy saved throughpairing as the paircondensate is forced to adapt or compensate. Given the complex relationship between magnetism andhigh-temperature superconductivity, the interactions of magnetic order with these superconductorsoffers animportant opportunity to shed light on the pairing in these systems, which remains among thegreatestunsolved problems in condensed matter physics.
Doped EuFe2As2[1-3]offers a unique opportunity tostudy this interaction. It consists ofhigh-temperature-superconducting FeAs layers, but the intermediate layeris a square lattice of magnetic Eu.
The FeAs and Eulayers both order magnetically in the undoped parentmaterial [1], but their magnetism isnearly decoupled [3,4]. Superconductivity can be induced by pressure [5] or bychemical substitution [6].
The latter can occur either onthe Eu site, which frustrates the magnetic order whileinjecting charge carriers
to the FeAs planes, or withinthe FeAs planes, which introduces disorder to the superconducting system but
leaves the magnetic sublatticelargely untouched. Doped EuFe2As2 has a number ofvery unconventional
properties, including a reentrant superconducting transition in the resistivity [6] and a reentrant spin-glass
phase [7], most likely as a result of thecompetition between superconductivity and magnetism.
The electronic structure of Eu(Fe0.88Rh0.12)2As2 measured by ARPES with a helium lamp (hν = 21.2 eV)
isdisplayed in Fig. 1. Figs. 1(a-d) show the Fermi surface and constant-energy contours at binding
energiesEb = 0, 20, 40, and 60meV taken at a temperatureof 15 K, just below the critical temperature.
Similar toEuFe2As2 with other dopants [8,9], there are hole pockets around the point and electron pockets
around theM points. The band dispersions centered around the𝛤̅ point [along cut #1 in Fig. 1(a)] and M[along cut#2 in Fig. 1(a)] at different temperatures are shown inFigs. 1(e-h) and Figs. 1(i-l), respectively.
Energydistribution curves (EDCs) are shown as a function of temperature at Fermi momenta near 𝛤̅and M
in Figs. 1(m) and1(n), respectively. Upon increasing temperature throughthe critical temperature Tc (from15 to 50 K), no significant changes in the band structure can be observedaround the 𝛤̅ or M point, we do
not observe a gap opening, and no features near the Fermi level EF can be unambiguously identified ascoherence peaks. This is mostlikely a consequence of being too close to Tc.
To improve the energy resolution and avoid any inhomogeneity of the cleaved surface, laser-ARPES witha
spot size below 5 μm was also used to measure theband structure around the 𝛤̅ point (Fig. 2 shows data
at13 K). Despite an energy resolution better than 5 meV,there are still no significant changes withtemperatureand no visible gap.A superconductor with a high transition temperature would normally have a
sizeable gap,but this close to the transition temperature it would bevery small. The lack of observablesignatures of a pairinggap may also arise from space charge effects, which wecannot completely rule out inthe laser-ARPES data, orEu acting as a magnetic impurity. The pair-breakinginduced by magnetic
impurities can lead to “gaplesssuperconductivity” [10, 11], a state which was discussed extensively inconnection with BCS superconductors.

- 12 -FIGURE 1.Electronic structure of Eu(Fe0.88Rh0.12)2As2 measured by ARPES with a photon energy hν = 21.2 eV. (a-d)
Constantenergycontours at 15K at different binding energies as labeled. (e-l) Energy-momentum images taken at different̅ − 𝛤̅ − 𝑀
̅ direction as indicated by dashed line #1 in (a), and (i-l) along the 𝛤̅ − 𝑀
̅ − 𝛤̅
temperaturesalong the (e-h) 𝑀
direction as indicated bydashed line #2 in (a). (m) Temperature-dependent EDCs extracted from images (e-h) at the Fermi
momentum of the holepocket, shown by the dashed line in (e). (n) Temperature-dependent EDCs extracted from images (i-l)
at the Fermi momentumof the electron pocket, identified by the dashed line in (i).

̅ − 𝛤̅ − 𝑀
̅ direction measured by
FIGURE 2.(a) Band structure of Eu(Fe0.88Rh0.12)2As2 crossingthe 𝛤̅ point along the 𝑀
μ-laserARPES (hν = 6.3 eV) at 13 K. (b) Energy distribution curves(EDCs) extracted from (a). EDCs at the Fermi momenta
areplotted in black.

The band dispersion measured at 13K in the s polarization geometry (electric field perpendicular to theplaneof incidence) is shown in Fig. 2(a). This geometry willselectively detect d xy and dyz orbitals, forpolarizationalong y with z being the surface normal. The difference in the Fermi wavevector kF between
Figs. 2(a) and1(e-h) is mostly likely due to either kz dispersion or differences in carrier doping at thesurface—the materialcleaves through the Eu charge reservoir layer. As seenmore clearly in the EDCs
shown in Fig. 2(b), there isa feature suggestive of a at band near the Fermi level,reminiscent of heavy
fermion physics. Such a feature canarise from aat 4f band interacting with the regular electrons at low
temperature (and having dipole symmetrythat matches dxy and dyz), or it can appear when a bandtop or
bottom is just above the Fermi level. A parabolic fit indicates that the top of the hole pocket at 𝛤̅ shouldlie
30meV above the Fermi level, and this apparent atband is most likely a tail from that band top, cut offbythe
Fermi function.
Based on specific heat and susceptibility measurements,an H-T phase diagram forEu(Fe0.88Rh0.12)2As2 isobtained and shown in Fig. 3. Thephase diagram combines the results of our measurements on
Eu(Fe0.88Rh0.12)2As2 for both orientations.The antiferromagnetic transition has been expanded bya factor of
5. Aside from the inclusion of superconductivity, this phase diagram bears a very close resemblanceto thatof (Eu0.88Ca0.12)Fe2As2 [12]. Such a high degreeof similarity is surprising given that the dopants in thelattercase are on the Eu site, disrupting the magneticorder, while here the Rh is in the superconducting
FeAsplanes, weakening the competing superconductivity.

- 13 -FIGURE 3.H-T phase diagram of Eu(Fe0.88Rh0.12)2As2 obtained from our specific heat and susceptibility data. Filled
symbols are for H∥c, and open symbols are for H⊥c; lines are guides to the eye. TF represents field-induced
ferromagnetism,THump identifies the low-temperature hump in the specific heat, Tρmarks the resistive superconducting
transition, and TN represents the bulk antiferromagnetic transition. TN, which has been expanded by a factor of 5 for clarity,
includes points based on the specific heat jump (triangles), susceptibility peak (squares), resistivity (diamonds), andsusceptibility jump (circles).

In summary, the antagonistic relationship between the Eu and FeAslayers in EuFe2As2 disrupts both thelong-range magneticorder and bulk superconductivity. The consequence forthe magnetism is that the
ground state can be tuned bydoping the FeAs layers, likely disrupting the interlayercoupling, and our work
indicates that the magnetism'sH-T phase diagram can be traversed with laboratorymagnets. The
superconductivity may also be tunable—our data suggest that the FeAs layers may feel astrong field evenunder zero applied field. The upper critical fields in the iron-based superconductors are normallyvery highand difficult to access, but the EuFe2As2 family may make the high-field region far more accessible.
Inparticular, ARPES is a strictly zero-field technique dueto the immediate destruction of momentum
informationby any applied magnetic field, so field-dependent ARPESis ordinarily completely impossible.
Our results suggestthat in the EuFe2As2 system, ARPES can be performed at a higher effective magnetic
field. Tuning the magneticlayers, for instance by Ca or Sr doping, should returnthe system to lower
effective field, allowing an effective-magnetic-field-dependent ARPES study and directly accessing theelectronic structure under conditions that areordinarily inaccessible to ARPES.

REFERENCES
1. H. Raffius et. al.,J. Phys. Chem. Solids54, 135 (1993).
2. Z. Ren et. al., Phys. Rev. B 78, 052501 (2008).
3. H. S. Jeevan et. al., Phys. Rev. B 78, 052502 (2008).
4. Y. Xiao et. al., Phys. Rev. B 80, 174424 (2009).
5. K. Matsubayashi et. al., J. Phys.: Conf. Ser. 215, 012187 (2010).
6. W.-H. Jiao et. al., Supercond. Sci. Technol. 30, 025012 (2017).
7. A. Baumgartner et. al., Phys. Rev. B 95, 174522 (2017).
8. S. Thirupathaiah et. al., Phys. Rev. B 84, 014531 (2011).
9. M. Xia et. al., J. Phys.: Condens. Matter 26, 265701 (2014).
10. A. A. Abrikosov and P. Gor'kov, Soviet Physics JETP12, 1243 (1961).
11. J. C. Phillips, Phys. Rev. Lett. 10, 96 (1963).
12. L. M. Tran et. al., Phys. Rev. B 98, 104412 (2018).

- 14 -

17BU002

BL-1/9B

Interplay of Exchange and Spin-Orbit Interaction forInterface States in Ni/W(110)Markus Donatha, Pascal Jona Grenza ,Eike Schwierb, Koji Miyamotob and Taichi OkudabaPhysikalisches Institut, University of Münster, Wilhelm-Klemm-Straße 10,48149 Münster, Germanyb
Hiroshima Synchrotron Radiation Center, Hiroshima University

Keywords: Nickel, tungsten, spin-orbit coupling, exchange interaction, interface states

Due to the high atomic number of tungsten and the ferromagnetic behavior of nickel, ultrathin Ni films on
W(110) are well suited for studying the interplay of spin-orbit coupling and exchange interaction. The focus
of our spin- and angle-resolved photoemission measurements were (i) the influence of Ni adsorbates on the
Dirac-cone-like surface state of W(110) [1], (ii) the spin dependence of electron states at the interface
between Ni and W, and (iii) the appearance of ferromagnetic order in the Ni overlayer.
We obtained the following results:(i)
The Dirac-cone-like surface state vanishes for a coverage above one monolayer [Fig. 1c)]. In
the submonolayer region, flat Ni-induced bands appear and overlap with the upper part of thesurface state [Fig. 1b)].
(ii)
Interface states near the Fermi level with parabolic-like dispersion and Rashba-type spin
splitting are observed for submonolayer [Fig. 2] and monolayer coverages [Fig. 3](iii)
Ferromagnetic behavior of the Ni film is observed for thicker films, e.g., at about 10 ML.

a)

b)

c)

E-EF

-1.0
-1.2
-1.4
-1.6
-0.4 -0.2

0

0.2
-1

k||(Å )

0.4

-0.4 -0. 2

0

0.2
-1

k||(Å )

0.4

-0.4 -0.2

0

0.2

0.4

-1

k||(Å )FIGURE 1. Angle-resolved photoemission measurements (second derivative) of a) clean W(110), b) 0.5ML Ni/W(110), andc) 1ML W(110) along the high symmetry line ΓH.

- 15 -FIGURE 2.

Angle-resolved photoemission measurements (second derivative) for 0.5 ML Ni/W(110) along ΓH.

a)

b)

c)

FIGURE 3. a) Angle-resolved photoemission measurements (second derivative) for 1ML Ni/W(110) along ΓH. b+c) Spindependent energy distribution curves (EDCs) and spin polarization for kǁ≈±0.6 Å-1 [see black lines in a)]. Blue and red symbols
mark spin-up and spin-down components along the Rashba direction.

REFERENCES
1.

K. Miyamoto, A. Kimura, K. Kuroda, T. Okuda, K. Shimada, H. Namatame, M. Taniguchi, and M.
Donath, Phys. Rev. Lett., 108, 066808 (2012).

- 16 -

17BU003

BL-1

Matrix elements in the resonant photoemission fromcerium oxide
Tomáš Duchoňa, David N. Muellera, Shiv Kumarb, Eike F. Schwierb, Slavomír
NemšákcaPeter-Grünberg-Institut 6, Forschungszentrum Jülich GmbH, Jülich, Germany
Hiroshima Synchrotron Radiation Center, Hiroshima University, Hiroshima, Japanc
Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, USAb

Keywords: cerium oxide, resonant photoemission, matrix elements

Cerium oxide, an industrially relevant material, has a remarkable ability to act either as a supply or a sinkof oxygen that allows it to balance oxygen-lean and oxygen-rich phases essential for chemical conversion[1,2]. The ability is realized by a redox cycle that utilizes a facile change of stoichiometry between two stableconfigurations—CeO2 (Ce4+) and Ce2O3 (Ce3+) [3]. The redox cycle of cerium oxide represents a dramatic
alteration of the occupancy of the valence shell. Specifically, the highest occupied subshell is changed within
the redox cycle via 4f0 and 4f1 electronic configurations [4]. Probing the 4f occupation is thus of great
importance for unraveling key elements of cerium oxide chemistry.
Resonant photoemission is becoming increasingly important as a probe of the 4f shell of cerium oxide.
The resonant enhancement at the 4d → 4f photoabsorption threshold uniquely allows for the detection ofvery low concentration of Ce3+ ions in the system. As such, a universal recipe for quantification of theresonant photoemission in cerium oxide is sought after. Lykhach et al. proposed an elegant solution for thequantification via a proportion coefficient calibrated to core level spectroscopy [5]. The simplicity of theapproach encouraged its use; however, the assumptions necessary are poorly understood.
The quantification process relies on comparison of intensities of two resonant features associated with
Ce4+ and Ce3+ ions. We have recently shown, based on results obtained at HiSOR, that the two features areof a disparate origin [6]. For a Ce3+ ion, it is an intra-atomic process; while for a Ce4+ ion, it is an interatomic process that carries information about cation–anion covalency. The two can be described within anominal electron configuration as follows:(Ce3+ ion)
Ce 4d104f1 O 2p6 + hν → Ce 4d104f0 O 2p6 + e−
Ce 4d104f1 O 2p6 + hν → Ce 4d94f2 O 2p6 → Ce 4d104f0 O 2p6 + e−
(Ce4+ ion)
Ce 4d104f0 O 2p6 + hν → Ce 4d104f0 O 2p5 + e−
Ce 4d104f0 O 2p6 + hν → Ce 4d94f1 O 2p6→ Ce 4d104f0 O 2p5 + e−
The above illustrates that fact the symmetry of the O 2p and Ce 4d and 4f states enters the quantification
process. Herein, we gauge the symmetry contribution using a Nb-doped cerium oxide (111) epitaxial filmgrown on gold support. The cation doping introduces a controlled amount of Ce3+ ions in the system that arestable against cleaning procedure consisting of annealing in oxygen. We measure the ratio of the intensity of
the Ce3+ and Ce4+ resonant features that forms the basis of the quantification process with respect to the- 17 -

symmetry of the experiment. The results are plotted in Figure 1. Two important points are immediately
apparent from the figure. First, while the Ce3+ resonant feature related to highly localized 4f electrons does
not show any measurable dispersion [6], the dispersion of the O 2p states governing the Ce4+ resonant featuresignificantly modifies the resulting ratio, up to a factor of 3 when moving away from the Γ point. Second,
the polarization of the exciting light introduces a sizeable factor, with s polarized light favoring the Ce4+
resonant feature.

FIGURE 1. The ratio of the intensity of the Ce3+ and Ce4+ resonant features measured along Γ → M high-symmetry directionof Nb-doped cerium oxide (111) surface with s and p polarized light (hν = 124.5 eV).

The results clearly illustrate that the quantification of resonant photoemission in cerium oxide as proposedin [5] is useful for general comparison only when the samples examined are disordered at the probing
lengthscale. Any other evaluation has to precisely account for the geometry of the experiment. It should be
noted that the geometry in question includes Ce–O bond length, a quantity that is known to differ, among
other influences, with a stoichiometry of cerium oxide. Understanding the effects pointed out in our study
provides an opportunity to reveal phenomena that would be otherwise hidden, or falsely interpreted, in thesignal variations originating from matrix elements.

REFERENCES
1. Montini, T.; Melchionna, M.; Monai, M.; Fornasiero, P. Chem. Rev. 2016, 116, 5987–6041.
2. Mullins, D. R. Surf. Sci. Rep. 2015, 70, 42–85.
3. Skorodumova, N. V.; Simak, S. I.; Lundqvist, B. I.; Abrikosov, I. A.; Johansson, B. Phys. Rev. Lett. 2002, 89, 166601.
4. Chen, B.; Ma, Y.; Ding, L.; Xu, L.; Wu, Z.; Yuan, Q.; Huang, W. J. Phys. Chem. C 2013, 2, 5800–5810.
5. Lykhach, Y.; Kozlov, S. M.; Skála, T.; Tovt, A.; Stetsovych, V.; Tsud, N.; Dvořák, F.; Johánek, V.; Neitzel, A.; Mysliveček,J.; Fabris, S.; Matolín, V.; Neyman, K. M.; Libuda, J. Nature Materials 2016, 15, 284–288.
6. Duchoň, T.; Aulická, M.; Schwier, E. F.; Iwasawa, H.; Zhao, C.; Xu, Y.; Veltruská, K.; Shimada, K.; Matolín, V. Phys.
Rev. B 2017, 95, 165124.

- 18 -

18AG026&18BG011

BL-1

Superstructure-Induced Splitting of Dirac Cones in
Silicene
Baojie Feng,1,2,* Hui Zhou,1,2 Ya Feng,3,4 Hang Liu,1,2 Shaolong He,3 Iwao
Matsuda,5 Lan Chen,1,2Eike F. Schwier,4 Kenya Shimada,4 Sheng Meng,1,† and
Kehui Wu1,2,‡
1

Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China
School of Physical Sciences, University of Chinese Academy of Sciences, Beijing 100049, China
3
Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201,
China
4
Hiroshima Synchrotron Radiation Center, Hiroshima University, 2-313 Kagamiyama, Higashi-Hiroshima
739-0046, Japan
5
Institute for Solid State Physics, The University of Tokyo, Kashiwa, Chiba 277-8581, Japan
2

Keywords: Silicene, Dirac cone, ARPES.

