Research Activities– Synchrotron Radiation Experiments –16AG040 & 17AG051

BL-1

The electronic structure of O-Fe(001) thin films grown on
MgO(001) studied by angle-resolved photoelectron
spectroscopy and density functional theory
Mingtian Zhenga, Eike F. Schwierb and Kenya ShimadababGraduate School of Science, Hiroshima University, Higashi-Hiroshima 739-8526, Japan
Hiroshima Synchrotron Radiation Center, Hiroshima University, Higashi-Hiroshima 739-0046, Japan

Keywords: Magnetism, Angle Resolved Photoelectron Spectroscopy, Density Functional Theory, Surface States

O-Fe(001) thin films grown on MgO(001) have attracted much interest within various areas of research.
It has been investigated for the magnetic tunneling junctions (Fe-MgO-Fe) where the interface is made upof O-Fe bonds [1]. The oxygen adsorbed Fe(001) surface can work as a highly efficient spin filter, and has
been used in the Very-Low-Energy Electron Diffraction (VLEED) based spin ARPES machines [2]. From amore fundamental point of view the correlated nature of the Fe 3d electrons is of interest.
In a previous study we found that O adsorption vastly reduces the scattering rate for photoelectrons,
allowing an unobstructed view on the electronic structure of surface [3] and bulk [4] states alike. Leading to
the extraction of self-energy information related to both electron-phonon and electron-electron(magnon)
many-body interactions. Here, we report a detailed comparison between high quality ARPES data and ourrecent density functional theory calculations (DFT).
In Figure 1 a) we show an ARPES spectrum at a selected photon energy of 50 eV along the ΓM direction measured in the p-polarization geometry (p-pol) at BL-1 of HiSOR [6]. This spectrum was chosenfrom several photon energy, polarization and high-symmetry direction dependent measurements as it showsmost of the bands present in the system. We symmetrized the spectrum with respect to the momentum origin
as a guide-to-the-eye in Figure 1b), while the following discussion is based on a detailed analysis of the raw
intensity of Figure 1 a). At least four bands can be identified to cross the Fermi level. Two electron-likebands around the Γ -point (A and B) and two almost linear bands close to the M -point (C and D). At abinding energy of ~250 meV, an additional hole-like band is situated at the Γ -point (E) and at 500 meV aflat band is visible at the M -point (F). In the current geometry ( ΓM and p-pol) the following orbitals canbe detected in the ARPES spectrum: Fe 3d (3z2-r2, xy, [x+y]z) and O 2p (z and x+y). These basis functions
do not change their sign under the symmetry operation of x ! y . The Fe 3d[x+y]z and O 2p[x+y] can beobtained by a linear combination of the orthogonal complex basis vectors 3dxz and 3dyz and 2px and 2py
respectively. This linear combination is necessary in order to obtain an orthogonal basis set of pure even andodd symmetries with regard to the ΓM high symmetry measurement direction.
Having obtained such high quality data, we used DFT to calculate the electronic structure of a 41 ML
thick Fe(001) slab covered with a p(1×1) O layer. DFT calculations were performed using the openMX code[7]. The slab was constructed from a fully relaxed bulk Fe lattice structure and the atomic positions in theslab where sub sequentially relaxed with residual force being smaller than 10-4 Bohr/Hartree. By analyzing
the overlap matrix elements as well as the LCAO coefficients of the resulting Kohn Sham Hamiltonian, thecontribution of each orbital of each atom to the spectral weight can be estimated. This method allows todetermine the orbital hybridization as well as spin structure of the surface and more bulk-like regions deeper
inside the slab. In the current analysis however, we focus on the contribution from the surface region alone.
By combining orbitals with even symmetry with respect to the ΓM high symmetry direction a simulation ofspectral weight can be obtained and compared to the high resolution ARPES measurements.

- 15 -FIGURE 1: a) Raw ARPES spectrum of O-Fe(001): p(1×1) surface obtained along the ΓM -direction in p-polarizationgeometry. b) Same as a) but symmetrized around k=0. c) Calculated orbital weight along the ΓM -direction with p-polarizationgeometry.

In Figure 1 c) the resulting orbital weight of the DFT calculation is plotted. The image plot is a summation
over all orbitals of even symmetry within the topmost 4 atoms including oxygen. Comparing the DFT band
structure with ARPES, it becomes clear that the electron pockets and hole pockets are well described
qualitatively, but not quantitatively in energy. While all bands (A-F) of the ARPES spectrum can be matchedto the DFT calculation, the theoretical binding energy of the electron- (A and B) and holepocket (E) aroundthe Γ -point is too high by a factor of ~3. The energy of the electron pockets at the M -point is also increased
by a factor of 3, if one considers the crossing point of C and D as a reference (ARPES ~300 meV vs DFT~900 meV). The binding energy of the flat band (F) around the M -point deviates by a factor of ~2.
This difference of calculated and experimental binding energies can be explained as a result of massenhancement due to the correlated nature of Fe 3d orbitals. Indeed, in a previous study [8], the mass
enhancement in bulk Fe(110) was estimated to be m/m0 ~ 3. However, mass enhancement usually does not
result in a change of the kF value of bands. There is however a discrepancy between bands A and C, whosekF values predicted by the DFT are much closer than in the ARPES spectrum. An explanation for thisdiscrepancy could be the effects of the charge transfer between oxygen and the finite iron slab as well as
effects related to a DFT functional dependent over- or under-estimation of exchange splitting. Since
calculated magnetic moment at the surface (~3 µB at the topmost Fe compared to ~2.1 µB inside the slab) issignificantly enhanced at the surface, the latter possibility has to be considered.
Such effects might also be responsible for the presence of the occupied hole-like band at the M -pointwith a binding energy of ~200 meV that is not found in the ARPES spectrum. The character of the band ishighly surface dependent and can be identified to be a surface state decoupled from a bulk state with abinding energy of ~ 1 eV (which was matched to band F in the ARPES after mass renormalization). Such a
surface band might become unoccupied as a result of more realistic description of the charge compensation
between the different orbitals at the surface and their magnetic moments.
Based on such promising matching between theory and experiment, the next steps of analysis will be thecomparison of charge transfer effects between oxygen and iron. An analysis of orbital hybridization basedon the calculated band structure as well as a comparison with fully relaxed clean Fe(001) slabs is underway
and expected to shine further light on the experimental findings.

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

A. Cattoni et al., Physical Review B 80, 104437 (2009).
T. Okuda et al., Review of Scientific Instruments 82, 103302 (2011).
M. Zheng et al., HiSOR Activity reports for proposal numbers 13-A-39, 14-A-52, 15-A-64.
HiSOR Activity reports for proposal numbers 16AG038
H. Iwasawa et al., J. Synchrotron Rad. 24, 836–841 (2017).
http://openmx-square.org
X.Y. Cui et al. Physical Review B 82, 195132 (2010)- 16 -

16AG047, 16BG031, 17AG019, 17BG012

BL-01

2-dimensional Surface Bands in Pb1-xSnxTe Thin Films
Ryota Akiyamaa Yusuke Otakib, Yuta Tomohiro, Keita Miyauchi, Ryosuke
Nakanishia, Ryo Ishikawab, Tomonari Yamaguchib, Hitoshi Satoc, Akio Kimurad,
Eike F. Schwierc, Koji Miyamotoc, Kenya Shimadac, Shuji Hasegawaa, andShinji KurodabaDepartment of Physics, The University of Tokyo, Tokyo, Japan
Institute of Materials Science, University of Tsukuba, Tsukuba, Japanc
Hiroshima Synchrotron Radiation Center, Hiroshima University, Hiroshima, Japand
Department of Physics, Hiroshima University, Hiroshima, Japanb

Keywords: Topological insulator, Topological crystalline insulator, Spin-orbit interaction, 2D material, Thin film

Topological insulators (TIs) are the new class of materials and have been investigated intensivly thesedays because of their abundant interesting and novel properties such as spin-momentum locking, spin-split
band, Dirac Fermion, and Majorana Fermion. In 3-dimensional TIs, the surface state is metallic whereas thebulk is insulating and has a linear band dispersion of a Dirac cone. Such surface state is called the topologicalsurface state (TSS), and the band is split for each spin except for the Kramers degeneracy points (k = 0). The
TSS is protected by the time-reversal symmetry and induced by the band inversion due to the strong spinorbit interaction.
Topological crystalline insulators (TCIs) are the group of TIs, but the TSS of TCIs is protected by the
crystal mirror symmetry instead of the time-reversal symmetry. As typical TCI materials, SnTe[1], SnSe[2],Pb1-xSnxTe[1], Pb1-xSnxSe [3] have been proposed and confirmed. The band characteristics of the TSS in
SnTe were demonstrated experimentally by the angle-resolved photoemission spectroscopy (ARPES) withbulk crystals of (001) and (111) orientation in 2012 and 2013, respectively[1,4]. On the other hand, theelectrical transport was measured in the thin films so far[5-8]. However, it has been difficult to obtain the
TSS of SnTe directly because SnTe tends to be heavily doped p-type. Especially, the detailed TSS of thethin film Pb1-xSnxTe (111) is unclear so far because there is almost no systematic investigation with changing
the incident photon energy (PE).
We have found that the Fermi level tuning by Sb doping can be realized and investigated the 2dimentional transport characteristic in Sb-doped Pb1-xSnxTe so far. In this research, we aim to observe thesurface band, particularly Dirac point, directly by ARPES. In TCIs, there is almost no work to check thechange of the Dirac cone by doping magnetic atoms. This is mainly because we can not see the surface bandaround the Dirac point clearly due to heavily p-type nature. Thus, the mechanism of the gap opening in TCI
is still unknown experimentally. If we investigate the gap with respect to such as quantum anomalous Hall
effect (QAHE), the effective tuning of the Fermi level is essential.
The band inversion of the valence and conduction band at L point occurs in Pb1-xSnxTe, and it is required
to realize the TSS. According to the previous reports, it occurs and the TSS persists with x >~ 0.25. We
confirmed that the carrier compensation occurs with doping Sb by electrical transport measurements. The
carrier density is changed with changing Sb concentration. Especially, Pb0.6Sn0.4Te:Sb can be tuned to bothp and n-type conduction. Thus, we tuned the Fermi level in the sample for ARPES to see the surface bandand to be topologically non-trivial by optimizing Pb content. Figures 1(a) and (c) show ARPES images of
Γ-M dispersion observed at PE of 60 eV in PbTe:Sb and Pb0.7Sn0.3Te:Sb thin films. Intriguingly, the valenceband shows Rashba-like splitting in the case of PbTe:Sb as shown in Fig. 1(a). Around this binding energy,
no similar bulk band is reproduced by calculations but some surface spin split bands exist. The precise band
coordination is sensitive for the termination and it has not been decided yet. Figure 1(b) and (d) represent
the enlarged and the contrast-tuned images to see the vicinity of the band gap well. The conduction band isobserved their band gaps are 0.25 and 0.3 eV in PbTe:Sb and Pb0.7Sn0.3Te:Sb, respectively. Although the- 17 -

intensity of the surface band in Pb0.7Sn0.3Te:Sb is not so strong, but it is surely observed in PE of 105 eV as
shown as white dot lines in Fig.2. This band is unchanged with changing PE, which indicates the 2dimensional band nature. It is suggested that the reason why the surface band at PE = 60 eV is not seenclearly is because the surface band of TIs is often easy to be seen at lower PE than 60 eV; the lower or higher
energy is better to see the surface band in this system.