Atomic scale engineering of two-dimensional materials could create devices with rich physical andchemical properties. External periodic potentials can enable the manipulation of the electronic band
structures of materials. A prototypical system is 3 × 3-silicene/Ag(111), which has substrate-induced periodic
modulations. Recent angle-resolved photoemission spectroscopy measurements revealed six Dirac conepairs at the Brillouin zone boundary of Ag(111), but their origin remains unclear [1]. We used linear
dichroism angle-resolved photoemission spectroscopy, the tight-binding model, and first-principles
calculations to reveal that these Dirac cones mainly derive from the original cones at the K (K’) points offree-standing silicene. The Dirac cones of free-standing silicene are split by external periodic potentials thatoriginate from the substrate-overlayer interaction.
Both the sample preparation and ARPES measurements were performed at the linear undulator beam line
BL-1. Monolayer silicene was prepared by evaporating silicon from a silicon wafer onto a Ag(111) substrate.
The linear polarization of the incident light could be switched between s and p by rotating the whole ARPES
system; this ensured pure s and p polarization without cross contamination as is the case with undulatorbased polarization switching [2].

- 19 -FIGURE 1. (a) Atomic structure of 3 × 3 silicene on Ag(111). Orange and blue balls represent Si and Ag atoms, respectively.
The white rhombus indicates a unit cell of the 3 × 3 superstructure. (b) BZs of silicene (red lines), Ag(111) (black lines), andthe superstructure (blue lines). (c) Photoemission intensity at the Fermi level as measured with 50 eV p-polarized photons. The
blue line indicates the momentum cut along which the ARPES intensity plots in (e) were taken. (d) The same plot as in (c) but—— —— ——

at a binding energy of 0.2 eV.(e)–(g) ARPES intensity plot along the blue line in (c), along the K -M - K direction, and along—— ——the Γ -M direction, respectively.(h) Schematic drawing of a pair of Dirac cones.

Figure 1(a) shows the structure model of 3 × 3 silicene on Ag(111). The white rhombus indicates a unit
cell of the superstructure which includes a 3 × 3 superlattice of silicene and a 4 × 4 superlattice of Ag(111).
From the Fermi surface in Figure 1(c), we found that there exist a pair of Fermi pockets symmetric to the——

M point of Ag(111). The size of the pockets increases at higher binding energies, as shown in Figure 1(d).
Figure 1(e) shows the band structure along the blue line in Figure 1(c), which shows a linear dispersion. The—— —— ——
band structure along the K - M - K direction also shows a linear dispersion [Figure 1(f)]. These results
indicate the existence of a pair of Dirac cones at each BZ boundary of Ag(111), which is consistent with—— ——previous ARPES results [1]. Along the perpendicular direction, i.e., the Γ -M direction, we found a reversed——

parabolic dispersion with the band top located at the M point. These results indicate the existence of asaddle point between the pair of Dirac cones [as illustrated in Figure 1(h)] and agree with previous ARPES
results [1,3]. The existence of a saddle point indicates that the pair of Dirac cones have a common origin andis reminiscent of the Weyl cone pair that are split from a Dirac cone after certain symmetry breaking. These——

results suggest the possibility that the pair of Dirac cones are split from one original cone at the M point ofAg(111).
To confirm this, we first discuss the possible existence of such an original Dirac cone (before splitting) at——

the M point of Ag(111). From the schematic drawing of the BZs in Figure 1(b), we can see that the K point——of the silicene 1 × 1 lattice is equivalent to the M point of Ag(111), because both of them are located at theΓ point of the superstructure (blue hexagons). In analogy to graphene, freestanding silicene naturally hosts——a Dirac cone at each K (K’)point; this Dirac cone will be folded to the M point of Ag(111) in the presenceof the superstructure. Another important fact is that the Dirac cones of a honeycomb lattice mainly derivefrom the pz orbitals of the outermost shell. We will show next that this can be experimentally confirmed by
our ARPES measurements.

FIGURE 2. (a) Left panel: schematic illustration of the experimental geometries of the ARPES measurements. The incident
light lies in the detection plane (colored yellow). The s and p polarization is defined as perpendicular and parallel to thedetection plane, respectively. Right panel: schematic drawing of the BZ of Ag(111) and the positions of the Dirac cones. The
blue line indicates the momentum cut that includes the Γ point and Dirac point. (b) ARPES intensity plots along the blue linewith p and s polarized light, respectively. The incident photon energy is 50 eV.

- 20 -

To disentangle the orbital composition of the Dirac cones, we performed linear dichroism ARPES. In our
ARPES measurement setup, the xz plane is the incident plane as well as the detection plane, as shown in
Figure 2(a). When the detection plane coincides with the mirror plane of the sample, there will be a selection
rule in the photoemission process according to the electric dipole approximation: linearly polarized lighttends to excite electronic states that have well-defined parities with respect to the mirror plane: the ppolarized light excites the even-parity states while the s-polarized light excites the odd-parity states [4]. In
silicene, the low-energy electronic states are dominated by the p electrons of Si, including the px, py, and pz
orbitals. In particular, the pz orbitals always have an even parity when we rotate the sample around the z axis,—— ——and thus can only be excited by p-polarized light. The photoemission intensity along the Γ - D direction
measured with p and s polarized light is shown in Figure 2 (b). One can see that the linearly dispersing Dirac
bands are only visible with p polarized light, while they almost disappear with s polarized light; this result
indicates that the Dirac cones mainly originate from even parity pz orbitals. These results further corroborate
our speculation that the Dirac cones originate from the original ones of the freestanding silicene.
To capture the physical mechanism of the splitting, it is convenient to use a simple tight-binding model inwhich the effects of the substrates are treated as perturbations. We start with a flat silicene structure, andonly the pz orbitals of Si are taken into account. When silicene is placed on Ag(111), a 3 × 3 superstructure
will form, and, more importantly, the sublattice symmetry will be broken [5]. In our tight-binding model, we
used an external perturbation to simulate the effects of the substrate. From this point of view, thesuperstructure has two prominent effects. First, the band structures will be folded into the BZs of the 3 × 3——

supercell because of the Umklapp scattering process, resulting in the appearance of a Dirac cone at the M
point of Ag(111), as discussed above. Second, it will expose a long-range periodic potential in silicene, which
has not yet been taken into account.

FIGURE 3. (a) Schematic illustration of the periodic potential in our tight-binding model. (b) and (c) Calculated band
structures using the tight-binding model before (b) and after (c) applying the periodic potential. (d) Schematic drawing of the
splitting direction of the Dirac cones. (e) and (f) Calculated partial density of states of Si and Ag atoms, respectively, using
DFT. (g) Schematic drawing of the Dirac cone pairs in the BZs of silicene and Ag(111). The red dots represent the positionsof the Dirac cones. The blue shaded ellipses highlight the Dirac cones that have been experimentally observed. The black
double arrows indicate the splitting of the Dirac cones.

Applying the periodic potential will break the sublattice symmetry and lead to a dramatic change in the
Dirac cone. With proper parameters, the Dirac cone will split along the Γ-K direction of the 3 × 3 supercell[Figure 3(d)], in agreement with our experimental results. Therefore, our tight-binding model could——

qualitatively explain the origin of Dirac cone pairs that are symmetric to the M point of Ag(111). It should
be noted that the periodic potential in our tight-binding model does not break the inversion symmetry, which- 21 -

protects the gapless nature of the Dirac cones [6]. Moreover, our calculated partial density of states near the
Fermi level are dominated by the pz orbitals of Si, as shown in Figure 3(e) and 3(f). These results agree wellwith our experimental results and support our tight-binding model that only considers the pz orbitals of Si.
On the other hand, although the contributions from the Ag orbitals are smaller than those from Si pz orbitals,
there are still considerable contributions from some orbitals of Ag, such as the s, pz, and dz2 orbitals. These
orbitals have out-of-plane components and are prone to hybridize with Si pz orbitals. This band hybridization,although weak, imposes a periodic potential in the silicene lattice and results in the splitting of the Diraccone.
In summary, we studied the electronic structures of (3 × 3)-silicene on Ag(111) using high-resolution
ARPES, tight-binding analysis, and DFT calculations. We confirmed the existence of six pairs of Dirac conesat the BZ boundary of Ag(111), but these Dirac cones were only detectable with p polarized light, indicatingthat the Dirac bands are mainly derived from the pz orbitals of silicon. Our tight-binding analysis and DFT
calculations revealed that these Dirac cones originate from the original Dirac cones of freestanding silicene.
Our results settle the long-debated question on the existence of Dirac cones in the silicene/Ag(111) system,
and, more importantly, provide a powerful route to tailor the physical properties of Dirac fermions in ahoneycomb lattice.
1. Y. Feng et al, Proc. Natl. Acad. Sci. USA 113, 14656 (2016).
2. H. Iwasawa et al, J. Synchrotron Radiat. 24, 836 (2017)
3. D. Tsoutsou et al, Appl. Phys. Lett. 103, 231604 (2013)
4. A. Damascelli et al, Rev. Mod. Phys.75, 473 (2003)
5. C.-L. Lin et al, Phys. Rev. Lett. 110, 076801 (2013).
6. See Supplemental Materials at http://link.aps.org/supplemental/10.1103/PhysRevLett.122.196801 for additional
experimental and theoretical analysis.

- 22 -

18AG027

BL-1

ARPES Measurements on Thin Films of Topological
Crystalline Insulator PbxSn1-xTe
Yuta Tomohiroa, Hiroshi Itoa, Takeru Shimanoa, Ryota Akiyamab , Eike F.
Schwierc, Akio Kimurad, Kenya Shimadac, Shuji Hasegawab, and Shinji KurodaaaInstitute of Materials Science, University of Tsukuba, 1-1-1 Tennoudai, Tsukuba 305-8573, Japanb
Department of Physics, The University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113-8654, Japanc
Hiroshima Synchrotron Radiation Center, Hiroshima University, 1-3-2 Kagamiyama, Higashi-Hiroshima
739-8511, Japand
Department of Physics, Hiroshima University, 1-3-2 Kagamiyama, Higashi-Hiroshima 739-8511, Japan
Keywords: Topological crystalline insulators, Dirac cone, Gap opening, Mirror reflection symmetry

SnTe has been attracting attention as a typical material of topological crystalline insulator (TCI)[1,2], inwhich the topological surface state (TSS) is protected by the mirror reflection symmetry[1], instead of thetime-reversal symmetry (TRS) in the conventional Z2 topological insulator (TI). PbxSn1-xTe, a mixed crystal
between SnTe and PbTe, exhibits the transition from topological to trivial phase at Pb content x ~ 0.7[3-5].
In order to investigate how the TSS in PbxSn1-xTe changes with the Pb content x, we have performed theangle-resolved photoemission spectroscopy (ARPES) measurement on PbxSn1-xTe thin films.
We prepared a series of PbxSn1-xTe (111) thin films with Pb content x = 0 (SnTe), 0.17, 0.23, which weregrown by molecular beam epitaxy (MBE) on a conductive CdTe template[6]. In the films with x = 0.17, 0.23,
Sb was doped as a donor impurity with a concentration about 1% in order to compensate the Sn vacancies
formed in the crystal. We performed the ARPES measurement at BL-1 of HiSOR. We used a home-madeportable vacuum chamber in order to bring the grown films without exposure to air. After having transferred
the grown films into the preparation chamber of BL-1, we annealed them at around 200oC to clean the surface.
Then we observed the unreconstructed diffraction pattern by low-energy electron diffraction (LEED).
Figure 1 shows the ARPES image of the SnTe (111) film along the Γ-M direction. The measurement wasdone at T = 20 K with an incident photon energy of 30 eV. As shown in the figure, one can see band
dispersions reaching the Fermi level (EF = 0) both at the M and Γ points. Since the SnTe crystal is heavily
p-degenerate due to the formation of Sn vacancies, the Fermi level is below the top of the bulk valenceband.
Then we observe only the lower part of the Dirac cone below the Fermi level.
Figures 2 and 3 the ARPES images of the PbxSn1-xTe films with x = 0.17 and 0.23, respectively, taken in
the same measurement condition as in Fig. 1. As shown in the figures, one can see a similar band dispersion
reaching the Fermi level at the M point. On the other hand, it does not reach the Fermi level at the Γ point.
This suggests that a gap opens in the Dirac cone at the Γ point while that at the M point remains gapclosed.
Similar transition was reported in another TCI material (Pb,Sn)Se[7]; In the (111) surface of (Pb,Sn)Se
with a Pb composition in the range of the TCI phase, it was observed that a gap is opened in the Dirac coneat the Γ point upon Bi-doping, while those at the M points remain gap-closed. This gap opening was
attributed to the phase transition from the TCI phase to the Z2 topological phase due to the breaking of themirror reflection symmetry of the crystal upon Bi-doping. In the present case, we observed the gap-opening
only at the Γ in the PbxSn1-xTe films with Sb doping. This gap-opening may be related to a similar phasetransition caused by the lattice distortion due to Sb-doping. Further studies analyzing details of the lattice
distortion would be needed in order to understand the origin of the observed gap-opening.

- 23 -FIGURE 1. ARPES image of a SnTe (111) film along the Γ-M direction. The measurement was done at T = 20 K with anincident photon energy of 30 eV.

FIGURE 2. ARPES images of PbxSn1-xTe (111) films with Pb content (a) x = 0.17 and (b) x = 0.23, both co-doped with Sb
with a concentration of 1%. The measurement was done with the same condition as in Fig. 1. The horizontal axis is taken alongthe Γ-M direction. The equi-energy mapping images at the Fermi level are shown in the top panel.

REFERENCES
1.
2.
3.
4.
5.
6.
7.

T. H. Hsieh, H. Lin, J. Liu, W. Duan, A. Bansil, and L. Fu, Nat. Commun. 3, 982 (2012).
Y. Tanaka, Z. Ren, T. Sato, K. Nakayama, S. Souma, T. Takahashi, K. Segawa, and Y. Ando, Nat. Phys. 8, 800 (2012).
S. –Y. Xu et al., Nat. Commun. 3, 1192 (2012).
Y. Tanaka, T. Sato, K. Nakayama, S. Souma, T. Takahashi, Z. Ren, M. Novak, K. Segawa, and Y. Ando, Phys. Rev. B
87, 155105 (2013).
C. Yan et al., Phys. Rev. Lett. 112, 186801 (2014).
R. Ishikawa, T. Yamaguchi, Y. Ohtaki, R. Akiyama, S. Kuroda, J. Cryst. Growth 453, 124 (2016).
P. S. Mandal et al., Nat. Commun. 8, 68 (2017).

- 24 -

BL-1 and μ-Laser

18AG030 and 18BU009

ARPES Study of The Evolution of Electronic Structures of
Yb-Doped SmB6
ShaozhuXiaoa, YaFenga, TaoWub, Eike F. Schwierc, Kenya Shimadac andShaolongHea,a

Ningbo Institute of Materials Technology and Engineering,Chinese Academy of Sciences, Ningbo,
Zhejiang 315201, Chinab
University of Science and Technology of China, Hefei, Anhui230026, China

cHiroshima Synchrotron Radiation Center, Hiroshima University, Higashi-Hiroshima 739-0046, Japan

Keywords:ARPES, SmB6, Kondohybridization.

SmB6, the first mixed valence material and first Kondo insulator identified in the 1960s, is a bad metal
at high temperature, and behaves with a metal to insulator transition as temperature decreased [1]. The
metal to insulator transition of SmB6 is due to the hybridization of the localized f electrons with conductiond electrons which opens a narrow band gap on the order of ~10 meV at low temperatures and the Fermi
level lies in the gap. This effect lead SmB6 to a resistivity raising with temperature decreasing and theresistivity behaves saturation at about 3 K instead of diverging as in an insulator, which was initiallythought to originated from in-gap states such as bulk impurity states [2, 3]. However, recent transport
experiments performed on SmB6 suggest that the residual conductivity is originated from thesurface-dominated transport state and SmB6 is identified to possess a conductive surface state with perfectinsulating bulk state [4-6]. Moreover, the presence of topological protected surface states in the gap of
SmB6 have been directly observed by angle resolved photoemission spectroscopy (ARPES) and quantum
oscillation experiments [7, 8]. Therefore, SmB6 is identified as an ideal Kondo topological insulator
candidate [9, 10].
The valence number of Sm ion is mixed between 2+ and 3+ [11] and is an important issue for the theoryof the proposed topological character of SmB6. For Yb-doped Sm1-xYbxB6, the valence of Sm Linearlychanges with increasing x, from 2.6+ to 3+ as x increased from 0 to 1 [12].Transport measurements showsthat Sm1-xYbxB6experience an insulator to metal transition with x increased up to about x=0.3 attemperature of 2 K. The theory and the relationship between the metal-to-insulator transition and Sm
valence variation are still remained to be uncovered.
Sm1-xYbxB6 is too stiff to be cleaved to obtain a smooth surface large enough. In this work, we
usedLaser μ-ARPES, which has small beam spot with size of several μm and high spatial resolution, toovercome this problem to study the electronic structures of Sm1-xYbxB6. Up to now, only the sample with x= 0.3 was successfully cleaved and measured, and the results are shown in Fig. 1. In Circular and P
polarization geometry, the hybridization between the flat f bands and the conductiond bands can be seen
(guide lines for eyes in Fig. 1b), which are further confirmed by the MDCs along the flat f bands at binding
energies of 18 meV and 172 meV (see Figs. 1d and 1e).

- 25 -FIGURE 1.Electronic structure of Sm0.7Yb0.3B6 measured byμ-Laser ARPES (hν = 6.3 eV) at 19 K in the (a) S polarizationgeometry, (b)Circular polarization geometry and (c) P polarization geometry, respectively. (d-e) Momentum distribution
curves (MDCs) extracted from image (b) along the flat f bands at binding energies of (d) 18 meV and (e) 172 meV.

REFERENCES
1. A. Menth, E. Buehler, and T.H. Geballe, Phys.Rev. Lett. 22, 295–297 (1969).
2. Allen J W, Batlogg B and Wachter P, Phys. Rev. B 20, 4807 (1979).
3. Sluchanko N E et al., Phys. Rev. B 61, 9906 (2000).
4. Kim D J et al., Sci. Rep. 3 3150 (2013).
5. Wolgast S et al., Phys. Rev. B 88, 180405 (2013).
6. D. J. Kim, J. Xia and Z. Fisk, Nature Materials. 13, 466 (2014).
7. H. Miyazaki et al., Phys. Rev. B 86, 075105 (2012).
8. G. Li et al., Science 346, 1208–1212 (2014).
9. N. Xu, H. Ding and M. Shi, J. Phys.: Condens. Matter. 28, 363001 (2016).
10. F. Lu, J. Z. Zhao, H.Weng, Z. Fang, and X. Dai, Phys. Rev. Lett. 110, 096401 (2013).
11. E. E. Vainshtein, S. M. Blokhin, and Y. B. Paderno, Sov. Phys. Solid State 6, 2318 (1965).
12. J. M. Tarascon et al., J. physique 41, 1135-1140 (1980).

- 26 -

18AU001

BL-1

The ARPES Studies on Nodal-line Semimetal LaSbTe
Yang Wang1 ,Wenjuan Zhao1, Guodong Liu1,*, E. Schwier2, K. Shimada2,
and Xingjiang Zhou1,*
National Laboratory for Superconductivity, Beijing National Laboratory for Condensed Matter Physics,
Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China
Hiroshima Synchrotron Radiation Center, Hiroshima University, 2-313 Kagamiyama, Higashi-Hiroshima,
Hiroshima 739-0046, Japan
1

2

Keywords: nodal-line semimetal, diamond fermi surface , ARPES.

The original title for this proposal is "ARPES studies on the electronic structure of 2D layered van der
Waals (vdW) ferromagnetic semiconductor CrI3". However, we found that the atomic-layer-thick 2D CrI3
is extremely sensitive to air. After being prepared, it got damaged in a short time in our lab. It is impossible
for us to safely transport the samples to HiSOR. Therefore, we used the beam time to investigate theinteresting nodal-line semimetal LaSbTe. In the following, we describe the research progress on thismaterial.

Materials with exotic topological properties have evoked great interests in the field of condensed
matter physics since the discovery of topological materials. Searching for new topological materials inexperiments is one of the direction of efforts all the time. In the past few years, the first-principle
calculations had predicted that LaSbTe is a new nodal-line semimetal when SOC is neglected[1]. However,there are no direct experiment evidences demonstrating it`s topological properties. From the point of thisview, we studied the electronic structure of LaSbTe for the first time via angle-resolved photoelectronic
emission spectroscopy experiments. We have measured the fermi surface and the band structures alonghigh symmetry directions with different photon energy. We found that it has a diamond-shaped topologicalfermi surface and Dirac band in Γ-M and X-M direction that are consistent with our band theory
calculation. Our results provide clear evidence that LaSbTe is a nodal-line semimetal.

In Fig.1(a), it shows the crystal structure of LaSbTe which has the tetragonal lattice formed by Sb atom.
Fig.1(b) shows the fermi surface and constant energy contour of LaSbTe by ARPES measurement with
55eV photon energy. We found that it has a diamond-shaped topological fermi surface arising from Sb
atom square similar to the case of ZrSiS [2][3].

-100 meV
～
meV

200

Figure 1. Fermi surface and constant energy contour of LaSbTe. (a) The crystal structure of LaSbTe. The red and yellow
balls stand for La and Te atoms respectively and the blue ball stands for Sb atom which forms a square lattice. (b)Fermi
surface and constant energy contour of LaSbTe by ARPES measurements with 55eV photon energy, demonstrating that it
possesses a topological diamond-shaped fermi surface.

- 27 --2～meFig.2(a) shows the band structure of LaSbTe by using first-principle calculation. According to the bandcalculation, LaSbTe is a Dirac nodal-line semimetal when SOC is neglected. Fig.2(b) shows the core level
photoemission spetrum of LaSbTe. In fig.2(c), the electronic band structures of LaSbTe which aremeasured by different photon energy along all high symmetry directions are shown. The Dirac band areobserved in Γ-M and X-M direction which is consistent with our band calculation. Our results indicate that
LaSbTe is a Dirac nodal-line semimetal.

Figure2. The band structure of LaSbTe. (a) The band structure of LaSbTe obtained by using first-principle calculation [1]. (b)The core level spectrum of LaSbTe acquired by ARPES expements with 55eV photon energy. (c) The electronic band
structure of LaSbTe along several high symmety directions with different photon energy. We found that there is a topological
Dirac band around X point along X-M direction which is contributed by bulk states. In Γ-M direction, another Dirac band isobserved in our measurements which is consistent with first-principle calculation [1].

REFERENCES
1. X. Dai et al., PHYSICAL REVIEW B 92, 205310 (2015).
2. Christian R. Ast et al, ncomms11696.
3. Guang-Hao Hong et al, CPB Vol.27,No.1(2018) 017105.

- 28 -

18BG001

BL-1

Excitonic correlation effect in multi-band superconductors
T. Mizokawaa, Y. Matsuzawaa, T. Moritaa, N. L.Sainib, T. Asanoc, T. Nakajimac,
R. Higashinakac, T. D. Matsudac, Y. Aokic, E. F. Schwierd, and K. ShimadadaDepartment of Applied Physics, Waseda University, Shinjuku-ku, Tokyo 169-8555, Japanb

Department of Physics, University of Roma "La Sapienza", Piazzale Aldo Moro 2,00185 Roma, Italyc

Department of Physics, Tokyo Metropolitan University, Hachioji 192-0397, Japand

HiSOR, Hiroshima University, Higashihiroshima, Hiroshima 739-0046, Japan

Keywords: BiS2 superconductors, excitonic correlation

Since the discovery of a BiS2-based superconductor by Mizuguchi et al. [1], a family of BiS2-based
superconductors including RE(O,F)BiS2 (RE=rare-earth element) have been investigated extensively bymeans of various experimental techniques [2-7]. In REOBiS2 with REO blocking layer, the F substitution
for O introduces electrons to the BiS2 layer. The theoretical calculations indicate that the Fermi surfaces
constructed from the Bi 6px and 6py orbitals cover the entire Brillouin zone when x is as large as 0.5[2]. The
orbital component of the Fermi surfaces has been identified by means of polarization dependent angleresolved photoemission spectroscopy (ARPES) in Ce(O,F)BiS2 which is consistent with the theoreticalpredictions [8]. However, the observed Fermi surfaces for CeO0.5F0.5BiS2 are rather small and located aroundX point of the Brillouin zone. Sugimoto et al. argued that Bi 6pz orbitals can accommodate some electrons
that do not contribute to the Fermi surfaces. If the Bi 6pz electrons and Bi 6s-S 3p holes have a kind of
excitonic correlation, the localized electron-hole pair can couple with asymmetric distortion of BiS5
pyramids. In order to understand the effect of electron-hole correlation in the BiS2-based superconductors,
we have performed ARPES on Pr(O,F)BiS2 (x=0.0, 0.3, and 0.5) at BL-1, HiSOR.
The electron pocket observed around X point is shown for x=0.5 in Fig. 1. The shape of the electron
pockets and the dispersion of the electron bands are symmetric around X point. This behavior is canonicalfor RE(O,F)BiS2 with RE=La, Ce, and Nd and is consistent with the band-structure calculation for x=0.2 [911]. On the other hand, as shown in Fig. 2, the shape of the electron pocket is considerably distorted for
x=0.3 indicating that the orbital component is different from the prediction of the band-structure calculation.
The difference between Ce(O,F)BiS2 and Pr(O,F)BiS2 would be derived from the mixed valence of the rareearth elements. In case of Ce(O,F)BiS2, Ce4+ component decreases with the F doping, indicating thatconsiderable amount of Bi 6pz electrons should exist in x=0.0. In going from x=0.0 to 0.5, the electrons in
the Bi 6pz orbitals are transferred to the Bi 6px and Bi 6py orbitals and the electron pockets at X point areformed. In case of Pr(O,F)BiS2, Pr4+ component does not change appreciably with the F doping [9]. The
present work has revealed that even PrOBiS2 has the electron pockets at X point suggesting that the Bi 6pz
orbitals do not accommodate electrons and only the Bi 6px and 6py orbitals form the electron pockets due tothe self-doping by Pr4+. With the F doping, the doped electrons partly contribute to increase the volume ofthe electron pocket and are partly trapped in the Bi 6pz orbitals. As shown in the right panel of Fig. 2, x=0.3
exhibits the anisotropic band dispersion down to -0.2 eV below the Fermi level. Therefore, one can exclude
possibility of partial elimination of the Fermi surfaces or pseudogap opening. Since the entire electron band
becomes anisotropic, one possible explanation is a kind of Jahn-Teller effect in which degeneracy between
the Bi 6px and 6py orbitals is lifted due to local lattice distortion from tetragonal to orthorhombic. The
distortion can be induced by excitonic correlation between Bi 6px/6py electron and Bi 6s-S 3p hole. In orderto understand dynamics of such correlation, time-resolved ARPES on Pr(O,F)BiS2 should be done in future.

- 29 -FIGURE 1. Left panel: Spectral weight map at the Fermi level (Fermi surface map) for PrO0.5F0.5BiS2. Right panel: Spectral
weight map at -0.2 eV for PrO0.5F0.5BiS2. h=30eV and T=20 K.

FIGURE 2. Left panel: Spectral weight map at the Fermi level (Fermi surface map) for PrO0.7F0.3BiS2. Right panel: Spectral
weight map at -0.2 eV for PrO0.7F0.3BiS2. h=30eV and T=20 K.

REFERENCES
1.

Y. Mizuguchi, H. Fujihisa, Y. Gotoh, K. Suzuki, H. Usui, K. Kuroki, S. Demura, Y. Takano, H. Izawa, and O. Miura, Phys.
Rev. B 86, 220510(R) (2012).
2. H. Usui, K. Suzuki, and K. Kuroki, Phys. Rev. B 86, 220501(R) (2012).
3. S. Demura, Y. Mizuguchi, K. Deguchi, H. Okazaki, H. Hara, T. Watanabe, S. J. Denholme, M. Fujioka, T. Ozaki, H.
Fujihisa, Y. Gotoh, O. Miura, T. Yamaguchi, H. Takeya, and Y. Takano, J. Phys. Soc. Jpn. 82, 033708 (2013).
4. T. Sugimoto, B. Joseph, E. Paris, A. Iadecola, T. Mizokawa, S. Demura, Y. Mizuguchi, Y. Takano, and N. L. Saini, Phys.
Rev. B 89, 201117(R) (2014).
5. S. Demura, K. Deguchi, Y. Mizuguchi, K. Sato, R. Honjyo, A. Yamashita, T. Yamaki, H. Hara, T. Watanabe, J. Denholme,
M. Fujioka, H. Okazaki, T. Ozaki, O. Miura, T. Yamaguchi, H. Takeya, and Y. Takano, J. Phys. Soc. Jpn. 84, 024709(2015).
6. T. Machida, Y. Fujisawa, M. Nagao, S. Demura, K. Deguchi, Y. Mizuguchi, Y. Takano, and H. Sakata,
J. Phys. Soc. Jpn. 83, 113701 (2014).
7. Masanori Nagao, Akira Miura, Satoshi Demura, Keita Deguchi, Satoshi Watauchi, Takahiro Takei, Yoshihiko Takano,Nobuhiro Kumada, Isao Tanaka, Solid State Commun. 178, 33 (2014).
8. T. Sugimoto, D. Ootsuki, C. Morice, E. Artacho, S. S. Saxena, E. F. Schwier, M. Zheng, Y. Kojima, H. Iwasawa, K.
Shimada, M. Arita, H. Namatame, M. Taniguchi, M. Takahashi, N. L. Saini, T. Asano, R. Higashinaka, T. D. Matsuda, Y.
Aoki, T. Mizokawa, Phys. Rev. B 92, 041113 (2015).
9.
Z. R. Ye, H. F. Yang, D. W. Shen, J. Jiang, X. H. Niu, D. L. Feng, Y. P. Du, X. G. Wan, J. Z. Liu, X. Y. Zhu, H. H. Wen,
and M. H. Jiang, Phys. Rev. B 90, 045116 (2014).
10. L. K. Zeng, X. B. Wang, J. Ma, P. Richard, S. M. Nie, H. M. Weng, N. L. Wang, Z. Wang, T. Qian, and
H. Ding, Phys. Rev. B 90, 054512 (2014).
11. N. L. Saini, D. Ootsuki, E. Paris, B. Joseph, A. Barinov, M. Tanaka, Y. Takano, T. Mizokawa, Phys. Rev. B 90, 214517(2014).
12. S. Dash, T. Morita, K. Kurokawa, Y. Matsuzawa, N. L. Saini, N. Yamamoto, Joe Kajitani, R. Higashinaka, T. D. Matsuda,
Y. Aoki, and T. Mizokawa, Phys. Rev. B 98, 144501 (2018)- 30 -

18BG005

BL-1

The Electronic Structure Investigation
on Pd Doped SrIrO3 Thin Film
Takashi Komesua, Prescott E. Evans a, Andrew J. Yost a
Eike F. Schwierb, Kenya Shimadab,
Le Zhanga, Xia Honga and P. A. DowbenaaDepartment of Physics and Astronomy, Nebraska Center for Materials and Nanoscience,
Theodore Jorgensen Hall, 855 N 16th, University of Nebraska, 880299, Lincoln, NE 68588-0299, U.S.A.
b
Hiroshima Synchrotron Radiation Center, Hiroshima University, Higashi-Hiroshima 739-0046, Japan
Keywords: High Resolution Angle Resolved Photoemission Spectroscopy, Electronic Structure, spin-orbital
interaction.

The goal of this research is to examine the surface electronic structure in a correlatedsemi-metallic oxide thin film, orthorhombic SrIrO3 (SIO), which possess strong spin-orbit
coupling (SOC). This effort is key to any effort to exploit the interfacial charge and control
quantum confinement to tune the relative energy scales of electron correlation. The band
structure, mapped at HiSOR, BL-1, using High Resolution Angle Resolved Photoemission
Spectroscopy (HR-ARPES), taken at 150 eV photon energy, show the itinerant character for
SrIrO3 [1], as seen in Figure 1. Indeed, there is clear evidence for an occupied density ofstates near the measured Fermi level, away from the Brillouin center, at 0.7 Å-1, as well as
significant dispersion of the occupied
bands, symmetric about the Brillouinzone edge, as is expected [1]. This
experimental
band
structureiscompletely
consistentwiththesemimetallic character of orthorhombic
SrIrO3 [1], but this measured band
structure is very different frommonoclinic SrIrO3 [2]. It is key tounderstand that we have now
experimentally identified some of the
bands predicted by theory, but missing
from prior experimental studies [1].
The SrIrO3 samples wereprepared by off-axis RF magnetron
sputtering, growing the SrIrO3 samples
on strontium titanate (SrTiO3). The
lattice match of the overlayer andsubstrate, as seen in Figure 2, is good.
Nb doped strontium titanate (SrTiO3)
Figure 1. The electronic structure of 5 nm think
substrates were used for electronic
orthorhombic SrIrO3 single crystal thin films
structure investigation. This work, inacquired at HiSOR BL1 beam line, taken with 150
combination with angle resolved X-rayeV photon energy, at room temperature.
photoemission, makes clear that the- 31 -

orthorhombic SrIrO3(001) (SIO) surface is
Sr-O terminated and that the surface
electronic structure is very different fromthe bulk.