FIGURE 1. ARPES images of Γ-M dispersion observed at the incident photon energy of 60 eV in PbTe:Sb(a,b) and Pb0.7Sn0.3Te:Sb (c,d), respectively.

FIGURE 2. ARPES images of Γ-M dispersion observed at the incident photon energy of 105 eV in

Pb0.7Sn0.3Te:Sb (c,d). The white dot lines represent the surface band.

REFERENCES
[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]

Y. Tanaka, Zhi Ren, T. Sato, K. Nakayama, S. Souma et al., Nat. Phys. 8, 800 (2012).
Z. Wang, J. Wang, Y. Zang, Q. Zhang, J.-A. Shi et al., Adv. Mater. 27, 4150 (2015).
P. Dziawa, B. J. Kowalski, K. Dybko, R. Buczko, A. Szczerbakow et al., Nat. Mat. 11, 1023 (2012).
Y. Tanaka, T. Shoman, K. Nakayama, S. Souma, T. Sato et al., Phys. Rev. B 88, 235126 (2013).
A. A. Taskin, F. Yang, S. Sasaki, K. Segawa, and Y. Ando, Phys. Rev. B 89, 121302 (2014).
B. A. Assaf, F. Katmis, P. Wei, B. Satpati, Z. Zhang et al., Appl. Phys. Lett. 105, 102108 (2014).
M. Safdar, Q. S. Wang, M. Mirza, Z. X. Wang, K. Xu, J. He, Nano Lett. 13, 5344 (2013).
R. Akiyama, K. Fujisawa, T. Yamaguchi, R. Ishikawa, and S. Kuroda, Nano Research 9, 490 (2016).

- 18 -

16BG053

BL-1

Observation of the Eu 4f 6 multiplet structure in EuPt2Si2
by angle-resolved photoemission spectroscopy
H. Anzaia, E. F. Schwierb, K. Shimadab, H. Namatameb, M. Taniguchib,A. Mitsudac, H. Wadac, and K. Mimuraaab

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, Kyushu University, Fukuoka 819-0395, Japan

Keywords: c-f hybridization, Eu compound, angle-resolved photoemission spectroscopy.

EuPt2Si2 with CaBe2Ge2-type structure exhibits an antiferromagnetic order at 15 K and keeps a nearly
divalent Eu state independent of temperature [1]. However, recent transport measurements have revealed thattemperature dependence of the electrical resistivity cannot be understood by single simple explanation based on
the Eu2+ ions [2,3]. Specifically, it is predicted that the competitive states among magnetic order, valencefluctuation and Kondo effect are associated with unusual response of the electrical resistivity [2,3,4]. To
understand the behavior of the electronic properties in EuPt2Si2, it is essential to observe the electronic structurenear Fermi level (EF) and unveil the detailed of Eu 4f and conduction bands.
Angle-resolved photoemission spectroscopy (ARPES) is a direct probe to measure the band dispersions inenergy and momentum spaces. Here, we report on a study of the electronic band structure of EuPt2Si2 obtainedby ARPES with synchrotron radiation. The experiments were performed at BL-1 of the Hiroshima Synchrotron
Radiation Center.
Figure 1(a) shows the photoemission spectra of EuPt2Si2 over a wide energy region taken with hν = 141 eV at
T = 15 K. Intense peak at |ω| ~ 100 eV originates from the Si 2p states. The spin-orbit split Pt 4f states areobserved at ~72 and ~76 eV. The well-defined peak is identified at ~1 eV near EF.
To understand the valence electronic structure, we performed ARPES measurements using hν = 70 eV
photons. Figure 1(b) shows ARPES spectra, I(k,ω), taken along the Γ-M direction of the surface Brillouin zone.
Three flat bands are observed at |ω| ~ 0.8, 1.0, and 1.3 eV. The bands at ~1.3 and ~1.0 eV are assigned to thesurface contribution from the topmost and subsurface region of Eu2+ ions, respectively [5]. In contrast, the bandat ~0.8 eV is the bulk components of the Eu 4f states [6].

Figure 1. (a) Photoemission spectra of EuPt2Si2 over a wide energy region. The data were obtained by using hν = 141! direction of the surface BrillouineV at T = 15 K. (b) ARPES spectra I(k,ω) near EF taken with hν = 70 eV along the Γ!-M
zone, as marked at the right side of panel (c). (c) Angle-integrated spectra for the data in (b). The dashed lines represent
the peak positions.

- 19 -

As shown by the angle-integrated spectra in Fig. 1(c), the photoemission intensity below |ω| ~ 0.8 eV seems
to be an oscillating character. A similar staircase-like structure near EF was also observed in the photoemission
spectra of Eu thin film on W(110) and assigned to the 7FJ multiplet derived from the electron configurations of
4f6 final states [7]. Thus, we assume that the peak at ~0.8 eV is the 7F6 terms.
The previous ARPES studies on EuNi2P2 with ThCr2Si2-type structure have revealed the hybridization effectbetween the 7FJ (J = 3-6) terms and Ni 3d states [8]. In current study, we have not detected the conduction
bands near EF and their hybridization with the Eu 4f bands. More detailed experiments over wide momentum
and temperature ranges are required to understand the energy balance among the antiferromagnetic ordering, thevalence transformation and the Kondo effect in EuPt2Si2.

REFERENCES
1. R. Nagarajan et al., Physica B 163, 591 (1990).
2. A. Mitsuda et al., Acta Phys. Pol. B 34, 1149 (2003).
3. Y. Ikeda et al., J. Magn. Magn. Mater. 310, e62 (2007).
4. A. Mitsuda et al., Physica B 403, 786 (2008).
5. T. Kinoshita et al., J. Phys. Soc. Jpn. 71, 148 (2002).
6. K. Mimura et al., J. Electron Spectrosc. Relat. Phenom. 137, 529 (2004).
7. G. Kaindl et al., Phys. Rev. B 51, 7920 (1995).
8. S. Danzenbächer et al., Phys. Rev. Lett. 102, 026403 (2009).

- 20 -

16BG055

BL-1

Low-energy heavy-fermion bands in YbCdCu4
revealed by angle-resolved photoemission spectroscopy
H. Shionoa, S. Ishiharaa, K. Mimuraa, H. Satob, E. F. Schwierb, K. Shimadab,
M. Taniguchib, S. Idetac, K. Tanakac, T. Zhuangd, K. T. Matsumotod,
K. Hiraokad, and H. AnzaiaabGraduate School of Engineering, Osaka Prefecture University, Sakai 599-8531, Japan
Hiroshima Synchrotron Radiation Center, Hiroshima University, Higashi-Hiroshima 739-0046, Japanc
UVSOR Facility, Institute for Molecular Science, Okazaki 444-8585, Japand
Graduate School of Science and Engineering, Ehime University, Matsuyama 790-8577, Japan

Keywords: c-f hybridization, Yb compound, angle-resolved photoemission spectroscopy.

In rare-earth compounds, the hybridization effect between conduction and localized-4f electrons (c-fhybridization) leads to the formation of heavy electrons, and it is characterized by a renormalized band structurenear the Fermi level (EF) [1]. According to the single-impurity Anderson model, a sharp resonance peak in theelectronic excitation spectrum appears at the energy scales of the Kondo temperature TK, which is a measure of
the hybridization strength [2]. The intensity of the Kondo resonance exhibits a strong temperature dependence
that reflects magnitudes of the c-f hybridization. Hence, temperature dependence of the Kondo resonance peaknear EF will provide important insights for understanding the heavy fermion state.
YbCdCu4 with C15b-type structure is known as a heavy fermion compound with the electronic specific heatcoefficient γ ~ 175 mJ/K2mol [3,4]. The Kondo temperature is estimated to be TK ~ 220 K from the magnetic
susceptibility measurements [5,6]. To understand the heavy fermion behavior of YbCdCu4, it is essential toobserve the electronic excitation spectrum and unveil the detailed of the Yb 4f and conduction bands. Angleresolved photoemission spectroscopy (ARPES) is a direct probe to measure the band dispersions in energy andmomentum spaces. Here, we report on a study of the hybridization effect in YbCdCu4 based on ARPES datanear EF. The experiments were performed at BL-1 of the Hiroshima Synchrotron Radiation Center (HSRC) and
BL7U of the UVSOR facility.

! direction at 15 K. The data were obtained using hν = 50
Figure 1. (a) ARPES spectra of YbCdCu4 taken along the Γ!-K
eV photons at BL-1 of the HSRC. (b) Momentum distribution curves near EF. (c) Angle-integrated spectra at various
temperatures. The vertical bars indicate the peak positions. (d) Energy shift of the Yb2+ 4f7/2 peak shown in panel (c),together with the data obtained in the UVSOR facility. Dashed line represents the result of the linear fits.

- 21 -

Figure 1(a) shows ARPES spectra of YbCdCu4 taken along the Γ-K direction of the surface Brillouin zone.
Two flat bands at |ω| ~ 0.02 and 1.3 eV are assigned to the spin-orbit split Yb2+ 4f7/2 and 4f5/2 states, respectively.
A dispersive band was observed over an energy range of 0.1 < |ω| < 1.2 eV. This conduction band intersects
with the 4f7/2 state and reaches EF, as seen from the peaks of momentum distribution curves in Fig. 1(b). Thus,
Fermi surface of YbCdCu4 at least consists of one hole band around the Γ point.
For quantification of the hybridization strength, we investigated the temperature dependence of the 4f7/2 state.
Figure 1(c) shows the temperature dependence of the 4f7/2 peak. We found an energy shift and an intensity
enhancement of the 4f7/2 peak with decreasing temperature. It is noteworthy that integrated spectral intensity ofthe peak is not conserved, and this behavior is incompatible with the simple thermal broadening of the metallic
density of states. In contrast, the temperature evolution of the 4f7/2 peak is consistent with the prediction of thesingle-impurity Anderson model [2]. The observed 4f7/2 peak near EF can therefore be assigned to the Kondo
resonance peak in YbCdCu4.
The sharpness of the 4f7/2 peak is sufficient even at 180 K and thus allows for precise determination of thepeak energy. Figure 2(b) shows the energy of the Kondo resonance peak as a function of temperature, together
with the data obtained in the UVSOR facility. The results coincide with each other within the error bars,
indicating the consistency of our experimental data. The peak energies decrease linearly with decreasing
temperature. The appearance of the Kondo resonance at kBTK in the limit of zero temperature represents theformation of the singlet ground state [2]. Thus, we have estimated the energy scale of the Kondo temperature at
T = 0 K. As shown by the dashed line in Fig. 2(b), the energy extrapolated to T = 0 K is estimated as ~ 26.5
meV, which corresponds to TK ~ 308 K. This value is approximately consistent with TK obtained by themagnetic susceptibility measurements [5,6]. This consistency demonstrates that the heavy fermion behavior of
YbCdCu4 stems from the hybridization between the observed 4f7/2 and conduction bands. Further experiments
over wide momentum and temperature ranges are required to understand the mechanism of the heavy fermion
behavior of YbCdCu4.