Figure 2. X-ray diffraction showing the closelattice match of orthorhombic SrIrO3 (0001),
here denoted as SIO, with the strontium
titanate (SrTiO3) substrate, denoted as STO.
As noted above, Figure 1 shows
experimental band structure mapping of
Figure 3. Low energy electron diffraction orthorhombic SrIrO3(001), as taken at HiSOR,of orthorhombic SrIrO3(001) thin films at room temperature. The surface Brillouingrown on Nb doped strontium titanate zone, from the band structure, is consistent(SrTiO3) substrates. The electron energy is with the (001) surface, as determined by low
energy diffraction (LEED) shown in Figure 3,
38 eV.
but not perturbed by the 2x2 surface
reconstruction hinted at in the low energy electron diffraction (there is little evidence of bandfolding), suggesting that only some of the surface is subject to a surface reconstruction atroom temperature.
References
1) Y. F. Nie, P. D. C. King, C. H. Kim, M. Uchida, H. I. Wei, B. D. Faeth, J. P. Ruf, J. P. C. Ruff, L.
Xie, X. Pan, C. J. Fennie, D. G. Schlom, and K. M. Shen, Phys. Rev. Lett. 114 (2015) 016401
2) T. Takayama, A. N. Yaresko and H. Takagi, J. Phys.: Condens. Matter 31 (2019) 074001- 32 -

16AU003, 17AU010, 18BU008

BL-5

Current activities of research and education on BL-5
(FY2018)
T. Yokoya, T. Wakita, Y. Muraoka and K. Terashima
Research Institute for Interdisciplinary Science, Okayama University
Research Laboratory for Surface Science, Okayama University, Okayama 700-8530, Japan
Keywords: photoemission spectroscopy, photoelectron emission microscopy

We present an overview of our resent research and educational activities on beamline 5 (BL5) in the fiscal year
2018. Our beamline has two experimental stations in a tandem way. The first station is equipped with an angleresolved photoemission spectrometer (ARPES), a low energy electron diffraction (LEED) apparatus and an Xray source. The hemispherical analyzer of ARPES spectrometer (HA54, VSW) has a mean radius of 50 mm andis mounted on a twin axis goniometer in ultra-high vacuum chamber. Using this goniometer, one can perform
ARPES and photoelectron diffraction (PED) measurements. It is also possible to perform resonant
photoemission spectroscopy (RPES) measurements by using photon energy tunability of synchrotron radiation
with X-ray absorption spectroscopy (XAS) measurement. With the X-ray source (XR2E2, FISONS), we canperform an X-ray photoelectron spectroscopy (XPS) measurement for the chemical state analysis and the PED.
At the second station, we have installed a photoelectron emission microscope (PEEM, ‘PEEM III’, Elmitec).
PEEM provides a magnified image of lateral intensity distribution of photo-emitted electrons from a sample
surface. The spatial resolutions are several ten nanometers with Hg lamp and a few micrometers withsynchrotron radiation. The sample is transferred between the ARPES and the PEEM chamber in-situ, and one
can perform measurements at both stations for the same sample.
In the recent researches on BL-5, we have studied the electronic structure of potassium doped aromatic
molecule (Kx picene) [1], iron-based superconductor (FeSexTe1-x) [2], transition metal di-oxide films such as
VO2 thin films which exhibits a first-order metal-to-insulator transition at 340 K [3], CrO2 thin films which are
known as a half-metallic material [4], and TaO2 film which is stabilized with a new technique developed in ourgroup [5]. We have also studied the electronic structures of a high quality boron doped diamond film whichshows a signature of the highest superconducting transition temperature of 25 K [6] and a high quality single
crystal of YbFe2O4 which is one of multiferroic materials [7], by utilizing RPES at B-K and Fe-M2,3 edges,
respectively. In this fiscal year, we are studying the charge states of Lu atoms in Lu2@C82 and Lu2C2@C82 by
the photon-energy dependence of the valence band photoemission spectra, as presented in this symposium.
We have used the BL-5 for education activity as well, for example, practical education for undergraduate
students of Okayama University. The students have an opportunity to study the synchrotron radiation
mechanism and to experience XPS measurement which is very useful for the surface science research. We
accepted more than 100 students from 2006 to 2012. From 2014, we have started to join the practical lecture forexperiments using the beamline end stations in HiSOR for both graduate school students of Hiroshima andOkayama Universities. In this February, we have started a new project for education under a Japan-Asia youth
exchange program in science supported by Japan Science and Technology Agency (JST), “Sakura Exchange
Program in Science”. We have accepted six students from Changchun University of Science and Technology inChina this year.

REFERENCES
1.
2.
3.
4.
5.
6.
7.

H. Okazaki et al., Phys. Rev 82, pp. 195114 (5 pp.) (2010).
Y. Yoshida et al., J. Phys. Soc. Jpn 78, pp. 034708 (4 pp.) (2009).
K. Saeki et al., Phys. Rev 80, pp. 125406 (5 pp.) (2009).
Y. Muraoka et al., MRS Proceedings 1406 (2012).
Y. Muraoka et al., Thin Solid Films 599, pp. 125-132 (2016).
H. Okazaki et al., Appl. Phys. Lett 106, pp. 052601 (5 pp.) (2015).
K. Fujiwara et al., Trans. Mater. Res. Soc. Jpn. 41, pp. 139-142 (2016).

- 33 -

18BU002

BL-06

Development of On-site Cleaning Method of Carbon
Contamination with Atomic Hydrogen
Masahito Niibe,a Takashi Tokushimaa,b,c, Tomohiko Konod, Yusuke Hashimotoc,
Yuka Horikawad, and Hiroaki YoshidaeaLaboratory of Advanced Science and Technology for Industry, University of Hyogo, Kamigori, Hyogo,
JapanbSanka High Technology Co. Ltd., Tatsuno, Hyogo, Japanc
MAX IV Lab. Lund Univ., Lund, SwedendGraduate School of Sciences and Technology for Innovation, Yamaguchi University, Yamaguchi, Japand
Department of Physical Science, Graduate School of Science, Hiroshima University, Higashi-Hiroshima,
Japan
Keywords: Contamination, Cleaning, Atomic hydrogen.

Carbon contamination of optical elements in synchrotron radiation beamlines is a serious problem [1-4].
In particular, for a mirror coated with easily-oxidizable-metal such as Ni, Cr, etc., carbon cleaning technique
by VU-O3 ashing is not applicable since Ozone cause surface oxidation resulting in reflectance lowering.
Therefore, the cleaning technique for this kind of mirrors is not established. Recently, we reported off-sitecleaning treatment using the atomic hydrogens is very effective in removing contamination of mirrors [5].
In response to this, we received requests from many people to develop atomic hydrogen cleaning equipmentthat can be used “on-site”. Therefore, we began to develop a small on-site cleaning device that can bemounted in a mirror vacuum chamber. Here we report on the first cleaning experiment conducted at the
HiSOR beamline BL-6 of Hiroshima University.
Fig. 1 shows the tip portion of the atomic hydrogen generating
apparatus. This device consists of tungsten (W) filament, current
introducing electrode, hydrogen gas introduction piping, hydrogen gas
source and pressure regulator, and DC power supply. These parts canbe easily installed in the vacuum chamber of the beam line by
fixing/positioning them to the short pipe with ICF-70 flange. By using
a container with a low-pressure hydrogen storage alloy as the gas
source, hydrogen gas can be easily supplied into the mirror chamber at
the synchrotron radiation experimental hall where it is difficult to
introduce a high-pressure dangerous gas cylinder. Hydrogen gas isbrought introduced in the vacuum chamber to contact with W filaments
heated to about 1700 °C for generation of atomic hydrogens by
catalytic reaction. As a preliminary experiment to confirm thegeneration of atomic hydrogens, the device was operated in a vacuum
chamber in front of a tungsten oxide powder sample. We have
FIGURE 1. Tip portion of atomic
successfully observed the color changes of the powder sample fromhydrogen generating apparatus.
green to black which is an evidence of the oxide reduction.
The first mirror cleaning experiment using atomic hydrogen was
carried out on the cleaning of the contaminated collector mirror of the monochromator in BL-6 of the HiSOR
facility. As the installation position of the W filament in two directions, laterally and above side from themirror, 4 to 5 times of cleaning were attempted, with input power 24 W to 40 W, gas pressure 10 Pa to 50
Pa, and processing time 1 hr. However, carbon contamination could not be removed visually.

- 34 -

Fig. 2 shows the spectra exited from the monochromator before and after the atomic hydrogen treatment.
It seems that a slight increase appears in the output of the spectrometer. However, errors due to beam position
fluctuations and others like are large, so it is difficult to make an assertive conclusion. Insufficient
improvement of the mirror reflectivity seems to be caused by insufficient input power and insufficient
generation of atomic hydrogens.
2018.11.20 15:41
2018.11.21 10:19
2018.11.21 19:00

8.00E-011
7.00E-011

PD Current (A)6.00E-011
5.00E-011
4.00E-011
3.00E-011
2.00E-011
1.00E-011
0.00E+000
200

400

600

800

1000

1200

1400

1600

1800

2000

Energy(eV)

FIGURE 2. Spectra emitted from the monochromator before and after the atomic hydrogen treatment.

In the future, we plan to improve the shape of the filament or heater so that we can introduce larger electric
power and generate more atomic hydrogen.

REFERENCES
1.
2.
3.
4.
5.

K. Boller, R.-P.Haelbich, H. Hogrefe, W. Jark and C. Kunz, Nuclear Instrum. Method. 208, 273-279 (1983).
T. Koide, S. Sato, T. Shidara, M. Niwano, M. Yanagihara, A. Yamada, A. Fujimori, A. Mikuni, H. Kato and T. Miyahara,
Nuclear Instrum. Method. Phys. Research A246, 215–218 (1986).
R. A. Rosenberg, Nuclear Instrum. Method. Phys. Research A291, 101–106 (1990).
R. Kohli and K.L. Mittal Ed., Developments in Surface Contamination and Cleaning (Elsevier, Amsterdam, 2015).
M. Niibe, T. Harada, A. Heya, T. Watanabe, and N. Matsuo, AIP Conf. Proc., 2054, 060010 (2019).

- 35 -

18BU003

BL-06

Ex-situ/In-situ Soft X-ray Absorption Investigation
Towards Corrosion of Cu and Passivation Behavior of Ti
Ying Jina, Qingrui Wanga, FeiFei Huanga, Yi-Tao Cuib, Hiroaki Yoshidac and
Takashi TokushimadaNational Center for Materials Service Safety, University of Science and Technology Beijing, No.30,
Xueyuan Road, Haidian District, Beijing P.R. CHINAb
SANKA High Technology Co. Ltd. 90-1,Kurimachi, Shingu-machi, Tatsuno, Hyogo,679-5155,Japanc
Department of Physical Science, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima, 7398526, Japand
MAX IV Laboratory Lund University, Box 117, 221 00 Lund, Sweden
Keywords: TC4, passive film, XAS, XPS, EIS, Mott-schottky

Passivation is the contributor for metals-based civilization since the active metals (Fe, Cr, Al, and Ti, etc.)
show excellent kinetic stabilities in oxidizing service environments [1]. Owing to their high strength-toweight ratios, superior fatigue strengths, modulus of elasticity and especially high corrosion resistance [2].
These metals and their alloys have been extensively used in kinds of fields. So copper (Cu), titanium (Ti)and their alloys are the most important materials having a wide range of application in many industries suchas electronic equipment, architectural engineering materials, marine equipment, and aeronautical materialetc. However, they may suffer damaging effects in corrosive environment.
This passive film provides Ti with the resistance to corrosion as long as the integrity of the film ismaintained. However, the nature, composition, and thickness of this protective film depends on theenvironmental conditions [3]. In order to investigate the effects of potentials and time on formation andfailure mechanisms of passivation comprehensively, as shown in figure 1, we have chosen commercial
samples of TC4 (Ti-6Al-4V) to do potentiostatic control at -0.6, 0, 0.5, 1.0, 1.5, 2.3 V (vs Ag/AgCl referenceelectrode) for 12 hrs, and at 1.0V for 2, 4, 8, 12, 24 hrs respectively in 0.5 M sulfuric to form the passive
film.
2.5

Potential (V)

2.0
1.5
1.0
0.5
0.0

-0.5
-1.0
0

2

4

6

8 10 12 14 16 18 20 22 24

Time (h)FIGURE 1. The conditions for potentiotatic control

To further understand the passivation, surface characterization techniques are often used. X-ray
Absorption Specstroscopy (XAS) is employed to reveal complex compositions and chemical states within
the film (as the figure 2 shows), also can support the data of X-ray Photoelectron Spectroscopy (XPS). Auger
Electron Spectroscopy (AES) is employed to reveal composition and thickness of passive Film. Beside,
Electrochemical impedance spectroscopy (EIS) is employed using a frequency range of 100 kHz to 10 mHz ,at 0 V (vs open circuit potential) and with a 10 mV amplitude of the alternating current (AC) signal to reveal- 36 -

corrosion resistance of the passive film. And Mott-schottky measurements is employed using a potential
fange from the film-formed potential to -1 V (vs Ag/AgCl reference electrode) at 1000Hz and with a 10 mV
amplitude of the AC signal to reveal semiconductive property of the passive film.

Ti 2p

Intenity

(a)

454

456

458

460

462

464

466

468

470

548

552

Energy(eV)O1sIntensity

(b)

524

528

532

536

540

544

Energy(eV)
FIGURE 2. The XAS (Ti 2p and O 1s) of TC4 in 0.5 M sulfuric acid

The main findings are summarized as follows:(a) The thickness of the passive film is proportional to the potential of titanium in potentiostatic control.
(b) The predominant oxide formed on titanium is TiO2, and with the increase of depth, the film is notcompletely oxidized because of the rapid increase of suboxides TiO and Ti2O3.
(c) The corrosion resistance of the passive film in passive region is proportional to the potential inpotentiostatic control. Suggesting that the worst corrosion resistance of passive film is formed in cathodic
region, and the medium corrosion resistance of passive film is generated in dissolve region, and the bestprotective passive film is formed in passive region.
(d) All the slopes of M-S plots are positive, implying the n-type semi-conductive character of the passive
film on TC4 [4]. And the semiconductive property of the passive film is also related to the potential ofpotentiostatic control.

REFERENCES
1. Macdonald D D. The Passive State in Our Reactive Metals-Based Civilization [J]. Arabian Journal for Science &Engineering, 2012, 37(5):1143-1185.
2. Pang J, Blackwood D J. Corrosion of titanium alloys in high temperature near anaerobic seawater [J]. Corrosion Science,
2016, 105:17-24.
3. Perini N, Corradini P G, Nascimento V P, et al. Characterization of AISI 1005 corrosion films grown under cyclic
voltammetry of low sulfide ion concentrations[J]. Corrosion Science, 2013, 74(Complete):214-222.
4. Li D, Wang J, Chen D, et al. Influence of passive potential on the electronic property of the passive film formed on Ti in
0.1M HCl solution during ultrasonic cavitation[J]. Ultrasonics Sonochemistry, 2016.

- 37 -

17AG009

BL-7

Photoelectron spectroscopy of YbCux systems
Hitoshi Yamaokaa, Hitoshi Satob, Naohito Tsujiic, and Kenya Shimadab,a

RIKEN SPring-8 Center, 1-1-1 Kouto, Sayo, Hyogo 679-5148, Japan
Hiroshima Synchrotron Radiation Center, Hiroshima University, Higashi-Hiroshima, Hiroshima 7390046, Japanc
International Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science,
1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japanb

Keywords: photoelectron spectroscopy, YbCux, temperature dependence

In rare-earth compounds some of elements such as Yb, Eu, Sm, and Ce often show the valence instability
between the two charge states. The 4f electrons of these elements have localized nature, while the electronic
state of the 4f electrons in the compounds is the result of the hybridization of the 4f electrons with conduction(c) electrons (c-f hybridization) which is characterized by the Kondo temperature. The valence of the rareearth elements of these compounds is a measure of the c-f hybridization and the Kondo temperature.
Temperature or pressure is a prominent tool to control the c-f hybridization or the Kondo temperature.
Here, we study the electronic structures of the Yb-Cu binary alloy systems, the compositions of YbCu,
YbCu2, YbCu4.5, YbCu5, and YbCu6.5, systematically. We measured the valence band spectra as shown in
Figs. 1(a)-1(c). The intensity is normalized by the area. The resonant increase of the intensity of the Yb3+
component was observed at the 4d-4f resonant energy of 182 eV. The data shown here were measured at theoff-resonant energy of 176 eV. The broad and large peak around 2-5 eV is the component of Cu 4d. The
intensity of the Yb3+ component in YbCu and YbCu2 is very small, suggesting nearly divelent Yb state ofthese compounds and supporting the results of the measurements of the PFY-XAS at SPring-8. The valenceband spectra of YbCu2 reproduced the spectra measured previously [1].
Another remarkable issue is the difference of the Cu 3d component. The electronic structure of the Cu 3dcomponent of YbCu, YbCu4.5, YbCu5, and YbCu6.5 are similar, but different from YbCu2. The width of the
Cu-3d component increased with increasing the Cu content. The Cu 3d component consists of mainly two
peaks and the separation of the two peaks of YbCu2 is much larger than others.
Temperature dependence of the valence band spectra were measured for YbCu, YbCu$_2$, YbCu4.5, andYbCu5 as shown in Figs. 1(d)-1(m). No temperature dependence was observed in YbCu and YbCu2 and theresults of PES agree with those of XES. While the valence band spectra of YbCu4.5 and YbCu5 show weaktemperature dependence. In YbCu4.5 the intensity of the 4f peak increased with decreasing the temperature.
This indicated the increase of the Yb devalent state and agrees with the previous measurements of the PFYXAS spectra [2].
On the other hand, the Yb3+ component showed a unique temperature dependence which has never been
observed in the other Yb compounds. The peaks of Yb3+ components are assigned as shown in Fig. 1(j) fromthe analogy to the spectra of Yb2Pd2Sn [3]. Shifts to the lower-biding energy were observed in the peaks of
3
P and 1D, while other peaks keep the same energy positions and the intensity of the peaks in 1I, 3H, 3F, and3
H increased and that in 1D decreased with decreasing the temperature. At present we do not understand themechanism of the phenomena. But this temperature-induced behavior may correlate to the existence of three
types of Yb sites and complex crystal structure in YbCu4.5.
In YbCu5 we observe a slight increase of the intensity of the Yb2+ component as shown in Fig. 1(l). But it
is difficult to see the temperature-induced change in the intensity of the Yb3+ component as shown in Fig.
1(m). The Yb valence changed approximately from 2.97 at 300 K to 2.94 at 18 K [4]. It may be too small toobserve as the change in the intensity of the Yb3+ component of the valence band spectra clearly.

- 38 -

(a)

(b)

(d)

(e)

(f)

(g)

(h)

(i)

(j)

(k)

(l)

(m)

(c)

FIGURE 1. (a) Comparison of the valence band spectra of YbCu, YbCu2, and YbCu4.5 at 8 K and that of YbCu6.5 at 200 K(hn=176 eV). (b) Expanded view of the spectra near the Fermi level in (a). (c) Expanded view of the spectra around Yb3+
component in (a). (d) Valence band spectra of YbCu at 8, 150, and 210 K and at hn =176 eV. (e) Expanded view of the spectra
near the Fermi level in (d). (f) Valence band spectra of YbCu2 at 8 and 150 K and at hn=176 eV. (g) Expanded view of thespectra near the Fermi level in (f). (h) Valence band spectra of YbCu4.5 as a function of temperature at hn =176 eV. (i) Expanded
view of the spectra near the Fermi level in (h). (j) Expanded view of the spectra around Yb3+ component in (h). (k) Valenceband spectra of YbCu5 as a function of temperature at hn=176 eV. (l) Expanded view of the spectra near the Fermi level in (k).
(m) Expanded view of the spectra around Yb3+ component in (k).