REFERENCES
1. A. N. Tahvildar-Zadeh, M. Jarrell, and J. K. Freericks, Phys. Rev. Lett. 80, 5168 (1998).
2. N. E. Bickers, D. L. Cox, and J. W. Wilkins, Phys. Rev. B 36, 2036 (1987).
3. K. Hiraoka K. Kojima, T. Hihara, and T. Shinohara, J. Magn. Magn. Mater. 140, 1243 (1995).
4. J. L. Sarrao, C. D. Immer, and Z. Fisk, C. H. Booth, E. Figueroa, and J. M. Lawrence, R. Modler,
A. L. Cornelius, M. F. Hundley, G. H. Kwei, and J. D. Thompson, F. Bridges, Phys. Rev. B 59, 6955 (1999).
5. T. Koyama, M. Matsumoto, T. Tanaka, H. Ishida, T. Mito, and S. Wada, Phys. Rev. B 66, 014420 (2002).
6. I. Felner and I. Nowik, Phys. Rev. B 33, 617 (1986).

- 22 -

17AG003

BL-1

Disturbance Influence on Conduction Pathway forElectron Carriers Doped near Anatase TiO2 Surface
Yoshihiro Aiura,a Kenichi Ozawa,b Eike F. Schwier,c and Kenya ShimadacaElectronics and Photonics Research Institute, National Institute of Advanced Industrial Science andTechnology (AIST), Tsukuba, Ibaraki 305-8568, Japanb
Department of Chemistry, Tokyo Institute of Technology, Meguro, Tokyo 152-8551, Japanc
Hiroshima Synchrotron Radiation Center, Hiroshima University, Higashi-Hiroshima, Hiroshima 7390046, Japan
Keywords: photoemission spectroscopy, electronic structure, TiO2, transparent conductive oxide, photocatalysis

TiO2 is one of the most important oxide materials used in applications such as photocatalysis [1].
Naturally grownTiO2 exists in three different polymorphs rutile, anatase, and brookite, and it is well known
that anatase TiO2 (a-TiO2) shows a higher catalytic activity than rutile phase. Since such photocatalysis is
closely related to the semiconducting properties, the microscopic electronic structure has been actively
studied by means of various spectroscopies. By irradiating ultraviolet (UV) light, it is well known thatoxygen vacancies are easily generated on TiO2 surfaces that are closely related to the photocatalysis [1].
Therefore, elucidating the microscopic behavior of the electrons doped by the oxygen vacancies near thesurface will be crucial to understand its photocatalytic properties. For a-TiO2, a metallic surface state, theso-called two-dimensional electron gas/liquid (2DEG/L), was reported to be formed upon irradiating with
UV-light [2]. Such UV-light irradiation induces oxygen vacancies in the conduction pathway near the surface,and so, the electronic behavior of the doped electrons is considered to be directly affected by the oxygen
vacancies. Recently, such a 2DEG/L was also observed by depositing potassium on the a-TiO2 surface [3].
Since the potassium ions are deposited on the surface, the doped electrons are supposed to be affected only
by loose and indirect disturbances in the conduction pathway. As inferred from a previous study of cuprate[4], the physical properties of oxides such as the semiconducting/conducting and catalytic properties are
considered to be sensitive to the conduction pathway. In order to understand the disturbance influence on the
conduction pathway for the electron carriers doped near the a-TiO2 surface, the electronic states induced bydepositing potassium were studied by means of photoemission spectroscopy (PES). Based on a simple band
picture [5], the electrons doped into a-TiO2 are supposed to conduct through a pathway of the antibonding
bands composed of the Ti 3dxy and oxygen 2p orbitals, which are located at the conduction band minimum.
The electrons doped by deposing potassium mainly occupy the metallic state just below the Fermi level (EF),and are considered to be the carriers responsible for the conductivity. The remaining small number ofelectrons are loosely trapped near the potassium ions at the surface, and form a (dispersion-less) localizedstate with a finite spectral intensity at EF, which contributes in part to the surface conductivity. Elucidating
the disturbance influence on the conduction pathway for the doped electron carriers of a-TiO2 is crucial forunderstanding the catalytically and semiconducting properties such as the mobility and dopability of theelectron carriers.
Single crystals of a-TiO2 grown naturally were used (SurfaceNet GmbH, Germany). Although the ideal
stoichiometric a-TiO2 should be an insulator based on band calculations [5], the naturally grown a-TiO2 usedin this study is a semiconductor, with a mobility of 1.3 cm2V-1s-1 estimated from Hall measurement. Since
no impurity was detected from X-ray fluorescence spectroscopy and energy dispersive X-ray spectroscopy,
it is believed that the conduction properties are a result of the electron carriers derived from the oxygen
vacancies in the bulk. The orientation of the a-TiO2 surface was set to be the naturally cleaving (101) plane.
The momentum direction shown in the angle-resolved photoemission spectroscopy (ARPES) images was
set to the [010] axis. To prevent the composition from deviating from the bulk value, clean surfaces wereobtained by fracturing the single crystal in situ below a pressure of 10-8 Pa. PES measurements were
performed using BL-1 of Hiroshima Synchrotron Radiation Center (HiSOR) [6]. In order to simultaneously
detect not only the valence bands including the gap states but also the shallow core levels such as O 2s, Ti- 23 -

3p, and K 3p, the photon energy was set as 100 eV. Linearly polarized synchrotron radiation (SR) in thehorizontal plane of incidence, i.e., that is, in p-polarization geometry, was used. The PES spectra werenormalized with the focusing mirror current. The valence bands and gap states were measured in the angularmode, whereas the shallow core levels such as K 3p, O2s, and Ti3p were recorded in the transmission mode.
The energy distribution curves (EDCs) of the valence band and gap states were obtained by integrated the
ARPES spectra along the acceptance angle of the energy analyzers. The spectra were taken at 20 K. The
instrumental energy resolution due to the energy analyzer and the SR was about 70 meV.

FIGURE 1. PES spectra of a-TiO2 (101) surface after potassium deposition. (a) Example of an ARPES image after potassium
deposition, corresponding to K #3 in Figure (f). The ARPES image was recorded along the [010] direction. (b) Potassium
deposition dependence of the gap states. The gap states are deconvoluted into metallic states and IGSs, as shown by the dottedlines. (c) and (d) Potassium deposition dependence of the metallic states and IGSs, respectively. (e) Potassium deposition
dependence of Ti 3p, O2s, and K 3p core levels, and the differential spectra between those observed after (K #1-#8) and before(K #0) potassium deposition. (f) Potassium-deposition dependence of the spectral intensities of the gap states (black line),
IGSs (blue line), and metallic states (red line).

The spectral change in the gap states upon depositing potassium was investigated. Nine PES spectra in
the gap states upon depositing potassium of a-TiO2, which are numbered K #0 to K #8, were successively
taken. As shown in Figure 1(e), the spectral intensity of the Ti4+ 3p core level monotonically reduces withthe deposition of potassium, whereas that of the Ti3+ 3p core level gradually increases. A successive reductionin the spectral intensity of the O 2s core levels is also shown, but this is considerably smaller compared withthe spectral enhancement observed in the K 3p core levels. Since oxygen vacancies were hardly formed near
the surface by irradiating the SR, the observed slight reduction in the spectral intensity of the O2s core levelis considered to be caused by the deposition of potassium on the surface. On the contrary, the clear changes
in the Ti 3p core level (Figure 1(e)) and in the gap states (Figure 1 (b)) are considered to be attributable tothe potassium deposition. The gap states were deconvoluted into metallic states and in-gap states (IGSs).
Figures 1(c) and 1(d) show the spectral contributions of the metallic states and IGSs, respectively. Since the- 24 -

spectra were taken at low temperatures, the clear dip structures of the metallic state, caused by electronphonon coupling, could be shown. The spectral width of the metallic states gradually increased withincreasing spectral intensity. The spectrum of the IGS just after cleaving (K #0) is almost symmetric aroundthe binding energy of -1.60 eV, but that after deposition (K #8), it shows an asymmetric shape, consisting ofa peak centered at about -1 eV and a tail structure on its high binding energy side. The changes in the
integrated spectral intensities of the gap states (black line), IGSs (blue line), and metallic states (red line) aresummarized in Figure 1(f). From this figure, it is seen that the integrated intensities monotonically increase
with the deposition of potassium.
As shown by the black line in Figure 1(c), the metallic state of the a-TiO2 surface before potassium
deposition shows a clear dip structure at the binding energy of about 0.1 eV. From recent PES studies of aTiO2 [2, 3], it is considered that the dip structure of the metallic state is derived from electron-phonon
coupling, as discussed considerably for cuprates [4]. The energy position of the dip structure is almost consistent with the energy of a longitudinal optical (LO) Eu phonon mode in the ab-plane and/or of a LO A2u
phonon mode along the c axis. The dip structure of the metallic state gradually becomes obscure withincreasing amount of deposited potassium. The metallic state of pristine a-TiO2 before potassium deposition
is formed by the original electron carriers in the bulk. After potassium deposition, on the other hand, themetallic state is derived from the electron carriers doped near the surface, in addition to the original bulk
contribution. Recently, such electron-phonon coupling strengths for a-TiO2 and SrTiO3 surfaces werereported to strongly depend on the carrier density [3]. In addition, it was shown that the surface phonon
energy of a-TiO2 depends on the orientation, and is somewhat different from that of the bulk. Therefore, it
is considered that the observed successive changes in the spectral shape of the dip structure in the metallic
state were caused by the increase in the electron carriers at the surface by depositing potassium, and/or by
the change in the surface phonon energy. For pristine a-TiO2, just after cleaving, the finite spectral intensity
in the vicinity of EF in the IGSs is shown by the black line in Figure 1(d). Observing it in detail, the spectral
intensity gradually increases from EF, and the shape is somewhat different from a stepped-like feature
characteristic of a Fermi-Dirac function. This spectral intensity near EF appears almost uniformly across theentire measured momentum space. For rutile TiO2, on the other hand, almost no spectral intensity wasobserved near EF. These results suggest that the finite spectral intensity of the IGSs near EF shown for pristine
a-TiO2 is closely related with the metallic state, i.e., it is a result of the secondary photoelectrons derived
from the inelastic scattering of the metallic states. Now, we shall discuss the change in the spectral intensityof the IGSs near EF caused by the deposition of potassium, shown in Figure 1(d). As the spectral intensityof the IGSs near EF increases with the deposition of potassium, the background intensity around the binding
energy of -3 eV also increases, but not so much compared with that of the IGSs. When potassium is heavily
deposited (K #8), especially, it is important to note that the background intensity becomes smaller than thatof the IGS near EF. This suggests that the net increase in the spectral intensity of IGS near EF due topotassium deposition is mainly caused by a phenomenon that is different from the inelastic scattering of themetallic state. As will be discussed later in detail, a dispersion-less state at - 0.83 eV is induced by potassium
deposition. The state at -0.83 eV is close to EF compared to the IGS derived from the oxygen vacancies inthe bulk of the pristine a-TiO2 before potassium deposition (-1.60 eV), and so, there is a finite spectral
intensity near EF. From another point of view, this suggests that the metallic state formed by potassium
deposition is less subject to the inelastic scattering, and does not contribute much to the background. It isconsidered that the electrons produced upon the depositing potassium are doped near the surface, and thatthe state at -0.83 eV is confined to the vicinity of the surface [3]. Those surface contribution induced bypotassium deposition should be hard to be accompanied by the background produced by the inelastic
scattering.
Subsequently, we will discuss the origin of the IGS at -1.60 eV and the state at -0.83 eV shown for thepotassium-deposited surface, which are extracted by subtracting the metallic contribution from the gap state.
It is seen from Figure 2 (c) that the IGS of the pristine surface just after cleaving consists of a dispersiveless state located at the binding energy of -1.60 eV and the background. On the basis of the spectral width,the IGS has no spectral intensity near EF, suggesting no contribution to electrical conduction. Previous softx-ray PES spectra of electron-doped SrTiO3 also showed the dispersion-less state located near the midpointof the band gap and discussed its origin in detail. In our previous PES studies on electron-doped SrTiO3, it
was shown that the spectral intensity of the gap state is closely related with the number of doped electrons,
suggesting that the gap state is not derived from the oxygen vacancies at the surface caused by the cleaving
process or the short SR irradiation before the first measurement. A similar IGS in the a-TiO2 was observedin the vicinity of the center of the band gap from resonant inelastic X-ray scattering measurement, and wasconcluded to be attributed to the oxygen vacancies in the bulk. Therefore, it seems reasonable to consider- 25 -

that the IGS located at -1.60 eV in our PES spectra also represents an intrinsic electronic structure derived
from the oxygen vacancies in the bulk and is unique to naturally grown a-TiO2. The IGS at -1.60 eV isconsidered to be a localized state, in which a fraction of the doped electrons are tightly trapped in the vicinityof the oxygen vacancies. Therefore, the IGS at -1.60 eV is hereafter referred to as a deep-trap state (DTS).
Theoretical calculations also predicted such a DTS for the a-TiO2 with oxygen vacancies in the bulk. Since
the DTS has almost no spectral intensity around EF, it is considered that the conductivity in the bulk isderived from the metallic state, while the DTS is closely related with the (electron) carrier generation
efficiency, or the dopability of the doped electrons.

FIGURE 2. (a) Difference spectra for a-TiO2 before and after potassium deposition. (b) Peak position and integrated intensityof the difference spectra. (c) IGS spectra before potassium deposition (blue solid circles). The Shirley background derived
from the metallic state is denoted by the green line. An empirical DTS (blue open circles) was estimated by subtracting thebackground (BG) from the IGS. The DTS was fitted using the Voigt function with the Shirley background (DTSVoigt, blue
thin line). The peak position of the DTS was estimated at -1.60 eV. (d) An example of the IGS spectra after potassium
deposition. The empirical shallow-trap state (STS; red open circles) was estimated by subtracting the BG and DTSVoigt fromthe IGS. The STS could be well fitted with the Voigt function multiplied with a Fermi-Dirac function (red line). The peak
position was estimated at -0.83 eV.

As shown in Figure 1 (d), the DTSs at -1.60 eV can still be observed as a shoulder structure even after
potassium deposition. This suggests that the electrons generated upon potassium deposition are doped only
close to the surface [3], and the bulk contribution observed in the PES spectra just after cleaving can also be
observed after potassium deposition. In order to extract the essential contribution of potassium deposition,
we subtracted the DTS before potassium deposition from the IGSs after potassium deposition. From thedifference spectra in Figure 3(a), it is clearly seen that another state gradually appears at -0.83 eV, which isalmost constant regardless of the amount of deposited potassium. The state at -0.83 eV appears over a wide
momentum area and shows no clear dispersion. Similar to the case of the DTS, a fraction of the electrons
doped due to potassium deposition are trapped in the vicinity of the potassium ions, which produces anotherlocalized state near the surface. The peak position of the localized state (-0.83 eV) induced by potassium
deposition is about half that of the DTS (-1.60 eV) induced by the oxygen vacancies. The difference in thepeak position of the localized state is considered to be caused by that in the valence between the oxygen
vacancies in the bulk (formally divalent) and the potassium ions at the surface (monovalent). The traps ofthe doped electrons in the vicinity of the monovalent potassium ions seem to be loose and incomplete, andso, the localized state at -0.83 eV induced by potassium deposition has a finite spectral intensity at EF. Thissuggests that the surface conduction observed after potassium deposition is mainly caused by the metallic
state, but is also partially attributed to the localized state at -0.83 eV. Hereafter, we refer to the localized state
at -0.83 eV as a shallow-trap state (STS). In Figure 2 (d), an example of the IGS spectra after potassium
deposition (black solid circles), together with the DTS (blue open circles) and STS (red open circles)
contributions, is shown. The DTS could be easily fitted by the Voigt function (blue line) with the background- 26 -

(black line). Similarly, the STS could be well reproduced by the Voigt function multiplied with a FermiDirac function (red line). Interestingly, the spectral shape of the STS is almost the same as that of the DTS,
as shown in Figure 2 (e), except for the finite spectral intensity of the STS.

FIGURE 3. Schematic view of the gap state of a-TiO2 due to the electrons doped by potassium deposition.

Figure 3 presents a schematic view of the gap state due to the electrons doped near the surface bypotassium deposition. Elucidating the microscopic relationship between the metallic surface state and the
IGS on the surface will be crucial in understanding the macroscopic semiconducting properties such as the(electron) carrier generation efficiency and mobility. The enhancement in the metallic state as a result ofpotassium deposition can be interpreted as being simply caused by the doping of the electrons into the surface
state called 2DEG/L. In addition to the metallic surface state, a fraction of the doped electrons are loosely
trapped around the monovalent potassium ions (-0.83 eV), and are shown as a STS having a finite spectral
intensity at EF, which is considered to contribute partially to the surface conductivity. Since the oxygen
vacancies in the bulk are formally divalent, on the other hand, it is supposed that the electrons around thesevacancies are more tightly trapped (-1.60 eV). As a result, the DTS reveals no spectral intensity at EF,
suggesting no contribution to the bulk conductivity. The oxygen vacancies in the bulk cause a direct disorderin the conduction pathway, and so, the intensity of the metallic state relative to the DTS, or the (electron)
carrier generation efficiency, is considered to decrease with increasing oxygen vacancies. On the other hand,the potassium ions on the surface do not directly disturb the (electron) carrier motion in the conduction
pathway, the ratio of the spectral intensities of the metallic state and the STS is hardly affected by the amountof deposited potassium. It seems that the (electron) carriers at the surface are effectively generated bypotassium deposition and the efficiency is hardly sensitive to the amount of deposited potassium.
In conclusion, we doped electrons near a-TiO2 surface by potassium deposition. Electrons are suppliedby the potassium ions outside the conduction pathway. Since there are formally no oxygen and tin vacancies
in the conduction pathway, the motion of the doped electrons is supposed to be only weakly and indirectly
influenced by the potassium ions. The electrons doped by potassium deposition mainly function as thecarriers responsible for the surface conductivity, and the few remaining are trapped around the deposited
potassium ions. Since the trap is loose and incomplete, the STS induced by the trapped electrons around themonovalent potassium ions has a finite DOS at EF. That is, there are two kinds of (electron) carrier sources:
the metallic state and the STS induced by potassium deposition. Minohara et al. showed that electrons canbe doped onto a-TiO2 by a heterointerface between a-TiO2 and the LaO-terminated polar surface of LaAlO3(001). [7] On the basis of the reported sheet carrier density, most of the electrons generated at the interface
are considered to behave as electron carriers. In addition, the mobility observed empirically is high (25 cm2V1 -1
s at room temperature), compared with that of the impurity/vacancy-doped a-TiO2 reported previously.
Although such semiconducting properties are thought to be originated from electron carriers doped into ideal
conduction pathway without the disturbance, it is hard to explain the temperature dependence of theresistivity merely based on the metallic state. Like the potassium-deposited a-TiO2 surface shown here, afraction of the electrons doped onto the interface are considered to be weakly and indirectly trapped/scatteredby the lanthanum/oxygen ions outside the conduction pathway. As a result, it is expected that weakly andindirectly trapped/scattered electrons also partially contribute to the interface conductivity, along with thecoherent electrons.

- 27 -

REFERENCES
1. A. Fujishima, X. Zhang, D. A. Tryk, Surf. Sci. Rep. 63, 515-582 (2008)
2. T. C. Rödel, F. Fortuna, F. Bertran, M. Gabay, M. J. Rozenberg, A. F. Santander-Syro, P. Le Fèvre, Phys. Rev. B 92,
041106(R) (2015)..
3. R. Yukawa, M. Minohara, D. Shiga, M. Kitamura, T. Mitsuhashi, M. Kobayashi, K. Horiba, H. Kumigashira, Phys. Rev.
B 97, 165428 (2018).
4. H. Eisaki, N. Kaneko, D. L. Feng, A. Damascelli, P. K. Mang, K. M. Shen, Z.-X. Shen, M. Greven, Phys. Rev. B 69,
064512 (2004).
5. R. Asahi, Y. Taga, W. Mannstadt, A. J. Freeman, Phys. Rev. B 61, 7459-7465 (2000).
6. H. Iwasawa, K. Shimada, E. F. Schwier, M. Zheng, Y. Kojima, H. Hayashi, J. Jiang, M. Higashiguchi, Y. Aiura, H.
Namatamea, M. Taniguchia, J. Synchrotron Rad. 24, 836–841 (2017).
7. M. Minohara, T. Tachikawa, Y. Nakanishi, Y. Hikita, L. F. Kourkoutis, J.-S. Lee, C.-C. Kao, M. Yoshita, H. Akiyama, C.
Bell, H. Y. Hwang, Nano Lett. 14, 6743−6746 (2014).

- 28 -

17AG009

BL-01

Angle-resolved Photoelectron Spectroscopy Study ofCe1-2-10 systems III
H. Yamaokaa, E. F. Schwierb, Y. Yamamotoc, K. Ohnod,
T. Nishiokad, K. Shimadab, and J. Mizukic,ac

RIKEN SPring-8 Center, Sayo, Hyogo 679-5148, Japan
Hiroshima Synchrotron Radiation Center, Hiroshima University, Higashi-Hiroshima, Hiroshima 739-0046, Japan

Graduate School of Science and Technology, Kwansei Gakuin University, Sanda, Hyogo 669-1337, Japand
Graduate School of Science, Kochi University, 2-5-1 Akebono, Kochi 780-8520, Japanb

Keywords: electronic structure, photoelectron spectroscopy, Ce1-2-10, DFT calculations, hybridization

Ternary intermetallic compounds of CeM2Al10 (M = Ru, Os, Fe) have attracted many interests becauseof their unusual physical properties [1,2]. An anomaly in CeT2Al10 was observed due to its anisotropic
magnetic order. Paramagnetic susceptibility exhibits large anisotropy in CeM2Al10, a > c > b at T > T0,
where i is the susceptibility along i-axis and T0 being the transition temperature to the antiferromagnetic(AFM) state. However, at T < T0 the AFM order occurs along the c-axis, not along the easy a-axis. Kondo
et al. suggested that a larger c– f hybridization along the a-axis could flip the magnetization easy axis towards
the c-axis [3]. Thus, the anomalies in CeT2Al10 may correlate to an anisotropy of the hybridization strength.
In CeM2Al10 a detailed study of the complex electronic structure, especially direct evidence of the anisotropyof the hybridization strength, has not been performed yet. By substituting Ru with Rh in Ce(Ru1-xRhx)2Al10( x~0.03-0.05) the axis of magnetization is flipped from c-axis to a-axis and the magnetic moment increases
to 0.8B/Ce. This makes the Rh substitution an ideal method to study the c– f hybridization anisotropy.
We performed XRD study for Ce(Ru1-xRhx)2Al10, indicating no structural change. We measure x-ray
absorption spectra at Ce L3 edge with partial fluorescence mode (PFY-XAS), corresponding to the highresolution absorption spectra [4]. The c-f hybridization effect appears as increase of f 0 intensity and decreaseof f 1 intensity in the PFY-XAS spectra, resulting increase of the Ce valence with the Rh substitution to Rusite. This may suggest a possible Lifshitz transition. We reported previously that the ARPES at the 4d–4f
resonance showed the anisotropy of the hybridization strength directly and the anisotropy changed drasticallywith a small amount of Rh-doping to the Ru site in CeRu2Al10. Here, we newly report the ARPES result for
CeFe2Al10. Figure 1(a) shows the ARPES results of CeFe2Al10 at h= 115 eV. The angle-integrated spectra
are shown in Fig. 1(b) for CeRu2Al10 and Fig. 1(c) for CeFe2Al10. Note that the intensity ratio of 4f 1 near the
Fermi level to 4f 0 around 2.5 eV is a measure of the c-f hybridization strength. In CeRu2Al10 strong
anisotropyofthehybridization strength isobserved
andthehybridization along thea-axis is much stronger
compared to that along
the c-axis. While theanisotropyofthehybridization strengthwasweakenedin
CeFe2Al10 as shown in
Fig. 1(c).
We also performed
DFT calculations for
CeRu2Al10. The results
are shown in Fig. 2. FIGURE 1. (a) ARPES results of CeFe2Al10 at h= 115 eV. (b) Angle-integrated spectra ofElectron doping the unit- CeRu2Al10. (c) Angle-integrated spectra of CeFe2Al10.