REFERENCES
1. A. Fujimori, T. Shimizu, and H. Yasuoka, Photoemission study of valence fluctuation in YbCu2, Phys. Rev. B 35, 8945
(1987).
2. H. Yamaoka, N. Tsujii, Y. Yamamoto, Y. Michiue, J.-F. Lin, N. Hiraoka, H. Ishii, K.-D. Tsuei, and J. Mizuki, Reentrant
valence transition in YbCu4.5 under pressure, Phys. Rev. B 97, 085106 (2018).
3. H. Yamaoka, P.Thunstrom, N. Tsujii, I. Jarrige, K. Shimada, M. Arita, H. Iwasawa, H. Hayashi, J. Jiang, H. Namatame,
M. Taniguchi, N. Hiraoka, H. Ishii, K.-D. Tsuei, M. Giovannini, and E. Bauer, The electronic structure and the valence stateof Yb2Pd2Sn and YbPd2Sn studied by photoelectron and resonant x-ray emission spectroscopies, Phys. Rev. B 86, 085137(2012).
4. H. Yamaoka, I. Jarrige, N. Tsujii, N. Hiraoka, H. Ishii, and K-D. Tsuei, Temperature dependence of the Yb valence in
YbCu5 and YbCu5-xAlx Kondo compounds studied by x-ray spectroscopy, Phys. Rev. B 80, 035120 (2009).

- 39 -

17AG020, 18AU003

BL-7

Observation of Singly Occupied Molecular Orbital in
2-iodo Nitronyl Nitroxide Radical
Hiroaki Anzaia, Yusuke Onoa, Ryosuke Takakuraa, Hitoshi Satob,
Toshiyuki Matsuia, Satoru Noguchic, and Yuko Hosokoshicab

Graduate School of Engineering, Osaka Prefecture University, Sakai 599-8531, Japan
Hiroshima Synchrotron Radiation Center, Hiroshima University, Higashi-Hiroshima 739-0046, Japanc
Graduate School of Science, Osaka Prefecture University, Sakai 599-8531, Japan

Keywords: organic radical crystal, electronic structure, photoemission spectroscopy.

Organic radicals that bear an unpaired electron have been focus of research on molecular magnetic materials
because of their potential for applications in spintronics devices [1]. The 2-iodo nitronyl nitroxide (INN) is atypical stable radical with spin-density localization on NO groups and forms a chain structure with contact
between the NO group and the I atom along the a-axis [2]. It is therefore considered that the intermoleculararrangement of the NO groups is responsible for the ferromagnetic interaction at low temperature.
The intermolecular interaction of INN can be interpreted by the model of charge transfer (CT) between
orbitals of two radicals where the electronic configurations of the singly occupied molecular orbital (SOMO)
and the next highest molecular orbital (NHOMO) are taken into account [3]. The ferromagnetic electron
coupling is favored when spin-triplet state is stabilized after the charge transfer, as shown in Fig. 1(a). Theintermolecular overlap and the energy difference of the molecular orbitals are closely related to the exchangeinteractions [4]. Hence, it is important to identify the SOMO and NHOMO in the valence bands. Previous
studies of photoemission spectroscopy on the nitronyl nitroxide radicals have reported the multiple electronic
features near Fermi level (EF) [5,6].
Here, we report the valence-band structure of INN using photoemission spectroscopy with synchrotron
radiation. Single crystals of INN with honeycomb lattice were grown by slow evaporation of concentrated
solutions. The chemical structure is shown in Fig. 1(b). The experiments were performed at BL-7 of Hiroshima
Synchrotron Radiation Center. The data were collected at T = 300 K with hn = 60 eV. Total energy resolution
was set to 33 meV. The samples were kept under a high vacuum of 4.7×10-9 Torr during the measurements.
No beam-induced degradation of the samples was observed.
(a)

(c)

NHOMO
before CT(b) INN

Triplet state after CTONIntensity (arb.units)SOMOhν = 60 eV
T = 300 K

I

+

N

O

Exp.
Fit
Backgroud

-15

-

-10

-5

0

Energy, ω (eV)Figure 1. (a) The charge transfer (CT) process within a radical dimer [3]. (b) Chemical structure of INN. (c) Valenceband spectra (circles) of INN taken with hn = 60 eV at 300 K. The thick blue and black curves represent the result offitting analysis and the extracted peak components, respectively. The dashed line is a background. Vertical bars indicate
peak positions of the SOMO and NHOMO.

- 40 -

Figure 1(c) shows the valence-band spectra of INN. The well-defined sharp and intense peak is observed at|w| ~ 3.6 eV. Several peaks are also visible at |w| ~ 7.0 and 10.0 eV. The multiple peak features are inagreement with the previous photoemission studies on the nitroxide radicals [5,6]. We note that there exists ashoulder structure on the low-energy side of the 3.6 eV peak. This first band just below EF can be attributed to
the SOMO, and thus the second peak at 3.6 eV is the NHOMO.
The line shape of the spectra is rigorously determined from a fitting analysis with Voigt functions, as shownby thick blue curves in Fig. 1(c). The energy of SOMO is estimated as |w| ~ 2.1 eV. The energy difference
between the SOMO and NHOMO is 1.5 eV, which is approximately 1.0 eV smaller than the value reported in
the organic radical with the intermolecular antiferromagnetic interactions [6]. Indeed, the formation of a chain
structure stacked NO group and I atom with short separations leads to an enhanced orbital overlap in the SOMO
and NHOMO [2,4]. We considered that the observed SOMO and NHOMO states play an important role in the
ferromagnetic interaction with neighboring spins. Further experiments in wide energy and temperature ranges
are required to provide the detailed evolution of the ferromagnetic interaction in INN.
In conclusion, we investigated the valence band structure in INN using synchrotron radiation photoemission
spectroscopy. The SOMO is observed at |w| ~ 2.1 eV, for the first time. The energy separation of the SOMO
and NHOMO is estimated to be ~1.5 eV, which is ~1.0 eV smaller than the value on the antiferromagnetic
compound. These findings indicate that the close intermolecular SOMO-NHOMO contact enhances theintermolecular ferromagnetic interactions of organic radicals.

REFERENCES
1. S. Sanvito, Chem. Soc. Rev. 40, 3336 (2011).
2. Y. Hosokoshi et al., Mol. Cryst. Liq. Cryst. 271, 115 (1995).
3. K. Awaga et al., Mol. Cryst. Liq. Cryst. 232, 27 (1993).
4. I. Novak et al., Chem. Phys. Lett. 413, 351 (2005).
5. I. Morishima et al., Chem. Phys. Lett. 16, 336 (1972).
6. H. Anzai et al., Physica B 536, 664 (2018).

- 41 -

18AG002

BL-7

Photoelectron spectroscopy of Yb4TGe8 (T = Cr, Mn, Fe, Ni)
Hitoshi Yamaokaa, Hitoshi Satob, Masahito Hikijic, Shunsuke Yamanakac,
Chishiro Michiokac, Naohito Tsujiid, Kenya Shimadab,
and Kazuyoshi Yoshimura ca

RIKEN SPring-8 Center, 1-1-1 Kouto, Sayo, Hyogo 679-5148, Japan
Hiroshima Synchrotron Radiation Center, Hiroshima University, Higashi-Hiroshima, Hiroshima 7390046, Japanc
Department of Chemistry, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan,dInternational Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science,
1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japanb

Keywords: photoelectron spectroscopy, Yb4TGe8, temperature dependence, zero thermal expansion

Materials with zero thermal expansion (ZTE) or negative thermal expansion (NTE) are very rare, and haveattracted much interest for their physical mechanism as well as potential technological applications. Yb4TGe8(T = Cr, Mn, Fe, Ni, Co) has been found to show ZTE upon cooling. Interestingly, it has been suggested that
the ZTE is not originated from the temperature-induced change in the Yb valance [1]. The crystal structureof Yb4TGe8 is CeNiSi2-type with the space group of Cmcm. Yb and Ge zigzag chains are on the Ge square
net forming the alternating layers stacked along the long b axis. It was claimed that ZTE was caused by the
3-dimensional geometrical mechanism of the temperature-induced structural modulation arising from thecharge transfer from T atoms to the Ge net, destabilizing the square-net structure of Ge [1]. In Yb4FeGe8,
Yb4CrGe8, and Yb4CoGe8, the x-ray absorption spectra have been measured at three or four temperaturepoints, indicating the valence fluctuation of Yb. However, detailed study of the temperature dependence of
the Yb valence has not been clarified yet. The study of the correlation between the electronic and crystal
structures of these ZTE compounds under pressure may provide clues to understand the ZTE mechanism of
the valence fluctuation systems. We have measured the temperature and pressure dependence of the Yb
valence at SPring-8. Here, complementary to the measurements at SPring-8, we study the temperaturedependence of the valence band and core-level spectra of Yb4TGe8 (T = Cr, Mn, Fe, Ni).
We performed photoelectron spectroscopy to observe the electronic structure below the Fermi level.
Figure 1(a) shows incident energy dependence of the valence-band spectra of Yb4MnGe8 at room
temperature around Yb 4d-4f resonance. Resonant enhancement of the intensity of the Yb3+ component wasobserved at 182 eV as in other Yb compounds. In Fig. 1(b) we show an example of wide band spectra of
Yb4FeGe8 at on- and off-resonant energies. The temperature dependence of the spectra shown below were
measured at off-resonant energy of 176 eV. Figure 1(c) shows a comparison of the valence band spectra of
Yb4TGe8 (T = Cr, Mn, Fe, Ni) at 8 K. The intensity of the Yb2+ component is very weak compared to that of
the Yb3+ component in these four compounds, indicating nearly Yb3+ mean valence state. The intensity of
the Yb2+ component of Yb4FeGe8 is larger than those of the other three compounds and the Yb valencefluctuation of Yb4FeGe8 is relatively stronger than the others. In Yb4CrGe8 there is a broad peak around 4
eV. Figures 1(c) shows a comparison of the Ge 3d core-level spectra of Yb4TGe8 (T = Cr, Mn, Fe, Ni) at 8
K. The peak of Ge 3d consists of 3d3/2 and 3d5/2. Intensity ratio of 3d3/2 to 3d5/2 components depends on T
and the intensity of 3d5/2 of the T = Fe sample is stronger than the others. The intensity of 3d3/2 at 8 K isstronger than that at 300 K as shown in Figs. 1(d) and 1(e). This may be caused by the increase of theintensity of the tail component above the binding energy of 30 eV at low temperatures. The intensity of theYb2+ component increased at low temperatures as shown in Figs. 1(f), 1(i), 1(f), and 1(o). The spectra of the
Yb3+ component shift to the lower binding energy at low temperatures. This temperature-induced behavior
has not been observed so far in the Yb compounds.

- 42 -

(b)

(a)

(c)

(d)

(e)

(f)

(g)

(h)

(i)

(j)

(k)

(l)

(m)

(n)

(o)

(p)

(q)

FIGURE 1. (a) Incident energy dependence of the valence band spectra of Yb 4MnGe8 around Yb 4d-4f resonant energy at
300 K. (b) Wide band spectra of Yb4FeGe8 at on and off resonances at 300 K. (c) Comparison of the valence band spectra of
Yb4TGe8 (T = Cr, Mn, Fe, Ni) at 8 K. (d), (e) Comparison of the Ge 3d core-level spectra of Yb4TGe8 (T= Cr, Mn, Fe, Ni) at
8 and 300 K.(f), (i), (l), (o) Temperature dependence of the valence band spectra of Yb2+ components of Yb4TGe8 near the
Fermi level. (g), (j), (m), (p) Temperature dependence of the valence band spectra of Yb 3+ components of Yb4TGe8. (h), (k),(n), (q) Temperature dependence of the Ge 3d core-level spectra of Yb4TGe8.

REFERENCES
1. S. C. Peter, M. Chondroudi, C. D. Malliakas, M. Balasubramanian, and M. G. Kanatzidis, J. Am. Chem. Soc. 133, 13840(2011).

- 43 -

18AG042

BL-7,9B

Electronic structure of Mn3Sn investigated by angleresolved photoemission
Shilong Wua†, Masashi Aritab, Kazuki Sumidaa‡,
Koji Miyamotob Hitoshi Satob, and Taichi OkudabaGraduate School of Science, Hiroshima University, 3-1-5 Kagamiyama, Higashi-Hiroshima 739-8586,
Japanb
Hiroshima Synchrotron Radiation Center, Hiroshima University, 2-313 Kagamiyama, Higashi-Hiroshima
739-0046, Japan
Keywords: Mn3Sn, Weyl semimetal

Weyl semimetals are novel topological materials that host Weyl fermions. This novel quantum matter has
been realized in certain inversion-symmetry breaking materials with large spin-orbit interaction such as
TaAs[1] and NbAs[2] systems. In these weakly correlated systems, some proof of Weyl semimetals such as
Weyl node or Fermi arc connecting Weyl points has been observed by ARPES measurement. Recently, it has
been expected that a non-collinear antiferromagnet Mn3Sn, which is a strongly correlated system, can be a
Weyl semimetal caused by the time-reversal-symmetry breaking and demonstrated the evidence bymagnetotransport measurement and ARPES observation[3]. However, not only because of the highly
correlated system but also uncleavable crystal structure obtaining the well-ordered surface is highly difficult
and the reported electronic structures observed by ARPES measurement are not clear enough to compare thedata with first principles calculation precisely.
For getting the atomically flat clean surface of the uncleaved metal surfaces, the repeated ion-sputtering andannealing procedure is widely utilized so far. Recently, it has been demonstrated that the procedure also
works to obtain a clean surface of some alloys such as YbB12 and SmB6 single crystals[4,5].
Here in this report, in order to obtain clearer evidence of the nature of Weyle semimetal in Mn3Sn, we have
tried to obtain the clean Mn3Sn(0001) surface by the sputtering and annealing procedure and observe theelectronic band structure by ARPES and spin-ARPES.
The Mn3Sn single crystal sample was grown by the bridgemann method. The orientation of the sample waschecked by the observation of the Laue pattern and mechanically polished with diamond paper. The polishedsample was sputtered with Ar+ ion at 550 V and post-annealed at 950 K in the ultra-high-vacuum chamber.
Figure 1(a) shows the low energy electron diffraction (LEED) pattern taken at Ek=37.2 eV. The clear LEED
spots and the absence of the C 1s or O 1s peak in the Auger spectrum (not shown here) indicate that the
sample surface was successfully cleaned by the procedure.
Figures 2 show the results of Fermi surface mapping as well as the band mapping along K-G-K and K-MK lines (red lines in the FS map) taken at hn=103 eV where the kz value corresponds to the bulk G point.
Hexagonal FS and observed band structures are in good agreement with the previous results reported
recently[3]. Although we have measured the well ordered clean surface, the observed band structure issimilar to the previous report and renormalized very much indicating that the material is a highly correlated
system. Similarly to the previous study, the Fermi arc has not been observed and no clear evidence of spinpolarization has been observed in our spin-ARPES measurement that was done near the kx, ky positions
where the existence of Fermi arc is expected (not shown here). Since our measurement was done around 50
K, the absence of spin-polarized states might come from the possible phase transition to the cluster glass- 44 -

phase with a ferromagnetic moment that appears below 50K[6]. Further investigation at a higher temperature
and with a magnetized sample is expected in the near future.

FIGURE 1. LEED pattern of Mn3Sn taken at Ek=37 eV after Ar+ sputtering and annealing.
Γ

K

K

K

Γ

M

K

Γ

1.0

Binding Energy (eV)Κ-1ky (Å )

Μ

0.0

Κ

Κ

Γ

-0.5

0.0

Binding Energy (eV)

0.0

Κ

0.5

-0.1
-0.2
-0.3
-0.4
-0.5

-1.0
-1.0

-0.5

0.0

-1

0.5

1.0

-1.5

-0.1
-0.2
-0.3
-0.4
-0.5

-1.0

kx (Å )

-0.5

0.0

0.5

1.0

-1

kx (Å )

1.5

-1.0

-0.5

0.0

0.5

1.0

-1

kx (Å )FIGURE 2. ARPES results of Mn3Sn surface taken at hn=103 eV.

REFERENCES
1. S.-Y. Xu et al., “Discovery of a Weyl fermion semimetal and topological Fermi arcs”, Science 349, 613–617 (2015).
2. S.-Y. Xu et al., “Discovery of a Weyl fermion state with Fermi arcs in niobium arsenide”, Nat. Phys. 11, 748-754(2015).
3. K. Kuroda et al., “Evidence for Magnetic Weyl Fermions in a Correlated Metal”, Nat. Mat. 16, 1090-1096 (2017).
4. K. Hagiwara et al., “Surface Kondo Effect and Non-Trivial Metallic State of the Kondo Insulator YbB12”. Nat. Commun.
7, 12690 (2016).
5. Y. Ohtsubo, et al., “Non-trivial surface states of samarium hexaboride at the (111) surface”, Nat. Commun. 10 2298-2304(2019).
6. S. Nakatsuji, N. Kiyohara, and T. Higo, “Large anomalous Hall effect in a non-collinear antiferromagnet at room
temperature”, Nature 527, 212–215 (2015).

†‡

present address: Koeln Universitat
present address: Tokyo Institute of Technology- 45 -

18AU005

BL-7

Elucidation of the electronic band of germanene
superperiodic structure
Osamu Kuboa, Seijiro Kinoshitaa, Hitoshi Satob and Taichi OkudabaGraduate School of Engineering, Osaka University,
2-1 Yamadaoka, Suita, Osaka 565-0871, Japanb
HiSOR, Hiroshima University,
2-313 Kagamiyama, Higashi Hiroshima, Hiroshima 739-0046, Japan
Keywords: germanene, ARPES

Monolayer sheets of group IV materials such as Si and Ge were also predicted to have Dirac coneanalogous to graphene. They are composed of buckled honeycomb structure which is predicted to introduce
very curious physical properties, such as quantum-spin Hall effect and tunable band gap by external electricfield [1,2]. These properties are more likely to be able to observe in germanene as compared with silicene
owing to its relatively large spin-orbit coupling [2]. Currently, formation of germanene has been reported
mainly on metal substrates such as Au(111), Pt(111), Al(111), and so forth. [3-5]. However, it is still
controversial whether the reported germanenes have Dirac cone. Recently, we have shown that ahoneycomb-like structure (germanene) of a Ge monolayer film with an √7×√7 period is formed on theAl(111) surface [6]. In this structure, however, the interaction with the substrate is strong and Dirac cone asa germanene did not appear. On the other hand, the interaction with the Al(111) substrate is reported to be
weak in the germanene with 3×3 periodicity reported in previous researches [4]. Thus, we have studied
electronic band structure of 3×3 germanene using angle resolved photoelectron spectroscopy (ARPES).