- 29 -

cell leads to (in first-order) a rigid-type band shift. This has significant influence on the Fermi Surface (seeFig. 2), but more importantly on the c-f hybridization at the Fermi level. For the undoped case the c-f
hybridization is strongest along GY followed by GX. Upon electron doping the c-f band along GY
becomes fully occupied and the GX direction dominates the c-f hybridization at the Fermi level (see Fig. 3).

FIGURE 2. The results of the DFT calculations with the Fermi surfaces of XZ plane (right figures) for Ce(Ru1-xRhx)2Al10.

FIGURE 3. The results of the DFT calculations: Rh-doping dependence of the c-f hybridization for Ce(Ru1-xRhx)2Al10.

REFERENCES
1. V. M. T. Thiede, T. Ebel, and W. Jeitschko, J. Mater. Chem. 8, 125 (1998).
2. T. Nishioka, Y. Kawamura, T. Takesaka, R. Kobayashi, H. Kato, M. Matsumura, K. Kodama, K.
Matsubayashi, and Y. Uwatoko, J. Phys. Soc. Jpn. 78, 123705 (2009).
3. A. Kondo, J. Wang, K. Kindo, Y. Oogane, Y. Kawamura, S. Tanimoto, T. Nishioka, D. Tanaka, H. Tanida,
and M. Sera, Phys. Rev. B 83, 180415 (2011).
4. Y. Zekko, Y. Yamamoto, H. Yamaoka, F. Tajima, T. Nishioka, F. Strigari, A. Severing, J.-F. Lin, N. Hiraoka,
H. Ishii, K.-D. Tsuei, and J. Mizuki, Phys. Rev. B 89, 125108 (2014).
5. elk version 4.0.16 (http://elk.sourceforge.net)- 30 -

17AG015

BL-1

Electronic structure of Pr1-xYxCoO3 in comparison withcopper delafossites studied by resonant photoemission
spectroscopy
D. Kanaia, R. Takayanagia, K. Takahashia, M. Okawaa,
Y. Kobayashib, and T. SaitohaaDepartment of Applied Physics, Tokyo University of Science, Katsushika, Tokyo 125-8585, Japanb
Department of Physics, Tokyo Medical University, Shinjuku, Tokyo 160-8402, Japan

Keywords: Pr1-xYxCoO3, electronic structure, resonant photoemission spectroscopy, magnetism

Perovskite-type cobaltites RECoO3 (RE: rare earths or Y) typically show a broad and strange
magnetic/electronic transition(s) with temperature [1-4], which has long been a major research interest on
the properties of RECoO3. So far, it has been established that the phenomenon essentially originates fromunusual spin-state evolution in the Co 3d electrons although it has not been completely understood. By
contrast, it has long been considered that the RE site acts merely as a charge reservoir and plays no importantrole in the magnetism and the electronic structure of RECoO3, even in the case of Ce, Pr, Nd, etc., whichhave magnetic 4f electrons. Considering the localized nature of the 4f electrons, it is reasonable to assume
that they are independent of the Co 3d electrons.
Recently, however, Kobayashi et al. found that the magnetism of Pr1-xYxCoO3 can’t be explained by sucha simple picture; Pr1-xYxCoO3 shows temperature dependent Van Vleck paramagnetism for the smaller Y
content (x≤0.4) while it shows a Curie-Weiss-like behavior for the lager content (0.5≤x≤0.9) [5]. Because
the effective moment deduced from the Curie-Weiss term was able to be explained by the 4f1 electron
configuration, they speculated that the Pr 4f electron of Pr3+ (4f 2) was partly transferred to the Co 3d state
due to the hybridization between Pr 4f and Co 3d via O 2p states in the diluted Pr (namely, large x) range.
In this work, we performed Pr 4d-4f resonant photoemission spectroscopy (RPES) measurements on Pr1xYxCoO3 (x=0.5 and 0.8) in order to examine the validity of this “Pr 4f−O 2p−Co 3d hybridization model”.
Polycrystalline samples of Pr1-xYxCoO3 (x=0.5 and 0.8) were prepared by the sol-gel method [5]. RPES
experiments were performed at the beamline BL-1 of the Hiroshima Synchrotron Radiation Center (HiSOR)
using a Scienta R4000 electron analyzer. The total energy resolution was 90-200 meV at 110-150 eV photon
energy range. Clean and fresh surface was prepared by fracturing the samples in situ right before themeasurements.
Figure 1(a) shows RPES spectra of Pr0.5Y0.5CoO3 in the Pr 4d-4f resonance region (hν =110eV-150eV).
The on- (123 eV) and off- (120 eV) resonance energies, highlighted by thick lines, were determined fromthe constant-initial-state (CIS) spectra deduced from the RPES intensity shown in Panel (a) (not shown for
x=0.8). The valence band continues from ~0.5 eV to about 8 eV and one can see the leading peak at 1 eVcharacteristic of the Co3+ oxides with the low-spin state [6,9].
Figure 1(b) shows the on-off difference spectra of Pr1-xYxCoO3 (x=0.5, 0.8) together with the on- and offresonant spectra. The difference spectra experimentally represent the Pr 4f partial density of states (PDOS).
It is noted that the experimental Pr 4f PDOS of this system (including PrCoO3 (x=0) [9]) has a double-peak(α and β) structure whereas that of PrMO3 (M = Mn or Ni) shows a single peak [7,8]. This fact indicates thatthe combination of Pr3+ and Co3+ is rather exceptional but it is similar to Y0.2Pr0.8Ba2Cu3O7-y [10]. In orderto understand the double-peak structure, Saitoh et al. proposed a Pr 4f−O 2p hybridization model based on
the Fehrenbacher-Rice model for PBCO [9,11]. They also inferred that this exceptional situation would
originate from a large Pr 4f−O 2p−Co 3d effective hybridization due to the large Co 3d DOS at the binding
energy of ~1eV [9]. According to their interpretation, α and β are mainly due to |4f 0L⟩ and |4f 1⟩ final state,and hence roughly correspond to 4f 1 and 4f 2 initial state electron configuration, respectively. (Here, Ldenotes a ligand state.) In Panel (b), one can observe that the intensity ratio and relative location of α and β- 31 -

change with x. In fact, α grows compared to β with increasing x, which is consistent with the evolution of
the magnetic property with x. Hence this change is possibly interpreted as an increase in the strength of thePr 4f−O 2p−Co 3d effective hybridization with x, although a further study should be needed.
Finally, we point out similarities between the electronic structure of Pr1-xYxCoO3 and a copper delafossite
CuCrO2. Yokobori et al. have proposed a Cu 3d−O 2p−Cr 3d charge transfer model in order to explain the
Cu2+ signal in the photoemission spectra of Cu+CrO2 [12], which is similar to the present case, the Pr4+
signature in the magnetic measurements in Pr3+CoO3. Here Cu 3d and Cr 3d corresponds to Pr 4f and Co 3d.
The two completely different systems thus realize each unusual valence state due to an unexpected but
similar effective hybridization mechanism.
This work was supported by KAKENHI from JSPS (Grants No.26400321 and No. 17K05502).

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

G. Thornton, F. C. Morrison, S. Partington, B. C. Tofield, and D. E. Williams, J. Phys. C 21, 2871 (1988).
M. Itoh, M. Mori, S. Yamaguchi, and Y. Tokura, Physica B 259-261, 902 (1999).
M. Itoh, J. Hashimoto, S. Yamaguchi, and Y. Tokura, Physica B 281-282, 510 (2000).
S. Yamaguchi, Y. Okimoto, and Y. Tokuram, Phys. Rev. B 54, R11022 (1996).
Y. Kobayashi, Y. Terakado, and K. Asai, J.Phys. Soc. Jpn. 83, 104704 (2014).
T. Saitoh, T. Mizokawa, A. Fujimori, M. Abbate, Y. Takeda, and M. Takano, Phys. Rev. B 55, 4257 (1997).
J.-S. Kang, C. G. Olson, J. H. Jung, S. T. Lee, T. W. Noh, and B. I. Min, Phys. Rev. B 60, 13257 (1999).
E. Sakai, K. Yoshimatsu, M. Tamamitsu, K. Horiba, A. Fujimori, M. Oshima, and H. Kumigashira, Appl. Phys. Lett. 104,
171609 (2014).
9. T. Saitoh, D. Ishii, A. Hachimura, M. Hirose, T. S. Naing, Y. Kobayashi, K. Asai, M. Nakatake, M. Higashiguchi, K.
Shimada, H. Namatamec, and M. Taniguchi, J. Magn. Magn, Mater. 310, 981 (2007).
10. J.-S. Kang, S. Haffner, C. G. Olson, J. H. Kim, M. B. Maple, S. K. Kwon, and B. I. Min, Phys. Rev. B 66, 052503 (2002).
11. R. Feherenbacher and T. M. Rice, Phys. Rev. Lett. 70, 3471 (1997).
12. T. Yokobori, M. Okawa, K. Konishi, R. Takei, K. Katayama, S. Oozono, T. Shinmura, T. Okuda, H. Wadati, E. Sakai, K.
Ono, H. Kumigashira, M. Oshima, T. Sugiyama, E. Ikenaga, N. Hamada, and T. Saitoh, Phys. Rev. B 87, 195124 (2013).

FIGURE 1. Resonant photoemission spectra of Pr0.5Y0.5CoO3 and Pr0.2Y0.8CoO3 taken at 300K. (a) resonant photoemission
spectra of Pr0.5Y0.5CoO3. The on- and off- resonant spectra are shown by thick lines; (b) difference spectra of Pr0.5Y0.5CoO3
and Pr0.2Y0.8CoO3 deduced from the on- and off-resonant spectra.

- 32 -

17AG016, 17AU007

BL-1

High Resolution Probing of the Energy Levels in Carbon
Nanotubes
Gunther Anderssona, Cameron Shearera, Joe ShapteraaFlinders Centre for Nanoscale Science and Technology, School of Chemistry and Physical Sciences,
Flinders University of South Australia, Bedford Park, SA 5042, Australia

Keywords: carbon nanotubes, valence electron structure.

Single walled carbon nanotubes (SWCNTs) are a remarkable material that can be metallic orsemiconducting depending upon the arrangement of carbon atoms in the cylinder, defined by chirality. [1] It
is now possible to prepare samples with defined chirality via multi-step purification of an initially mixed
ensemble. [1] The density of states (DOS) of the SWCNTs depend on their chirality and can range frommetallic to semiconducting. The DOS determine how the SWCNTs can be used in electronic devices likesolar cells.
The purpose of the visit at the beamline B1 in December 2017 was measuring the density of states ofsingle wall carbon nanotubes. The measurements require low excitation energies in the range of a few 10 eVwith high intensity. These conditions are provided at BL-1 at HiSOR.
Semiconducting SWCNTs with a large range of chiralities have been measured at BL-1. In Figure 1, a
spectrum of P2 SWCNTs are shown measured at an excitation energy of 30 eV and a pass energy of theanalyser of 20 eV. A range of further samples have been measured. We are currently drafting a manuscriptfor publication.

FIGURE 1Valence electron spectrum of P2 SWCNTs measured at an excitation energy of 30 eV.

REFERENCES
1. B. Liu, F. Wu, H. Gui, M. Zheng and C. Zhou, ACS Nano, 2017, 11, pp. 31.

- 33 -

17AG026

BL-1

ARPES study of multi-band Bi chalcogenides
T. Mizokawaa, Y. Matsuzawaa, T. Moritaa, N. L. Sainib,
T. Asanoc, T. Nakajimac, R. Higashinakac, T. D. Matsudac, Y. Aokic,
E. F. Schwierd, K. Shimadad, H. Namatamedab

Department of Applied Physics, Waseda University, Shinjuku-ku, Tokyo 169-8555, Japan

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-based superconductor, electron-lattice interaction, valance fluctuation

The discovery of a BiS2-based superconductor by Mizuguchi et al. [1] has been followed by extensive
experimental and theoretical studies on the BiS2-based superconductors including RE(O,F)BiS2 systems(RE=rare earth element) [2-7]. In a typical RE(O,F)BiS2 system, the F substitution for O in the RE(O,F) layer
introduces electrons to the electronically active BiS2 layer. The theoretical calculations indicate that the Fermi
surfaces are constructed from the Bi 6px and Bi 6py orbitals [2]. The orbital character of the Fermi surfaces has
been investigated by means of angle-resolved photoemission spectroscopy (ARPES) in Ce(O,F)BiS2, and the
ARPES results are consistent with the theoretical predictions [8]. In Ce(O,F)BiS2, the Ce3+/Ce4+ valencefluctuation is suppressed by the F doping and the decrease of the Pr valence tends to reduce the number ofelectrons in the BiS2 layer. In order to understand the effect of the rare earth site in the BiS2-based
superconductors, we have performed angle-resolved photoemission spectroscopy on Pr(O,F)BiS2 at BL-1,
HiSOR. We cleaved the single crystalline samples in situ under ultrahigh vacuum in order to obtain clean (001)surface. The total energy resolution including both monochromator and electron energy analyzer was set to 21
meV. All the measurements were taken at 30 K.
Figure 1 shows the ARPES data across the electron pocket around the X point for x=0.3 and x=0.5 of PrO1xFxBiS2. Here, kx is the wave number along the X direction. In going from x=0.3 to 0.5, the bottom of theelectron band is shifted towards the Fermi level, inconsistent with electron doping by the F substitution for O.
Although the F doping is expected to introduce electrons in the BiS 2 layer, the Pr valence may decrease by the F
doping just like Ce(O,F)BiS2 and the doped electron can be compensated. Indeed, the Pr3+/Pr4+ valencefluctuation is seen in core level spectra and the Pr4+ component is reduced with the F doping.
As for the Fermi surfaces, rectangular Fermi pockets around X point are clearly observed at x=0.5 and theirarea is much smaller than the value expected from the F doping level. The missing electrons can beaccommodated by the Bi 6pz orbitals with strong mixture of Pr 4f. In this scenario, the change of orbital
anisotropy of Bi 6p (from 6px/6py to 6pz) is coupled with the Pr valence change (from Pr 4+ to Pr3+). Interestingly,
some straight portions of the Fermi pockets disappear at x=0.3 although the area of the Fermi pockets would beconsistent with the F doping level. The partial deletion of the Fermi surfaces suggests that charge-orbital
fluctuations are stronger in the low doping level. In addition, the spectral shape near the Fermi level indicatesstrong electron-lattice coupling.
The present ARPES results show that Pr(O,F)BiS2 exhibits interesting interplay between the Bi 6p orbital
degeneracy and the Pr valence fluctuation. The strong electron-lattice coupling associated with the Bi 6p orbital
degeneracy may play important roles in the emergence of the superconductivity.

- 34 -FIGURE 1. Band dispersion along the X direction taken at 30 K for x=0.3 (left panel) and x=0.5 (rightpanel).

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).

- 35 -

17AU024

BL-01

Laser ARPES study on surface-doped black phosphorus
Woo Jong Shina, b and Keun Su Kimaab

Department of Physics, Yonsei University, Seoul 03722, Korea
Department of Physics, Pohang University of Science and Technology, Pohang 37673, Korea

Keywords: black phosphorus, kink, angle-resolved photoemission spectroscopy

Two-dimensional (2D) van der Waals crystals, such as graphene, transition-metal dichalcogenide (TMD),
and black phosphorus (BP), have been widely studied not only for their novel quantum phenomena, but also
for their potential for application in nanoelectronics [1, 2]. Among them, BP is a narrow-gap semiconductorthat shows a high charge carrier mobility together with high on-off current modulation ratio. Owing to thispromising device characteristics, there is renewed interest in the study of few-layer BP and its applications[3]. BP is also attractive in that it has rather simple lattice and band structures for the study of fundamental
quantum phenomena and manybody interactions. Recently, it was reported that BP decorated with alkali
metal atoms exhibits an unexpected superconductivity with the critical temperature of 3.8 K [4]. This
motivates us to study the possible strong electron-phonon interactions in alkali-metal doped black
phosphorus.
Angle-resolved photoemission spectroscopy (ARPES) is a powerful technique, which can not only
measure the band dispersion of quasiparticles, but also provide information on electron-phonon interactions.
The latter is the effect of band renormalization that typically appears as an abrupt kink in the band dispersion
at the energy of the involved phonons. By systematically analyzing the measured ARPES spectra, one canextract the electron-phonon self-energy that contains the strength of electron-phonon coupling. Consequently,
electron-phonon coupling constant of surface-doped BP can possibly be quantified [5].

FIGURE 1. ARPES spectra taken from pristine BP along the armchair direction: (a) Data taken with thesynchrotron radiation at the Advanced Light Source (hν = 104 eV). (b) Data taken with the laser at HiSOR (hν = 6.3 eV),which is also directly compared with that taken with the synchrotron radiation in (a). (c) Data taken after the deposition of K
atoms on the surface of BP.

- 36 -FIGURE 2. Series of constant-energy maps taken for the valence band of pristine BP at the energy shown at the bottom.

To address this issue, we employ high-resolution ARPES apparatus equipped with the Ti:sapphire laser
at HiSOR. The ARPES spectral taken from pristine BP with laser-ARPES is shown in Fig. 1(b) and directlycompared with that taken at the Advanced Light Source (ALS) with the synchrotron radiation [Fig. 1(a)].
The ARPES data taken with the laser show a much narrower spectral width than that with the synchrotron
radiation, which indicates the improved energy and momentum resolutions owing to the low excitationenergy of the LASER and its small spot size. After doping, however, the spectral feature gets broadened,diminishing the merit of high energy and momentum resolutions. This makes the expected kink feature notapparent in our data as shown in Fig. 1(c). Furthermore, ARPES spectra in Fig. 1(c) is strongly asymmetricwith respect to the Gamma point, which is not expected at all for surface-doped BP. This could be due to aslight misalignment of the sample or the effect of light polarization. Indeed, the symmetry of constant-energy
maps taken for the valence states of pristine BP is slightly tilted with respect to vertical and horizontal axes,
as shown in Fig. 2. We expect that the possible use of a fully motorized 6-axis manipulator might improve
this experimental situation for future studies.

REFERENCES
1.
2.
3.
4.
5.

K.S. Novoselov et al., Science 306, 666-669 (2004).
K.S. Novoselov et al., PNAS 102, 10451-10453 (2005).
Xi Ling et al., PNAS 112, 4523-4530 (2015).
R. Zhang et al., Nat. Comm. 8, 15036 (2017).
A. V. Fedorov et al., Nat. Comm. 5, 3257 (2014).

- 37 -

AU026 BG U009

BL-1

Growth and ARPES Measurement of HydrogenatedSilicene on Ag(111)
Ya Feng1,2, Baojie Feng2,Shaolong He1and Kenya Shimada2
1

Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo,
Zhejiang 315201, China
2
Hiroshima Synchrotron Radiation Center, Hiroshima University, Higashi-Hiroshima, Hiroshima 7390046, Japan
Keywords: Silicene, magnetism , ARPES, Hydrogenation.

Silicene, analogous to graphene, is a one-atom-thick two-dimensional crystal of silicon, which is expected
to share many of the remarkable properties of graphene [1-4]. For example, it harbors the characteristic lowenergy Dirac cone states [5-9]. On the other hand, the buckled honeycomb structure of silicene, along withenhanced spin-orbit coupling, endows silicene with considerable advantages over graphene in that the spinsplit states in silicene are tunable with external fields [10-14]. The unique advantages of silicene and its
compatibility with the traditional silicon industry make it an attractive materials platform for next generation
nanoelectronics applications.
Moreover, the theoretical calculations show that silicene will be magnetic after adsorption of hydrogenatoms and the Curie temperature can reach 300K [15]. Once combined with advanced nanotechnology basedon silicon, the magnetic silicene possibly becomes a very promising nanotechnology application on
spintronics. The long-range ferromagnetic ordering may simulate its application potential in spintronics.

FIGURE 1. STM images with increasing Hydrogen atoms coverage.

ab
Ky(1/Å)Si 3x3Si 1x1
Ag（111）Kx(1/Å)FIGURE 2. Brillouin zone of silicene/Ag(111) and constant energy contours of pure silicene/Ag(111).

- 38 -

Last year we grew hydrogenated 33 silicene/Ag(111) on STM system and grew it again and didmeasurement on BL-1 in HiSOR. Figure 1 shows the STM images of hydrogenated silicene with different
amount of hydrogenation. Although the hydrogenation is successful, more hydrogen atoms make lesssilicene on Ag(111).

Ky(1/Å)Kx(1/Å)FIGURE 3. The constant energy contours of hydrogenated silicene.

FIGURE 4. Compare of Dirac cone
before and after hydrogenation.

According to the growth condition on STM system, we grew 33 silicene/Ag(111) and hydrogenated itin the prep-chamber and measured it by ARPES in situ. We got similar results as before[16] which showed
twelve Dirac cone on the edge of Brillouin Zone of Ag(111). Figure 2.b shows the constant energy contoursof pure silicene. There are twelve Dirac points on the Fermi surface, areas of which enlarge with higher
binding energies. Figure 3 shows the constant energy contours of hydrogenated silicene. Figure 4 showsband structures along the momentum line acrossing the Dirac points before and after hydrogenation. The
Dirac points are indeed corrsponding to linear dispersions. However, no matter from Fig.2 or Fig.3, we cannot see obvious difference between the pure and the hydrogenated silicene but only weekness of the Dirac
bands of silicene after hydrogenation. This result is consistant with the phenomena observed by STM that
more hydrogen atoms makes less silicene. Maybe we hydrogenated the silicene too much that it became
more than half hydrogenation. Some hydrogen atoms insert between the silicene and Ag(111), which makessome silicene seperated from Ag(111). Next time, we should control the amount of hydrogen atoms more
carefully to avoid more hydrogenation.