FIGURE 1. LEED image obtained from germanene on Al(111). White circles indicate 1×1 spots. (b) Schematic of Structuralmodel of Al(111)3×3-Ge surface. (c) ARPES intensity obtained from germanene on Al(111) along Γ-K direction.

- 46 -

Figure 1(a) shows a LEED image for an Al(111)3×3-Ge surface, which was formed in an ultrahighvacuum chamber by Ge deposition onto an atomically clean Al(111) surface at a substrate temperature
about 100 °C with an evaporation rate of 0.03 ± 0.01 ML/min for 30 min. A proposed 3×3-Ge structuralmodel [7] is shown in Fig. 1(b), in which the precise atomic coordinates are determined by our low
energy ion scattering spectroscopy [8].
Figure 1 (c) shows the results of ARPES measurement for 3×3-Ge surface with incident light energyof 53 eV obtained along the Γ-K direction of Al (111). This direction is assigned to a Brillouin zone ofAl(111)3×3 phase as the direction of γ-k-m-k-γ (described in small letters above Fig. 1(c)). In addition
to the surface band of Al(111) (black line in the figure), various band dispersions not observed before
Ge deposition were observed on the surface. It has been assumed that Ge honeycomb structure forms
2×2 superstructure in the Al(111) 3×3-Ge, and Dirac cone is expected to be expressed at its k point. In
fact, a linear band dispersion is seen around the k point indicated by an arrow in Fig. 1(c). When theincident light energy was changed, however, the linear dispersion around k point disappeared alongwith the surface state of Al(111), which suggests that this band dispersion is derived from Al layer but
not from surface Ge layer.

REFERENCES
1.
2.
3.
4.
5.
6.
7.
8.

C.-C. Liu, et al., Phys. Rev. Lett. 107, 076802 (2011).
M. Ezawa, New J. Phys. 14, 033003 (2012).
M. E. Dávila, et al., New J. Phys. 16, 095002 (2014).
M. Derivaz, et al., Nano Lett. 15, 2510 (2015).
L. Li, et al., Adv. Mater. 26, 8420 (2014).
S. Endo, et al., Appl. Phys. Express 11, 015502 (2018).
Y. Fukaya, et al., 2D Mater. 3, 035019 (2016).
O. Kubo, et al., unpublished.

- 47 -

18AU019

BL-7

Photoemission spectroscopy study
for half-metal Heusler compounds
K. Gotoa, b, Y. Sakurabab, H. Satoc and K. Honoa, baGraduate school of Pure and Applied Science, University of Tsukuba, 1-1-1 Tennodai Tsukuba 305-0047,
Japanb
National Institute for Materials Science, 1-2-1 Sengen, Tsukuba 305-0047, Japanc
Hiroshima Synchrotron Radiation Center, Hiroshima University, 2-313 Kagamiyama, Higashi-hiroshima
739-0046, Japan

Keywords: Half-metal Heusler compounds, ARPES, Photon energy scan

Co-based Heusler compounds such as Co2MnSi and Co2FeGa0.5Ge0.5 (CFGG) are predicted to have highspin-polarization of the conduction electron at the Fermi level. These materials have been attracting muchattention for spintronics devices of tunneling magnetoresistance (TMR) and current perpendicular to plane
giant magnetoresistive (CPP-GMR) devices [1-4]. However, observed large reduction of MR ratio withtemperature [1, 4] limits a potential of applications using half-metallic Heusler compounds. To understand acause of this problem, the spin-polarized electronic structure of bulk, surface and interface and its
temperature dependence should be observed by spin and angle-resolved photoemission spectroscopy(SARPES) which is the most powerful method for direct measurement of the electronic structure (energy,
moment and spin) in the solids. However, CFGG has 3-dimensional Brillouin Zone (BZ) and it is not clearthat which photon energy is suitable for observation of the specific band structure. First, a photon energy
scan using synchrotron radiation for CFGG should be performed to understand which photon energy is
optimal for specific kz point in the BZ.
In this study, the CFGG thin film sample which has L21 structure was prepared by the magnetron
sputtering method equipped at National Institute for Materials Science (NIMS) and it was delivered toHiroshima Synchrotron Radiation Center (HiSOR) with keeping high vacuum (~3.0×10-10 mbar) level by
the suitcase chamber. The surface quality was confirmed by low-energy electron diffraction (LEED). The
ARPES measurement was carried out at BL-7 of the HiSOR. The sample was kept at about 8.0 K during theexperiment. A photon energy scan was performed, ranging photon energy hν from 27-107 eV and the Fermi
surface was observed.

Figure 1 shows the Fermi surface of CFGG along kz direction obtained by a photon energy scan. In
the free-electron final-state approximation, the conversion for the perpendicular momentum kz given by
𝑘𝑧 = √2𝑚[(ℎ𝜈 − 𝜙 − 𝐸𝐵 ) cos2 𝜃 + 𝑉0 ]/ℏ, where 𝜃 is the emission angle of the electron, 𝜙 is the work
function and 𝑉0 is the inner potential. In the conversion of experimental data to Fermi surface, 𝑉0 = 9 eV
was used tentatively. However, the distinctive structure around high symmetry points for comparison withthe calculation to determine the proper value of the inner potential cannot be obtained in experimental Fermi
surface. The cause of this problem is considered that the sample surface was not clean because of oxygen- 48 -

and carbon. The surface cleaning should be performed by short annealing before ARPES measurement.

FIGURE 1. Measured Fermi surface of CFGG along kz direction. A photon energy scan was performed, ranging from 27 –107 eV. Converting the photon energy into kz with the inner potential 𝑉0 of 9 eV.

In summary, the photon energy scan was performed for half-metal Heusler compound CFGG thin filmand Fermi surface along kz direction was obtained to determine the proper value of the inner potential 𝑉0 .
However, the symmetric structure around high symmetry points cannot be observed because of the impurity
on the sample surface. The short annealing should be performed for Heusler thin film samples to clean thesurface before ARPES measurement.

REFERENCES
1.
2.
3.
4.

Y. Sakuraba et al., Appl.Phys. Lett. 88, 192508 (2006)
T. Ishikawa et al., J. Appl. Phys. 103, 07A919 (2008)
S. Li et al., Appl. Phys. Lett. 103, 042405 (2013)
J. W. Jung et al., Appl. Phys. Lett. 108, 102408 (2016)- 49 -

18BG029

BL-7

Photoelectron Spectroscopy of Thin-film Beta Tungsten
H.T. Leea, H. Yamaokab, A. Nagakuboa and H. Satoca

Graduate School of Engineering, Osaka University, Suita 565-0871, Japanb
RIKEN SPring-8 Center, Hyogo 679-5148, Japanc
Hiroshima Synchrotron Radiation Center, Hiroshima University, Higashi-Hiroshima 739-0046, Japan
Keywords: Electronic structure, Photoelectron spectroscopy, Spintronic devices, Tungsten

Thin film β-tungsten (β-W) is attracting extensive attention due to its giant spin Hall effect andpromising application in spintronics as memory devices. Large spin currents have been used to set themagnetization of adjacent magnetic layers using the spin torque effect [1]. The transfer efficiency ofthe charge-to-spin current is defined by the spin-Hall angle which is ~0.3 for β-W [2] in comparison
to <0.07 for α-W at 300 K. Utilization of the large spin-Hall angle of β-W appears promising for
applications in spin torque devices. However, the reason for the large magnitude of the spin Hall anglein β-W is presently not known. Clarifying whether the giant spin Hall effect arises from extrinsic or
intrinsic properties is the first step towards engineering β-W based spin torque devices. Extrinsic
effects arise from spin-dependent scattering with impurities or band structure peculiarities. Many
studies have examined the magnitude of the spin Hall angle as a function of W structure, but to date,hardly any experimental measurements of the electronic states of β-W have been reported. It isespecially important to characterize the band structure near the Fermi level as the large spin Hall
angles reported are experiments performed at very low currents.
In this work, the experimental goal was to characterize the electron density of states of depositedβ-W films by measuring high-resolution valence band spectra near the Fermi level and compare toreference α-W. Thin (~50 nm) α-W and β-W films on silicon substrates were produced using physicalvapor deposition technique. The mechanical and structural properties were characterized using a
combination of X-ray diffraction and picosecond ultrasonic measurements [3]. The elastic constantsof pure α-W and β-W were 5–10% lower than the reported bulk values for α-W. The calculated elastic
constants using density functional theory agreed within 10% of the experimental values.
In 2017, photoelectron spectroscopy (PES) measurements of thin α-W and β-W films were
performed. Figure 1 (a) shows the results for α-W as a function of in-vacuo Ar sputtering time. The
tungsten oxide layer formed due to air transport was removed following ~25 minutes of Ar sputtering(1 keV) as determined from the W 4f core level spectra. Figure 1(b) compares the photoelectron
spectrum between α-W and β-W samples. Fine difference in the structure between -2 and -4 eV wasobserved. Specifically, α-W clearly showed a double peak structure, which was less prominent in β-W.
However, such comparisons in peak shape and magnitudes were complicated by the fact that there
was an additional broad peak observed between -4 and -6 eV in experiments. This signal continuously
decreased with increasing sputtering time as illustrated in both Figures 1(a) and (b) - suggesting
partial contribution from surface oxygen contamination.
In 2018, PES measurements on 99.999% pure bulk-W sample was performed to clarify whether
the oxygen signals arose due to intrinsic oxygen impurities in deposited thin films or from surface
contamination during sputtering in the preparation chamber. Figure 2 (a) shows the results for bulk
α-W as a function of in-vacuo Ar sputtering time. Comparing the spectrum between 5- and 25-minutes
sputtering, it is clear that the contamination decreased then increased. This suggests that the Ar
sputtering process is a source of contamination, either indirectly by recoil implantation of surface
impurities or direct implantation from the Ar ion source itself. While the contamination is reduced
following longer sputtering time, it is nevertheless never reduced to zero. Figure 2 (b) compares thephotoelectron spectrum between bulk and thin film α-W samples. Qualitatively, the width and energyof the contamination peak in the region between -4 and -6 eV are in good agreement, although the- 50 -

absolute magnitudes differ due to differences in sputtering time. The fact that there is qualitatively
very little difference in the spectrum between a 99.999% pure bulk-W and a much less pure thin film
supports the notion that the impurities are being introduced by the Ar sputtering process itself. While
this is a disappointing result, it should be remembered that β-W thin films are almost exclusively
produced using physical vapor deposition methods with unknown control of oxygen impurities. Since
large spin Hall angles reported in other thin film materials generally arise due to spin-orbit interactionof impurities [4], or resonant/surface skew scattering effects with impurity sites [5], our results
indicate that the role of oxygen impurities on β-W surfaces should be clarified in the reported large
spin Hall angles of β-W layered systems.
(a) a - W (thin film)
1 min
5 min
25 min70 minIntensity (a.u.)

Before sputtering

-16

-14

-12

-10

-8

-6

-4

-2

0

E below EF (eV)FIGURE 1. Photoemission spectra for (a) thin film α-W sample as function of Ar sputtering time, and (b) comparison of
α-W and β-W samples showing the difference in peak structure between -2 and -4 eV.
1.0(a) a - W (bulk)

Intensity (a.u.)

0.8

Intensity (normalized)

25 min70 min5 min
110 min
120 min

Before sputtering(b)a-W (bulk) - 110 min
a-W (thin film) - 190 min

0.6

0.4

0.2

-16

-14

-12

-10

-8

-6

-4

-2

0.0
-16

0

-14

-12

-10

-8

-6

-4

-2

0

E below EF (eV)E below EF (eV)FIGURE 2. Photoemission spectra for (a) 99.999% pure bulk α-W sample as function of Ar sputtering time, and (b)comparison of bulk and thin film α-W samples showing qualitatively similar peak structure between -4 and -6 eV.

REFERENCES
1.
2.
3.
4.
5.

L. Liu et al., Science 336, 555 (2012).
C-F. Pai et al., Appl. Phys. Lett. 101, 122404 (2012).
A. Nagakubo et al., “Elastic constant of alpha and beta tungsten films studied by picosecond ultrasonics” USE2017
N. Nagaosa et al., Rev. Mod. Phys. 82, 1539 (2010).
G-Y Guo et al., Phys. Rev. Lett., 102, 036401 (2009).

- 51 -

16BG043

BL-9A

Angle resolved photoemission study of Sm1-xYbxB6
M. Arita1, H. Sato1, K. Shimada1, H. Namatame1, M. Taniguchi1, H. Tanida2, Y. Osanai3, K. Hayashi3, F. Iga4
1Hiroshima Synchrotron Radiation Center, 2Graduate School of Advanced Sciences of Matter,

Hiroshima University, Kagamiyama, Higashi-Hiroshima 739-0046, Japan
3Graduate School of Science and Engineering, 4College of Science, Ibaraki University, Mito, Ibaraki, 310-8512, Japan

1)
2)
3)

T. Takimoto et al., J. Phys. Soc. Jpn. 80 123710 (2011).
N. Xu et al., Nat Commun 5 (2014).
J. Jiang et al., Nat. Commun, 4, 3010(2013)- 52 -

0.0

Energy (eV)

-0.1

-0.2

-0.3

-0.4

-0.5
-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

-1

kx (A )

Fig.1 ARPES image of Sm0.9Yb0.1B6.

0.0

-0.1

Energy (eV)

SmB6 is well known as Kondo insulator, the energy
gap of which opens below ~100 K. Recently, it has
attracted great interest as candidate material of
topological Kondo insulator [1]. The spin-polarized
surface band structure near EF has been observed by spin
resolved ARPES for SmB6 (001) [2] .
Although YbB6 is one of candidate for topological
Kondo insulators, c-f hybridized gap has not been found[3]. It is controversial whether there is the topologicalsurface state or not.
To clarify the impurity effects for the energy gap andthe surface states in SmB6 and YbB6 near EF, we havemeasured the electronic band structures of Sm1-xYbxB6(x=0, 0.1 and 0.8) by means of ARPES.
＿
Figure 1 shows the ARPES image for x = 0.1 around with h= 26 eV at 8 K. The localized bands stem from
Sm2+ 4f bands at E ~ - 20 and – 160 meV. The bands
crossing the 4f bands are derived from rare-earth 5dbands, which are hybridized with the 4f bands strongly.
Between EF and - 20 meV, the bands exist and cross EF at
k ~±0.35 Å-1, similar to that in SmB6 [2]. There is nochange in the band structure with the substitution from Sm
to Yb in x ≤ 0.1. Thus, they are attributed to the
topological surface states as in SmB6, and robust for the
impunity of Yb ion.
＿
Figure
2
shows
the
ARPES
image
for
x
=
0.8
along


＿
X line with h= 26 eV at 8 K. Though the whole band
structure is similar to that for YbB6, the weaknon-dispersive bands derived from Sm2+ 4f are observed
at E ~ - 20 and - 160 meV same as in SmB6. The 5dderived bands cross EF at kF~ ± 0.25 and 3.8 Å-1,
respectively.However the bands are not hybridized with
Sm 4f bands clearly. Moreover, the surface band andenergy gap are not observed for SmB6 and Sm0.9Yb0.1B6
near EF. Thus, c-f hybridization gap and the surface
topological bands are not found for Sm0.9Yb0.1B6.

-0.2

-0.3

-0.4

-0.5
-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

-1

kx (A )

Fig.2 ARPES image of Sm0.2Yb0.8B6.

16BG043, 17BG007

BL-9A

Linear polarization dependence of angle resolved photoemission study
on SmB6
M. Arita1, H. Sato1, K. Shimada1, H. Namatame1, M. Taniguchi1, H. Tanida2, Y. Osanai3, K. Hayashi3, F. Iga4
1Hiroshima Synchrotron Radiation Center, 2Graduate School of Advanced Sciences of Matter,

Hiroshima University, Kagamiyama, Higashi-Hiroshima 739-0046, Japan
3Graduate School of Science and Engineering, 4College of Science, Ibaraki University, Mito, Ibaraki, 310-8512, Japan

Rare-earth hexaboride forms CaB6-type cubic crystal structure. The unusual physicalproperties appear originated from localized 4f bands. SmB6 is well known as Kondo insulator,
the energy gap of which opens below ~100 K. Recently, it has attracted great interest as a
candidate material of topological Kondo
insulator [1]. The spin-polarized surface band structure＿near the Fermi level (EF) around X has been observed by spin resolved ARPES
for SmB6 (001)＿[2]. On the other hand, the existence of the topological surface state around  is controversial,
because the band structures observed by
ARPES are obscure.
In order to clarify the topological＿surface states in SmB6 around  , wehave measured the electronic band
structures of SmB6 by means of ARPES
with the linear polarized lights paralleland perpendicular to the mirror plane,that is, P and S polarized lights.
Figures 1 (a) and (b) show the Fermi
surfaces (FS’s) for h= 26 eV at 8 Kwith P and S polarized lights, respectively.
＿
The elliptical FS’s centered at X point
are derived from the topological surface
state in thebulk c-f hybridization gap.
＿
Around  , two circular FS’s are Fig.1 Fermi surfaces of SmB6 measured with (a) P and (b)observed with S polarized light in first S polarized lights.
BZ and with P polarized light in second
BZ. Figure 2＿ shows
the ARPES E-k image for S polarized＿
light along  - X in first BZ at 8 K. The localized bands
at E ~ - 20 meV stem from Sm 4f bands. Between EF and4f band, there are two metallic bands which cross EF at k~±0.1 and ~ ±0.3 Å-1. Though the metallic state remains
below 300 K in inner band, the structure crossing EF inouter band disappears above 60 K. The temperaturecorresponds approximately to the temperature of the bulk
c-f gap opening. It is considered that the inner band stems
from the bottom of the bulk conduction state and the outerband is derived from the topologicalsurface state, which＿forms the circular FS centered at  below 60 K.
1)
2)

T. Takimoto et al., J. Phys. Soc. Jpn. 80, 123710 (2011).
N. Xu et al., Nat. Commun. 5, 4566 (2014).

- 53 -Fig.2＿＿ARPES image of SmB6 along - Xwith S polarized light.