REFERENCES
1. Novoselov KS, et al. Electric field effect in atomically thin carbon films. Science 306:666–669 (2004).
2. Novoselov KS, et al. Two-dimensional gas of massless Dirac fermions in graphene. Nature 438:197–200 (2005).
3. Zhang Y, Tan YW, Stormer HL, Kim P. Experimental observation of the quantum Hall effect and Berry’s phase in graphene.
Nature 438:201–204 (2005)
4. Castro Neto AH, Guinea F, Peres NMR, Novoselov KS, Geim AK. The electronic properties of graphene. Rev Mod Phys
81:109–162 (2009).
5. Bansil A, Lin H, Das T. Colloquium: Topological band theory. Rev Mod Phys 88:021004 (2016).
6. Takeda K, Shiraishi K. Theoretical possibility of stage corrugation in Si and Ge analogs of graphite. Phys Rev B Condens
Matter 50:14916–14922 (1994).
7. Guzman-Verri GG, Lew Yan Voon LC. Electronic structure of silicon-based nanostructures. Phys Rev B 76:075131 (2007)
8. Cahangirov S, Topsakal M, Akturk E, Sahin H, Ciraci S. Two- and one- dimensional honeycomb structures of silicon andgermanium. Phys Rev Lett 102:236804 (2009).
9. Lebegue S, Eriksson O. Electronic structure of two-dimensional crystals from ab initio theory. Phys Rev B 79:115409(2009).
10. Ni Z, et al. Tunable bandgap in silicene and germanene. Nano Lett 12:113–118 (2012).
11. Drummond ND, Zolyomi V, Fal’ko VI. Electrically tunable band gap in silicene. Phys Rev B 85:075423 (2012).
12. Ezawa M. A topological insulator and helical zero mode in silicene under aninhomogeneous electric field. New J Phys 14:033003 (2012).
13. Ezawa M. Valley-polarized metals and quantum anomalous Hall effect in silicene. Phys Rev Lett 109:055502 (2012).
14. Stille L, Tabert CJ, Nicol EJ. Optical signatures of the tunable band gap and valley-spin coupling in silicene. Phys Rev B
86:195405 (2012).
15. Zheng, et al. Nanoscale Research Letters 7:422 (2012)
16. Ya Feng, Defa Liu, BaojieFeng and XuLiu et.al. PNAS 113: 51:14656-14661.(2016)- 39 -

17BG019

BL-1

Photoemission Study of the Dirac cone state splitting at
the Dirac point for Gd-doped Topological Insulators
D. Estyunina, A.V. Korolevaa, A.M. Shikina,
S. Kumarb, E.F. Schwierb, A. Kimurab,cabSaint Petersburg State University, 198504 Saint Petersburg, Russia
Hiroshima Synchrotron Radiation Center, University of Hiroshima, Higashi-Hiroshima 739-0047, Japanc
Graduate School of Science, Hiroshima University , Higashi-Hiroshima 739-8526, Japan

Keywords: Topological Insulators, ARPES, Electronic Structure.

In recent years, magnetically-doped topological insulators (TIs) have attracted enhanced interest due topossibility of effective realization Quantum Anomalous Hall effect (QAHE) [1,2]. For the effective
realization of the QAHE in magnetically-doped TIs the Dirac gap should be located at the Fermi level (FL)[1,2] and the developed surface ferromagnetism (FM) should survive in the insulating regime, i.e. should
be independent on bulk carriers.
The key challenge to realizing the QAHE is to achieve simultaneously a low bulk carrier density
together with an appropriate ferromagnetic ordering. Unfortunately, the incorporated transition metal (TM)atoms act as not only magnetic dopants. The doping by magnetic TM impurities leading to opening the
Dirac gap is also followed by the charge carrier doping effects simultaneously. TMs are usually divalentand, therefore, substitution of Bi+3 with TM+2 creates hole dopants into the system. On the other hand, if
the doping elements are rare-earth metals such as Gd, which are trivalent, no charge carriers are doped andonly magnetic moments can be induced [3]. Gd has an equal number of bonding electrons to Bi, therefore,
the isoelectric substitution with Gd into Bi2Te3 takes place. Under Gd-doping the lattice parameters of thehost TI are not changed due to the Gd substitution.
The current work is devoted to study of the features of electronic structure and magnetic properties of
Gd-doped TIs with stoichiometry Bi2-xGd0.06SbxTe3, for x is between 0.75 and 0.95, when the Dirac gap islocated in the region of the FL and analysis of the gap open at the Dirac point (DP) at different positions of
the DP relative to the FL.
Fig. 1.a shows the ARPES dispersion map measured for Gd-doped topological insulators withstoichiometry Bi1.19Gd0.06Sb0.75Te3 at temperature 15K at photon energy of 28 eV under photoexcitation bylinear p-polarized synchrotron radiation SR. In Fig. 1.b the ARPES dispersion maps in the form d2N/dE2
are also presented for better visualization of the electronic structure in the region of the DP and possible
splitting of the Dirac cone states at the DP developed due to influence of the magnetic doping. One candistinguish formation of the dip at the DP testifying to splitting of the Dirac cone states at the DP. Fig. 1.cshows the EDC-profile cut directly at the DP (at kII=0). The fitting of the EDC demonstrates
decomposition on two spectral components related to the Dirac cone states at the DP (red lines) separated
by the energy of about 30 meV and the spectral component corresponding to the valence band states (blackline). We relate this splitting (30 meV) to the splitting between the upper and lower Dirac cone states due
to spontaneous out-of-plane magnetic moment developed for Bi1.19Gd0.06Sb0.75Te3 at temperature of 15 K.

- 40 -

Fig.1. The ARPES dispersion maps for the DC states measured for Bi1.19Gd0.06Sb0.75Te3 at temperature 15K and photon
energy of 28 eV in the form of N(E) – (a) and d2N/dE2 – (b) presentations. (c) - the corresponding EDC-profile measured
directly at the Г-point (kII=0) with fitting on the spectral components related to the DC states (red lines) and the upper edge
VB state contribution (grey line)

It means that the Gd-doping open the Dirac gap at the DP for Bi1.19Gd0.06Sb0.75Te3 at temperature of 15 K
up to 30 meV. In future experiment we will study how the Dirac gap will depend on the Gd-concentrationand the position of the DP relative to the FL for different stoichiomentry of Gd-doped TI.

REFERENCES
1. C.-Z. Chang et al., Science 340, 167 (2013).
2. C.-Z. Chang et al.,, Nat Mater 14, 473 (2015).
3. J. Kim et al., Sci. Rep 5, 10309 (2015)- 41 -

17BG020

BL-1

Resonant Photoemission Study of Mn-doped TopologicalInsulators
A.M. Shikina, D. Estyunina, A.V. Korolevaa, I.I. Klimovskikha,
S. Kumarb, E.F. Schwierb, A. Kimurab,cabSaint Petersburg State University, 198504 Saint Petersburg, Russia
Hiroshima Synchrotron Radiation Center, University of Hiroshima, Higashi-Hiroshima 739-0047, Japanc
Graduate School of Science, Hiroshima University , Higashi-Hiroshima 739-8526, Japan

Keywords: Topological Insulators, ARPES, Electronic Structure.

During recent decades active research into fundamentally new structures – topological insulators (TIs) –is one of the priority areas in the physics of nanosystems. An intriguing electronic structure configurationof TI defines the researchers' special expectations for their application in spintronics, optoelectronics,
quantum computations and other technologically important fields. Recently, it has been demonstrated thatat very low temperatures (~ 1 K) the topological insulators with time-reversal symmetry broken due to themagnetic doping (Cr, V) exhibit the possibility of the quantum anomalous Hall effect (QAHE) realization[1,2]. Such effect becomes possible due to the local magnetic gap opening at the Dirac point caused bylifting Kramers degeneracy between up and down spins.
Significantly larger Dirac point gap has been observed in Mn-doped topological insulator, and thehedgehog out-of-plane spin structure, linked to the magnetic interaction has been confirmed [3]. However,recent detailed investigations of the electronic structure of Mn-doped TIs reveal the survival of the Diracpoint gap up to the room temperature, and moreover, similar gap values had been observed fornon-magnetic doping of TIs [4]. Resonant photoemission on the Mn-edge confirms the presence of Mn dstates near the Dirac point, that can induce the apparent gap to the ARPES spectrum via the hybridization
with topological surface states.
Nevertheless, more recent studies demonstrate the possibility of transformation of the TI’s surface
layers under the Mn doping with formation of the novel magnetic compounds [5]. Such a structure ischaracterized by the large magnetic Dirac point gap that is caused by surface magnetic ordering. Thus, themain aim of the present work is the investigation of the electronic structure of highly Mn-doped
topological insulators, including the resonant photoemission analysis in order to distinguish the Mn
impurity states at the Dirac point and reveal the surface magnetic ordering.
The experimental ARPES images of Mn-doped Bi2Te3 taken at the photon energies of 48 and 50 eV
are shown in Fig. 1a,b. These photon energies correspond to off- and on- Mn-edge resonance, relatively.
One can clearly see the dispersion relations of the valence band of topological insulator in the region of
0.5-5 eV of binding energy. In the region of BE of 0-0.5 eV the Dirac cone states are located takes plcewhile having the very low intensity at these photon energies. Fig. 1 c shows the normalized difference
between the ARPES dispersion maps measured off- and on- Mn-edge resonance (a and b), where one candistinguish the well pronounced Mn resonant state at ~3.8 eV. It is well visible in Fig. 1.d. Additionally,
Mn impurity states are visible at the BE of 1 eV. Most import that there is no any enhancement of theintensity visible in the region of the Dirac cone, i.e. 0-0.5 eV. It means that the gap open at the Dirac point
for highly Mn-doped topological insulator is not related to the hybridization effect. It is rather related to the
induced magnetic moment influence.

- 42 -

Fig.1 ARPES images of Mn-doped Bi2Te3 taken at 48 (a) and 50 eV (b). (c) - Normalized difference between images aand b.. (d) - EDC spectra at the Gamma-point at photon energy of 48 (red) and 50 eV (blue). Normalized difference between
the spectra is presented by black curve.

Thus, our results show the large resonant signal from the Mn impurity states of highly Mn-doped
topological insulator Bi2Te3 located in the region of the bulk valence band of TI at the binding energy of
3.8 eV. The absence of the resonant features in the region of the Dirac point allows to ascribe the gap open
at the Dirac point to influence of magnetic ordering. Detailed investigation of the Dirac cone structure will
be carried in the future experiments.

REFERENCES
1. C.-Z. Chang et al., Science 340, 167-1702 (2013)
2. S. Qi et al., Phys. Rev. Lett. 117, 056804 (2016)
3. S. Y. Xu. et al., Nat. Phys. 8, 616 (2012)
4. J. Sanchez-Barriga et al., Nat. Commun. 7, 10559 (2016)
5. T. Hirahara et al., Nano Lett., 7, 3493–3500 (2017)- 43 -

17BG021

BL-1

Probing Correlated Topological Phase in CeBi
Peng Li, Zhongzheng Wu, Yang Liu*
Center for Correlated Matter and Department of Physics, Zhejiang University, Hangzhou, China

Keywords: Topological Insulator/semimetal, Electron Correlation.