17AG034

BL-9A

Probing bulk kz dispersion of a correlated semimetal CeSb
by low h ARPES
Kenta Kurodaa, Shuntaro Akebia, Ryo Noguchia, So Kunisadaa, Masashi Aritab,
Hideaki Kitazawac, Shik Shina, Hiroyuki Suzukia, Takeshi Kondoaa

The Institute for Solid State Physics (ISSP), The University of Tokyo, Kashiwa, Chiba 277-8581, Japan
Hiroshima Synchrotron Radiation Center, Hiroshima University, Higashi-Hiroshima, 739-0046, Japanc
National Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba 305-0047, Japanc

Keywords: Strong correlation, c-f mixing, magnetic transition

CeSb is a correlated semimetal with a rocksalt lattice, where its Ce3+ ions provoke Kondo transport[1] and at low temperature coordinate fourteen magnetostructures in the phase diagram [2, 3]. Even at zerofield, the seven phases with various long-period magnetostructures appear one after another below the Néel
temperature (TN) ~17 K. The magnetic ground state appears below ~8.5 K, as antiferromagnetic (AF) phasewith the double-layer stacking of the Ising-like ferromagnetic (001) planes. Various antiferro-paramagnetic(AFP) phases initially establish below the TN, where the double-layer modulation is periodically locked by
the paramagnetic (P) layers. This variety in the magnetic modulation is understood as “devil’s staircase”
behavior [4] that can be phenomenologically explained by presence of competing different magnetic
interactions.
Microscopically, the key mechanism for the above anomalies is for long time considered to be astrong hybridization between the localized Ce 4f moment and the itinerant carriers (c-f mixing). However,despite 40 years of intense researches, the low energy electronic structure corresponding to the devil’s
staircase has been so far elusive. Therefore, direct observations of the quasiparticle structure by angleresolved photoemission spectroscopy (ARPES) across the TN is strongly required, because it has been widely
applied for the strongly correlated systems and substantiated the impact of the c-f mixing on their
quasiparticle structures. Previous ARPES measurements on CeSb with surface-sensitive photon energy (h)have mapped the electronic structures in ordered phase, and suggested the imapact of the c-f mixing.
However, a critical issue was recently made for its surface sensitivity of ARPES measurements to investigate“intrinsic” bulk structures of CeSb [5]: either kz broadening effect [6] in the surface-sensitive ARPES or
possible surface resonant states can hinder the bulk responses.
In this study, we have performed relatively bulk-sensitive ARPES measurement by low-energy
photons from synchrotron at HiSOR BL9A to access the intrinsic bulk structures of CeSb. Figure 1(a)summarizes the ARPES images at 30 K above TN acquired with various h ranging from 6.4 to 7.4 eV. Forh= eV, we observe, in the paramagnetic phase, the two-hole bands originating from the Sb 5p state [5].
With increasing h, our data clearly shows that the top of the hole bands shift downward in energy. Thisbehavior becomes more clearly visible in Fig. 1(b) where the energy distribution curves (EDCs) at k||=0 are
presented. All of these data provide a signature for the kz dispersion, demonstrating that the bulk-sensitive
ARPES with low h, in sharp contrast to the surface-sensitive ARPES, enables us to detect the intrinsic bulk
states. More experiments with low h ARPES particularly at low temperature below TN and with high energy
resolution is necessary to experimentally investigate the microscopic mechanism of the devil’s staircase.

- 54 -FIGURE 1. (a) ARPES images of electronic structures at paramagnetic state by using various photon energies from 6.4 eV to
7.4 eV. (b) Their energy distribution curves (EDCs) at k||=0. The clear kz dispersions of the hole bands are traced by the circles.

REFERENCES
1. T. Kasuya, Y. Haga, Y. Kwon, and T. Suzuki, Physics in low carrier strong correlation systems. Physica B: Con-densed
Matter 186, 9 (1993).
2. J. Rossat-Mignod et al., Phase diagram and magnetic structures of CeSb. Phys. Rev. B 16, 440 (1977).
3. P. Fischer, G. Meier, B. Lebech, B. D. Rainford, and O. Vogt, Magnetic phase transitions of CeSb. I. zero applied magnetic
field. Journal of Physics C: Solid State Physics 11, 345 (1978).
4. J. von Boehm, and P. Bak, Devil's stairs and the commensurate-commensurate transitions in CeSb. Phys. Rev. Lett. 42, 122(1979).
5. K. Kuroda et al., “Experimental Determination of the Topological Phase Diagram in Cerium Monopnictides”, Phys. Rev.
Lett. 120, 086402 (2018).
6. V. Strocov, J. Electron Spectrsc. Phenom. 130, 65 (2003).

- 55 -

17BG018

BL-9A,BL9B

Semiconductor To Metal Transition And Spin-orbit
Coupling In Boron Doped Graphene Nanoribbons
Alexander Grüneis, Boris Senkovskiy
Physics Institute II, Zülpicher Strasse 77, Köln, Germany
Keywords: ARPES, graphene nanoribbons, on-surface polymerization

In our works at BL9A and BL9B we have investigated the electronic band structure of in-situ grown N=7
and N=14 armchair graphene nanoribbons (AGNRs). Samples were made by the following procedure. We
have cleaned a Au788 crystal by repeated sputtering and annealing and evaporated the organic precursor
material onto the clean Au788 surface. Following the annealing of the sample with precursor molecules, weobserved the fusing to an N=7 AGNRs with the well-known band structure [1].
Interestingly, we found that further annealing of the sample resulted in a new band structure that agrees
well with N=14 AGNRs. This finding is not totally unexpected since the are reports of the lateral fusion of
two N=7 GNRs to a N=14 GNR. However, to date there is no ARPES information. The ARPES that wemeasured is shown in the Figure 1. These data have been measured at BL9A. It shows that the N=14 valenceband is reaching the Fermi level and that these GNRs are expect to conduct electricity much better as thewide gap N=7 GNRs. It is thus clear that the metallicity of the GNR sample can be increased by theconversion of N=7 to N=17 AGNRs.
In the time following the beamtime at HiSOR we have performed many more epxeriments (UHV Raman,
Li doping and UHV transport) that proof unambigously the arge area formation of N=14 AGNRs and amanuscript is currently in preparation [2].

FIGURE 1. (a) and (b) ARPES spectra taken at BL9A (T= 9 K , E = 25 eV) of in-situ synthesized N=14 armchair graphene
nanoribbons overlaid to a calculation. (a) and (b) are taken at different perpendicular wavevector that can selectively enhance
the intensity of the linear band (labelled by VB1). (c) depicts a energy distribution curve (EDC) at the Gamma point. The
nanoribbon related bands VB5 and VB6 are indicated.

REFERENCES
1. Boris V Senkovskiy et al 2018 2D Mater. 5 035007.
2. Boris V Senkovskiy et al in preparation.

- 56 -

17BG026/18AG023

BL-9A

High-resolution ARPES Study of Ca3Ru2O7
D. Ootsuki a, A. Hishikawa a, Y. Takasuka a, D. Shibata a, Y. Shinya a,
N. Kikugawa b, M. Arita c, H. Tamatame c, M. Taniguchi c, and T. Yoshida aaGraduate School of Human and Environmental Studies, Kyoto University, Sakyo-ku, Kyoto 606-8501,
Japan,b
National Institute for Materials Science, Tsukuba, Ibaraki 305-0003, Japan,c
Hiroshima Synchrotron Radiation Center, Hiroshima University, Higashi-Hiroshima, 739-0046, Japan.
Keywords: Ru oxides, Angle-resolved photoemission spectroscopy, metal to insulator transition

Perovskite-type ruthenium oxides has rich physics such as spin-triplet superconductivity, orbital ordering,
antiferro/ferromagnetism, and metal to insulator transition with structural phase transition. Among them,
Ca3Ru2O7 is a quasi-2-dimensional semimetal showing an antiferromagnetic transition at TN = 56 K and astructural phase transition at Ts = 48 K [1-3]. Previous ARPES study revealed the presence of the tiny Fermi
surfaces at M and M’ points and the momentum-dependent gap at the low temperature phase [4]. However,the Fermi surface at the high temperature phase is not determined by the ARPES measurement.
In this work, we have performed high-resolution angle-resolved photoemission spectroscopy (ARPES)
measurements in order to reveal the origin of the antiferromagnetic transition and the structural transition of
Ca3Ru2O7. The ARPES spectra are were taken at BL-9A of Hiroshima synchrotron radiation center (HiSOR).
The total energy resolution was ~10 meV for the incident photon energy h = 21 eV. The sample surface of
Ca3RuO7 was obtained by cleavage in ultrahigh vacuum of ~1.4×10-9 Pa . The Fermi level was calibrated
by using the Fermi edge of the gold reference samples.
Figure 1 shows the temperature dependence of the Fermi surface for Ca3Ru2O7. At the low temperaturephase, the large square contour with rounded corners connecting the M points and the more intense squarecomposed of four broad triangular feature are observed, which is in good agreement of the previous ARPES
study by Baumberger et al [4]. On the other hand, the large square contour disappears as well as the intense
inner square contour in the high temperature phase. The band dispersions along the -X line are displayedin Fig. 2. The broad hole-like band around -point is observed at the high temperature phase (Fig. 2 (b)). In
going from the high temperature phase to the low temperature phase, the “M”-shaped band dispersion
appears around -point (Fig. 2 (a)), corresponding to the intense inner square contour as shown in Fig. 1 (a).
These observations suggest the band folding due to the structural and/or antiferromagnetic transitions.

FIGURE 1. Fermi surface maps of Ca3Ru2O7 measured at (a) T = 20 K and (b) T = 65 K, respectively. The data were acquired
using h = 21 eV with circular polarization. The ARPES spectra were integrated within±10 meV.

- 57 -FIGURE 2. Band dispersions of Ca3Ru2O7 taken at (a) T = 20 K and (b) T = 65 K taken at h = 21 eV with circular polarization.
The data are corresponding to the cut #1 and #2 in Figs. 1 (a) and (b), respectively.

REFERENCES
1. G. Gao et al. Phys. Rev. Lett. 78, 1751 (1997).
2. Y. Yoshida et al. Phys. Rev. B 69, 220411 (R) (2004).
3. N. Kikugawa et al. J. Phys. Soc. Jpn. 79, 0244704 (2007).
4. F. Baumberger et al. Phys. Rev. Lett. 96, 107601 (2006).

- 58 -

17BU025

BL-09

ARPES Study on the New candidate Weyl
semimetal in XSi (X=Co,Rh)
Cong Li1 , Jianwei Huang1, Desehng Wu1, Wei Wu1, Guodong Liu1, Jianlin
Luo1, K. Shimada2 and Xingjiang Zhou1
National Laboratory for Superconductivity, Beijing National Laboratory for Condensed Matter Physics,
Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China
Hiroshima Synchrotron Radiation Center, Hiroshima University, 2-313 Kagamiyama, Higashi-Hiroshima,
Hiroshima 739-0046, Japan
1

2

Keywords: Weyl semimetal, RhSi4, ARPESThe Weyl semimetal is now a very hot topic in condensed matter physics field. So far, Dirac Fermionsand Weyl Fermions have been discovered experimentally. A brand-new kind of double Weyl Fermions
with Chirality have been predicted in 2017 by Shoucheng Zhang’s group in Stanford[1], Zahid Hasan’sgroup in Princeton[2], and one group in Institute of Physics, Beijing[3]. But so far, no experimental
confirmation has been made yet mainly because of the lack of high quality samples. In order to study
the electronic structure of the new candidate Weyl semimetal, we need use different photon energy to
study this material.
Figure 1 shows the Crystal structure and electronic properties of CoSi family. CoSi is belong cubic
crystal system with No.198 space group, the crystal structure as shown in Fig. 1a. The corresponding
BZ is shown in Fig. 2b. The projected BZ of the (001) surface is marked in red lines. The band structureof Co(Rh)Si along the high symmetry line without spin orbital coupling as shown in Fig. 1c. It exists
spin-1 excitations at Gamma point and double Weyl Fermions (WF) at R point. The 3D Fermi surfaceof Co(Rh)Si at calculated Fermi level is show in Fig. 1c. Because of nontrivial topology possessed byhole and electron pockets in the bulk, the Fermi arc surface states can be observed on the side surface.
The electronic spectra for the (001) surface is shown in Fig. 1d. We can see that topological surface
states (marked by SS) emerge from projections of spin-1 excitation and a double Weyl fermion at Γ andR points, which are stable in a large energy window. The Fermi surface of Co(Rh)Si at Fermi level are
shown in Fig. 1e. Fig. 1f shows the Laue spot of RhSi sample. The clear Laue spot indicated the highquality single crystal of RhSi.
Figure 2 shows the band structure of RhSi. In order to confirm the material is on the light path, wemeasured the large energy range photoemission spectra (EDC) with 150 eV photon energy. Two peakcan be clearly observed on the inset of Fig. 2a, one is corresponding to the 99.8 eV and anothercorresponding to the 99.2 eV, which match the core level of 2p1/2 and 2p3/2 orbital, respectively. So we
confirm the RhSi sample is on the light path. Fig. 2b shows the Fermi surface of RhSi sample. And theband structure of RhSi measured with 150 eV photon energy and 50 eV photon energy are show in theFig. 2c-2e. The band structure and Fermi surface are looks not clearly. So more experiment should be
do for further analysis.

- 59 -

Figure 1. Crystal structure and electronic properties of Co(Rh)Si. (a) The crystal structure of Co(Rh)Si, the yellow
solid ball stand for the Co(Rh) atom, while the blue solid ball stand for Si atom. (b) The Brillouin zone (BZ) for
the Co(Rh)Si. The projected BZ of the (001) surface is marked in red lines. (c) The band structure of Co(Rh)Si
along the high symmetry line. It exists double Weyl Fermions in R point in BZ. The inset shows the 3D Fermi
surface of Co(Rh)Si at calculated Fermi level. (d) The corresponding electronic spectra for the (001) surface. Blacksolid curve stand for the bulk band. (e) The Fermi surface on the (001) surface of CoSi family.[1] (f) The Laue spot
figure of RhSi.

Figure 2. Electronic structure of RhSi. (a) The large energy range photoemission spectra measured with 150 eV
photon energy. Zoom in the EDC as shown in the inset of (a). (b) The Fermi surface of RhSi. (c) The band structureof RhSi measured with 150 eV photon energy along momentum cut marked in (a). (d) and (e) The band structureof RhSi measured with 50 eV photon energy along the momentum cut marked in (a). (d) shows the larger range
than (e).

REFERENCES[1] Peizhe Tang, Shoucheng Zhang et al., Phys. Rev. Lett. 119(206402) 2017[2] Guoqing Chang, M Zahid Hasan et al. Phys. Rev. Lett. 119(206402) 2017[3] Tiantian Zhang, Zhong Fang et al., Phys. Rev. Lett. 120(016401) 2018

- 60 -

18AG007

BL-9A

Electronic correlation effect in electron-hole systems
with orbital degeneracy
T. Mizokawaa, R. Matsumotoa, T. Mitsuokaa, T. Shimaiwaa, N. L. Sainib,
R. Jhac, R. Higashinakac, T. D Matsudac, Y. Aokic, and M. AritadaDepartment of Applied Physics, Waseda University, Shinjuku-ku, Tokyo 169-8555, Japanb

Department of Physics, University of Roma "La Sapienza", Piazzale Aldo Moro 2,00185 Roma, Italyc

Department of Physics, Tokyo Metropolitan University, Hachioji 192-0397, Japand

HiSOR, Hiroshima University, Higashihiroshima, Hiroshima 739-0046, Japan

Keywords: Wyle semimetal, excitonic correlation

In Weyl semimetals, a Dirac point is split into two Wely points in three-dimesional momemtum space
due to time reversal symmetry or space inversion symmetry breaking [1-5]. In particular, the Weyl ponits
and the surface Fermi arcs are identified in As-terminated TaAs by means of angle-resolved photoemission
spectroscopy (ARPES) [6-10]. Among the various Weyl semimetals, MoTe2 without space inversion
symmetry is known as a type-II Weyl semimetal where the Weyl cones are turned over in the momentum
space [11-14]. It is expected that Coulomb interaction between electron and hole plays important roles insuch semimetals or narrow gap semiconductors. In the present work, we have studied the electronic structureof MoTe2 by means of ARPES in order to clarify the effect of electron-hole correlation.
Figure 1 (a) shows the Fermi surface map of MoTe2 taken at photon energy of 23.8 eV. A series of Femi
pockets are observed along the kx axis. As shown in Figure 1(b), the left and right Fermi pockets are reducedin size with increasing binding energy. Therefore, these Fermi pockets have electron character. On the other
hand, the Fermi pocket around the zone center has hole character. The existence of electron and hole pockets
is consistent with the band-structure calculation. However, the two electron pockets are observed at kx = ±0.3 - 0.4 Å-1 whereas they are predicted at kx = ± 0.4 - 0.5 Å-1 in the calculation. In addition, the observed
hole pocket is much smaller than the theoretical prediction. This situation is very similar to that reported for
WTe2[15] in which the excitonic correlation between electron and hole is responsible for upward anddownward energy shifts of the electron and hole bands, respectively. Since the Weyl points are located above
the Fermi level in WTe2 and MoTe2, optical control of the excitonic correlation can be employed to tune theWeyl point at the Fermi level.

FIGURE 1. (a) The observed Fermi surfaces of MoTe2 taken at 23.8 eV. (b) Constant energy maps at selected energies.

- 61 -FIGURE 2. ARPES results near the Fermi level. ARPES intensity maps measured at photon energy of 23.8 eV along CUT1
to CUT8 direction indicated in Fig.1.

Figure 2 shows the band dispersions near the Fermi level for CUT1 to CUT8 as indicated in Fig. 1. The
electron pocket on the right side is clearly observed in CUT6-CUT8. The bottom of the electron band reaches-0.04 eV below the Fermi level. A surface band stems from the electron band bottom and reaches the holeband around -0.06 eV. Ref. 14 has reported domain dependence of the surface band which is not clear in thepresent MoTe2 crystal. The relationship between the electron-hole correlation and the surface band should
be clarified in future by means of space-resolved ARPES and time-resolved ARPES.

REFERENCES
1.
2.
3.
4.
5.
6.

H. Weyl, Z. Phys. 56, 330 (1929).
H. B. Nielsen and M. Ninomiya, Phys. Lett. 130B, 389 (1983).
S. Murakami, New J. Phys. 9, 356 (2007).
Xiangang Wan, Ari M. Turner, Ashvin Vishwanath, and Sergey Y. Savrasov, Phys. Rev. B 83, 205101 (2011).
A. A. Burkov, M. D. Hook, and L. Balents, Phys. Rev. B 84, 235126 (2011).
S.-M. Huang, S.-Y. Xu, I. Belopolski, C.-C. Lee, G. Chang, B. Wang, N. Alidoust, G. Bian, M. Neupane, C. Zhang, S. Jia,
A. Bansil, H. Lin, and M. Z. Hasan, Nat. Commun.6, 7373 (2015).
7. B. Q. Lv, H. M. Weng, B. B. Fu, X. P. Wang, H. Miao, J. Ma, P. Richard, X. C. Huang, L. X. Zhao, G. F. Chen, Z. Fang,
X. Dai, T. Qian, H. Ding, Phys. Rev. X 5, 031013 (2015).
8. S.-Y. Xu, I. Belopolski, N. Alidoust, M. Neupane, G. Bian, C. Zhang, R. Sankar, G. Chang, Z. Yuan, C.-C. Lee, S.-M.
Huang, H. Zheng, J. Ma, D. S. Sanchez, B. Wang, A. Bansil, F. Chou, P. P. Shibayev, H. Lin, S. Jia, M. Z. Hasan, Science
349, 614 (2015).
9. B. Q. Lv, H. M. Weng, B. B. Fu, X. P. Wang, H. Miao, J. Ma, P. Richard, X. C. Huang, L. X. Zhao, G. F. Chen, Z. Fang,
X. Dai, T. Qian, and H. Ding, Nat. Phys. 11, 724 (2015).
10. S.-Y. Xu, I. Belopolski, D. S. Sanchez, M. Neupane, G. Chang, K. Yaji, Z. Yuan, C. Zhang, K. Kuroda, G. Bian, C. Guo,
H. Lu, T. R. Chang, N. Alidoust, H. Zheng, C. C. Lee, S. M. Huang, C. H. Hsu, H. T. Jeng, A. Bansil, T. Neupert, F. Komori,T. Kondo, S. Shin, H. Lin, S. Jia, M. Z. Hasan, Phys. Rev. Lett. 116, 096801, (2016).
11. Yan Sun, Shu-Chun Wu, Mazhar N. Ali, Claudia Felser, and Binghai Yan, “Prediction of Weyl semimetal in orthorhombic
MoTe2”, Phys. Rev. B 92, 161107(R) (2015).
12. L. Huang, T. M. McCormick, M. Ochi, Z. Zhao, Michi-To Suzuki, R. Arita, Y. Wu, D. Mou, H. Cao, J. Yan, N. Trivedi,
and A. Kaminski, Nat. Mater. 15, 1155 (2016).
13. Ke Deng, GuoliangWan, Peng Deng, Kenan Zhang, Shijie Ding, EryinWang, Mingzhe Yan, Huaqing Huang, Hongyun
Zhang, Zhilin Xu, Jonathan Denlinger, Alexei Fedorov, Haitao Yang, Wenhui Duan, Hong Yao, YangWu, Shoushan Fan,
Haijun Zhang, Xi Chen, and Shuyun Zhou, Nat. Phys. 12, 1105 (2016).
14. M. Sakano, M. S. Bahramy, H. Tsuji, I. Araya, K. Ikeura, H. Sakai, S. Ishiwata, K. Yaji, K. Kuroda, A. Harasawa, S. Shin,and K. Ishizaka1, Phys. Rev. B 95, 121101(R) (2017).
15. R. Matsumoto, T. Sugimoto, T. Mizokawa, N. L. Saini, M. Arita, R. Jha, R. Higashinaka, T. D. Matsuda, and Y. Aoki, Phys.
Rev. B 98, 205138 (2018).

- 62 -

Proposal No.: 18AU016

BL-9A

Electronic structure of electron-doped
J=1/2 Mott insulators
Yong Hua, Zhiyuan Weia and Junfeng Heaa

University of Science and Technology of China, Hefei, Anhui 230026, China.

Keywords: Iridates, Mott insulators, Electronic structure

Perovskite strontium iridates, the so-called spin-orbit coupled Mott insulators, have attracted much recent
attention by the interplay of spin orbit coupling (SOC) and Coulomb interaction. The splitting of t2g energy
level by spin orbit coupling effectively reduces the width of each sub-band (J=3/2 and J=1/2), that a moderate
onsite Coulomb Repulsion U is sufficient to split the J=1/2 band into upper and lower Hubbard bands.
Theoretically, many exotic phenomena, including superconductivity, have been expected when this spinorbit coupled Mott insulator gets doped [1]. Experimentally, one important finding in electron-doped iridates
is the spectroscopic evidence for negative electronic compressibility [2]. Negative properties of materials,
although rarely existed, have attracted substantial general interests by their roles in changing the ways people
think about materials. As a prominent example, negative electronic compressibility (κe) does appear in somequantum materials when it is countered by the positive compressibility of other subsystems of the materials.
However, it was only found in very few dilute 2D systems in which the electron exchange energy dominates
over kinetic energy. Recently, we found most likely the first experimental evidence for 3D negative κe in(Sr1-xLax)3Ir2O7, which is driven by a dominance of the correlation energy of electrons at a relatively highdensity [2]. Our finding might open the door to an uncharted territory of negative κe which potentiallyfeatures a variety of bulk correlated metals. It is natural to ask whether the negative electronic compressibility
also exists in the single layer electron-doped iridates, (Sr1-xLax)2IrO4. To this end, we proposed systematic
ARPES study on the electronic structure of electron-doped J=1/2 Mott insulators (Sr1-xLax)2IrO4.

FIGURE 1. Fermi surface and band structure of the UD80 K Bi2212 sample. (a) Fermi surface of the UD80 K Bi2212. (b)Band structure along five momentum cuts around anti-nodal direction (cut 1 to 5 as labelled in a). Upper panel shows the raw
data, the spectra in the lower panel have been divided by the corresponding Fermi-Dirac distribution functions. The data weretaken at ~95 K.

ARPES measurements were carried out at HiSOR BL-9A on the La-doped Sr2IrO4 samples. However,
we didn’t get clear band dispersion on Sr2IrO4 samples with the Xe lamp (no synchrotron available), possibly
due to the matrix element effects in ARPES measurements. But these initial tests provided us some very
useful information for the follow-up beam time in other places. For this beam time, we carried out the back-- 63 -

up plan and measured Bi-2212 samples (UD80K). The Fermi surface mapping and band dispersion are
shown in Fig. 1. More systematic data analysis will be performed in the near future.

REFERENCES
1. F. Wang et al., Phys. Rev. Lett. 106, 136402 (2011).
2. J.-F. He et al., Nat. Mat. 14, 577-582 (2015).

- 64 -

18BG050

BL-9A

Photoemission study of mechanical polished FeSi [111] surface
M. Arita1, K. Shimada1, T. Kanomata2
1Hiroshima Synchrotron Radiation Center,Hiroshima University, Kagamiyama, Higashi-Hiroshima 739-0046, Japan
2Research Institute for Engineering and Technology, Tohoku Gakuin University

Energy (eV)

FeSi is a nonmagnetic semiconductor at low temperatures.
With increasing temperature, the magnetic susceptibility rises
and the temperature dependence of the electric resistivity showsa crossover from the semiconducting state to the metallic state at~300 K [1,2]. Thus FeSi can be viewed as the Kondo insulator[3]. In order to investigate the electronic structure andelectron-electron correlation effect in FeSi, the ARPES
measurements on FeSi [111] surface were carried out at BL-9A.
In previous our study, the ordered surface was obtained bycleavage, but it is difficult to obtain the surfaces on which
ARPES spectra could be observed [4]. In this paper, FeSi [111] Fig.1 LEED image of FeSi[111] for E = 110 eV.
clean surface could be obtained by mechanical polishing andannealing. The sharp LEED image can be seen from the
0.02
surface (Fig. 1). We examine a difference of theobserved spectra due to a difference of the surface
0.00
treatment procedure.
-0.02
Figures 2 shows the ARPES image for h= 8.4 eV at
10 K. The clear hole-like band, a maximum of which-0.04
exists about E = - 25 meV, is observed around kx= 0. As-0.06
the spectral intensity is enhanced near EF steeply, whichsuggests the strong renormalized band exists near EF due-0.08
to the electron-electron interaction.
To clarify the temperature dependence of the
-0.10
-0.2
-0.1
0.0
0.1
0.2
-1
renormalized band structure, we measured thekx (A )
temperature dependence of the normal emission spectra Fig.2 ARPES image of FeSi [111] withh= 8.4 eV.
with h= 8.4 eV. Figures 3 shows the spectra between
10 and 350K, which corresponds to the energy
distribution curve (EDC) at kx = 0. The strong peakstructure is found at 10K. The peak width broadens andthe energy is shifted toward EF with increasing
temperature. Though the spectral intensities at EF aresuppressed below 100 K strongly, the full gap structure
is not observed. It is considered that the measured
sample is the p-type semiconductor or that the metallic
surface state exists. The spectral intensity at EF increases
above 100 K and the peak and gap structures almost
Energy (eV)disappear at 350 K. These spectral structures and Fig.3 Temperature dependence oftemperature dependence are consistent with the EDCs at kx= 0 taken with h= 8.4 eV.
10 K

50 K

Intensity (arb. units)

75 K

100 K

150 K

200 K
250 K

300 K

350 K

-0.20

- 65 -

-0.15

-0.10

-0.05

0.00

0.05

0.10

0.15

temperature dependences of the electrical resistivity and the magnetic susceptibility [5,6,7] andcorrespond to the result of the previous study [4].
Compared with the spectral features measured for the cleaved surface, the band dispersion isobserved more clearly and the EDC peak intensity relative to the intensity around E = -0.15 eV
is higher for the polished surface. Thismeans the atomic order for the polished surface in a wide
area is better than that of cleaved surface. Therefore, it is expected that the ARPES spectra
having little deviation can be obtained for FeSi [111], because the clean surface with good
reproducibility can be prepared by the mechanical polishing.
[1] V. Jaccarino et al., Phys Rev. 160, 476 (1998).
[2] M.B.Hunt et al., Phys Rev. B 50,14933 (1994).
[3] G. Aeppli,et al., Comments Condens. Matter Phys. 16,155 (1992).
[4] M. Arita et al., Phys Rev. B 77,205117 (2008).
[5] K. Koyama et al., J. Phys. Soc. Jpn. 68, 1693 (1999).
[6] K. Koyama et al., J. Phys. Soc. Jpn. 69, 219 (2000).
[7] E. Arushanov et al., Phys. Rev. B 55, 8056(1997).

- 66 -

18BG051

BL-9A

Electronic structure of van der Waals ferromagnet Fe3GeTe2
X. Xu1, Y. J. Chen1, L. X. Yang1, Y. L. Chen1
1

Physical department, Tsinghua University, Beijing 100084, China

Keywords: van der Waals ferromagnet, Electronic structure, ARPES

Van der Waals compounds represent a large variety of layered materials with extremely weakinterlayer coupling. On one hand, they provide fertile platform to explore and realize various intriguing
emergent phenomena, such as charge/spin density wave, superconductivity, magnetism, and topologicalquantum phases. On the other hand, they can deliver exotic two-dimensional materials and heterostructures with extraordinary properties, such as quantum Hall effect and Dirac fermions in graphene,
valley transport in atomically thin transition metal dichalcogenides, quantum spin Hall effect inmonolayer topological quantum materials, and high temperature superconductivity in FeSe-SrTiO3
interface. These extraordinary properties both in thin films and bulk form of the Van der Waals
compounds not only provides significant scientific implications in condensed matter physics, but also
promise broad application potentials in the electronics and spintronics devices.
Recently, low-dimensional ferromagnets have attracted great research attention. Many different
types of van der Waals ferromagnetic materials, such as Cr2Si2Te6/Cr2Ge2Te6, CrI3/CrBr3, and Fe3GeTe2(FGT) [1-7], have been thinned to their physical limit to realize 2D ferromagnets. Among them, Fe 3GeTe2
has been intensively studied due to the realization of tuneable room temperature ferromagnetism in its
monolayer form. The survival of 2D ferromagnetism against the thermal fluctuation in FGT is protectedby the intrinsic magnetocrystalline anisotropy [6, 7]. Thickness dependent measurements have revealed
2D to 3D crossover of the ferromagnetism [6]. Besides the 2D ferromagnet in atomically thin FGT, bulk
FGT also exhibits fertile and intriguing properties, such as electron correlation [6], Kondo lattice physics[8], strongly enhanced electron mass, and extremely large anomalous Hall effect induced by topologicalnodal lines [9], coherent-incoherent crossover. However, despite the intensive research effort, thedetailed electronic structure of bulk FGT, especially its evolution, is still essentially lacking.

Figure 1 (a) Perspective view and (b) top view of crystal structure of Fe3GeTe2. (c) Resistivity andits derivative with respect to the temperature as functions of temperature. (d) Magnetic moments
as functions of the temperature along c and ab directions. (e) RMCD measurement of theferromagnetism in monolayer Fe3GeTe2. (f) RMCD map at magnetic field of 0.2 T at 78 K. (g)
RMCD sweeps at different temperatures in the monolayer Fe 3GeTe2. Panels (c) and (d) are from
Ref. [9]. Panels (e-g) are from Ref. [6].
FGT crystallizes into a layered hexagonal structure with space group of P63/mmc (No. 194). It
consists of stacking FeTe-FeGe-FeTe sandwich layers that are bonded by weak van der Waals interaction
as shown in Figs. 1(a) and 1(b). There are two different positions for the Fe atoms. Fe1 and Te form
pyramid, while Fe2 and two Te atoms form Te-Fe-Te vertical lines. There are two FeTe-FeGe-FeTe- 67 -

sandwich layers rotated 90 degrees about c axis in the unit cell of FGT. In the ferromagnetic state, themagnetic moment of all the Fe atoms aligns along the c axis, indicated by the orange arrows in Fig. 1(a).
Figure 1(c) shows the 3D Brillouin zone and its (001) surface projection. The crystal can be easily cleaved
along (001) direction, inducing clean and flat surface with large scale. Transport measurements suggest
a ferromagnetic transition near 220 K with large anisotropy (Figs.1(c) and (d)). Ab-initio calculations of
the electronic structure of FGT in the ferromagnetic and paramagnetic states (Fig. 1(h)) show thecomplicated band splitting due to the exchange interaction [9]. By thinned to its monolayer limit, FGT
still shows strong ferromagnetism at low temperatures with a thickness dependent ferromagnetictransition, as shown in Figs. 1(e)-(g) [9].

Figure 2 ARPES measurements on Fe3GeTe2. (a) Fermi surface mapping. (b) DFT calculation offermi surface from Ref. [9]. (c) Band dispersion along K. (d) DFT calculation from Ref. [9].

In this report, we study the electronic structure of FGT using high-resolution ARPES at beamline
9A in Hisor. The measured fermi surface consists of a hole pocket around the center of the Brillouin zone
and blurred spectral weight from the electron dispersion near the K points, which shows overall
consistence with previous reports (Figs. 2(a) and 2(b)) [9]. The cut in Fig. 2(c) shows the electron
dispersions along KGK. The bands show strong polarization dependence due to their 3d orbital nature.
Therefore, not all the bands in the calculation (Fig. 2(d)) are observed in our measurement using photons
with linearly horizontal polarization. We have measured the temperature evolution of the electronic
structure of FGT (not shown here), further studies and analysis of our data will provide important insights
to the intriguing physical properties of FGT.

REFERENCES[1]C. Gong et al., Nature. 546, 265–269 (2017).
[2] B. Huang et al., Nature. 546, 270–273 (2017).
[3]Y. Liu, C. Petrovic, Phys. Rev. B 96, 054406 (2017).
[4] G. Lin et al., Phy. Rev. B. 95, 245212 (2017).
[5] B. Liu et al., Sci. Rep. 6, 33873 (2016).
[6] Z. Y. Fei et al., Nat. Mat. 17, 778 (2018).
[7] Y. J. Deng et al., arXiv:1803.02038 (2018).
[8] Y. Zhang et al., Sci. Adv. 4:eaao6791 (2018).
[9] K. Kim et al., Nat. Mat. 17, 794 (2018).

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