Recently topological insulators and semimetals have attracted considerable research interest. Interplay
between nontrivial band topology and strong electron correlation could induce exotic electronic phase,including weyl-kondo semimetal [1]. Here, we focus on CeBi, a kind of topological insulator with partial
gap, to find evidence of correlated topological states via angle resolved photoemission spectroscopy(ARPES). Rare earth mono-bismuthides have a simple rocky-salt structure, and the electronic structure athigh temperature has been well studied by several groups [2-4] and in our former paper [5], which features
two holes pockets at Γ̅ point, one electron pocket at 𝑋̅ point in momentum space with non-trivial band
topology. CeBi has exotic magnetic properties where a PM to type-I AFM transition takes place at 25 K andtype-I AFM to type-II AFM transition occurs at 11 K [6]. Meanwhile with decreasing temperature, f-electron
could hybridize with conduction electrons and become partially itinerant, as proposed by previous theoretical
calculations. Actually, transport experiments have revealed the kondo-like features, so our main purpose is
to find the evidence of the (weak) hybridizations between f and conduction electrons.
The cross-section of f orbital is generally small in low photon energy range. So we did the resonant
photoemission experiments which occur at ~122 eV (4d to 4f transition), at BL-1 in HiSOR. Because of thelarge enhancement of f-orbital features, resonant photoemission experiment can be used to identify the felectron spectral feature. Fig. 1 shows the results from on and off resonance ARPES experiments, where weuse 115 eV as off resonance photon energy and 122 eV as on resonance photon energy. Fig.1(a) shows theenergy-momentum dispersion on 115 eV and 122 eV, respectively. Compared with off resonance data, 4f
electron peaks at binding energy 2.82eV and 0.3eV show up under resonance condition, which corresponds
to 4f0 and 4f1 final state, respectively. We also integrate the energy distribution curve (EDC) as Fig.1 (b),one can see the obvious enhancement of the 4f peaks, i.e., on-resonant intensity (122eV) is more than 10times larger than off-resonant intensity (115eV). A zoom-in view near the Fermi level (the right-bottom panel)reveals the fine structure such as the spin-orbit coupling satellite of Kondo resonance state at 0.3 eV [7],although the Kondo resonance peak right below the fermi level is rather weak due to the low carrier density(corresponding to a low single-impurity Kondo temperature).
We also performed temperature dependent measurements, from 39 K (PM phase) to 14.2 K (AFM phase).
The left and middle panels in fig. 2 show on resonance spectrum taken above and below magnetic transitions,
respectively. Small yet obvious intensity weight changes can be found in EDC, where the local 4f0 state’s
intensity decreases upon cooling down towards AFM phase, while more itinerant 4f1 state intensity grows
up. More importantly, if one integrates each EDC, one could find the overall spectral weight to be conserved,
indicating that the increase of 4f1 spectral weight originates from the decreasing intensity from 4f0. In otherwords, more f electrons of CeBi become itinerant when temperature goes down from PM to AFM. However,no direct signature of f-c hybridization could be observed in the momentum-energy dispersion relations,likely due to the rather weak Kondo interaction in such a low carrier density system. Nevertheless, the subtle
spectral weight shift indicates (at least) weak Kondo hybridization at low temperature, whose origin remains
to be revealed.

- 44 -FIGURE 1. Off and on resonant measurements of CeBi. (a) off and on resonant spectrum at 115 eV and 122 eV, respectively.
And all measurements ware at PM state (38K). (b) the integrated EDC of (a), and the right-bottom panel is the zoom-in version
near the fermi level.

FIGURE 2. Temperature dependence measurements on 122eV, from high temperature 39K (PM state) to low temperature
14.2K (AFM state), the right panel is the integrated EDC of these two cases.

- 45 -

REFERENCES
1. Hsin-Hua Lai, Sarah E. Grefe, Silke Paschen, and Qimiao Si, “Weyl-Kondo semimetal in heavy-fermion systems”, PNAS,
2018, pp. 93-97.
2. Jayita Nayak, Shu-Chun Wu, eg. “Multiple Dirac cones at the surface of the topological metal LaBi”, Nature
Communications 8, 13942 (2017).
3. H. Kumugashira, S.-H. Yang, eg, “High-resolution angle- resolved photoemission spectroscopy of CeBi”, Phys. Rev. B
54, 9341 (1996).
4. Kenta Kuroya, M. Ochi, eg, “Experimental Determination of the Topological Phase Diagram in Cerium Monopnictides”,
Phys. Rev. Lett. 120, 086402 (2018).
5. Peng Li, Zhongzheng Wu, Yang Liu, eg, “Tunable Electronic Structure and Possible Topological Surface States in rare
Earth Mono-Bismuthides with Partially Filled f shell”, arXiv: 1802.03111(2018).
6. H. Bartholin, D. Florenoe, Wang Tcheng-si, and O. Vogt, “Magnetic Properties of CeBi”, Phys. stat. sol. (a) 24, 631 (1974).
7. Sooyoung Jang, Robert Kealhofer, et al., “Direct visualization of coexisting channels of interaction of CeSb”, arXiv:
1712.05817 (2017).

- 46 -

17BG036

BL-1

ARPES study of Gd: Bi2Se3 topological insulators
Rahul Singha, Shiv Kumarb, Eike F. Schwierb, Kenya Shimadab, Anup K.
Ghoshc and Sandip ChatterjeeaaDepartment of Physics, Indian Institute of Technology (BHU), Varanasi-221005, India
Hiroshima Synchrotron Radiation Center, Hiroshima University, Higashi-Hiroshima 739-0046, Japanc
Department of Physics, Banaras Hindu University, Varanasi-221005, Indiab

Keywords: Topological insulators, electronic structure, negative magnetoresistance.

In recent years magnetic three-dimensional topological insulators (MTIs) based on Bi2Te3 and Bi2Se3
have been interesting subject due to their fundamental interesting phenomena viz. quantum anomalous Hall
effect, three-dimensional Weyl fermion etc. The strong spin-orbit coupling (SOC) leads nontrivial band
topology in these material, in which spin of an electron is locked perpendicular to its momentum in chiralspin structure, whereas electrons with the opposite momenta have opposite spin that makes topologicalinsulator potentially very promising material that could be used in spintronics and for quantum computing
applications. [1-4]The transport studies of topological surface state (TSS) have revealed such as Aharonov-Bhomoscillation in Bi2Se3 nano-ribbon [5] weak antilocalization in Bi2Se3 and Bi2Te3 film and two dimensional(2D) Shubnikov-de Hass (SdH) oscillations are powerful means to distinguish between bulk and surface
charge carrier via angle-dependent. [6] Recently, magnetotransport measurement on Bi2Se3 epitaxial layers
in the magnetic field shows negative magnetoresistance (NMR) when the magnetic field applied parallel to
I (I ‖Bx). [7] However, the recent explosion of interest in 3D massless Dirac fermions in 3D Dirac Weyl
semimetals shows an extremely large positive magnetoresistance under perpendicular magnetic field andmore specially, the negative longitudinal magneto-resistance (NLMR) predicated to appear in Weyl
semimetals when the magnetic and electric field coaligned to axial anomaly. [8] Consequently, NLMR is
not a unique property of Weyl semimetal. We have observed first-time NLMR in Gd0.5(Bi2Se3)0.95 sample
which persist until 2.5 tesla under perpendicular magnetic field parallel to c-axis up to room temperature.
10(a)

MR% (ohm-cm)

8(b)

Gdx(Bi2Se3)1-x
2K
5K
10K
15K
25K
50K
100K
200K
300K

6
4
2
0
-2
0

20000

40000

60000

80000

H(Oe)

FIGURE 1: Fig (a) shows the longitudinal resistance vs temperature of Gd0.5(Bi2Se3)0.95 sample. Fig. (b) displays the band
structure near Fermi surface of Gd0.5(Bi2Se3)0.95 by angle-resolved photoemission spectroscopy (ARPES) at 18 K.

The band dispersion measured by Laser-ARPES along high symmetry direction (K-Γ-K) shows that
Gd0.5(Bi2Se3)0.95 sample is n-type, which is consistent with Hall-resistivity data. The binding energy of Diracpoint relative to the Fermi level − 0.302 eV, it is due to e- doping in the sample, in this case, e- is generated
by Gd-intercalation.

- 47 -

In summary, we have synthesized and study the bulk and surface state electronic properties of
Gd0.5(Bi2Se3)0.95 using Laser-ARPES. We have also studied the magnetotransport properties of Gdintercalated Bi2Se3 TI material and observed NMR exist when we have applied magnetic field perpendicular
to the electric field.

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

J. Y. Ando, J. Phys. Soc. Jpn. 82, 102001 (2013)
R. Yu, W. Zhang, H.-J. Zhang, S.-C. Zhang, X. Dai, and Z. Fang, Science 329, 61 (2010).
X.-L. Qi, R. Li, J. Zang, and S.-C. Zhang, Science 323, 1184 (2009).
S.-W. Kim and M.-H. Jung, Appl. Phys. Lett. 112, 202401 (2018).
G. Li, D. Liang, R. L. J. Qiu, and X. P. A. Gao, Appl. Phys. Lett. 102, 043104 (2013).
Y.-Y. Lv, B.-B. Zhang, X. Li, K.-W. Zhang, X.-B. Li, S.-H. Yao, Y. B. Chen, J. Zhou, S.-T. Zhang, M.-H. Lu, S.-C. Li,
and Y.-F. Chen, Phys. Rev. B 97, 115137 (2018).
7. S. Wiedmann, A. Jost, B. Fauqué, J. van Dijk, M. J. Meijer, T. Khouri, S. Pezzini, S. Grauer, S. Schreyeck, C. Brüne, H.
Buhmann, L. W. Molenkamp, and N. E. Hussey, Phys. Rev. B 94, 081302 (2016).
8. F. C. Chen, H. Y. Lv, X. Luo, W. J. Lu, Q. L. Pei, G. T. Lin, Y. Y. Han, X. B. Zhu, W. H. Song, and Y. P. Sun, Phys. Rev.
B 94, 235154 (2016).

- 48 -

17BU004(HiSOR proposal number)

BL-1(HiSOR beamline number)

ARPES Study on the New Fermion in ZrTe and theelectronic structure of Monolayer MoTe2
Qiang Gao1,2, Wenjuan Zhao1,2, Wenliang Zhu1,2, Genfu Chen1, Yuan Huang1,
Ya Feng3, E. Schwier3, K. Shimada3, Guodong Liu1*, Xingjiang Zhou1,4*1National Laboratory for Superconductivity, Beijing National Laboratory for Condensed Matter Physics,
Institute of Physics, Chinese Academy of Sciences, Beijing, 100190, China
2
University of Chinese Academy of Science, Beijing, 100049, Peopleʼs Republic of China
3
Hiroshima Synchrotron Radiation Center, Hiroshima University, 2-313 Kagamiyama, Higashi-Hiroshima,
Hiroshima 739-0046,, Japan
4
Collaborative Innovation Center of Quantum Matter, Beijing 100871, China
Keywords: new fermion,

ZrTe, 2D material, transition metal dichalcogenids (TMDCs), mono-layer MoTe2, ARPES.

The topological quantum materials, representing a new state of condensed matter, have attracted
remarkable interest in recent years. Many of them have been predicted in theory and realized in experiment,
including three-dimensional or two-dimensional topological insulators [1–3], three- dimensional Diracsemimetals [4–6]，three-dimensional Weyl semimetals [7–9], and nodal line semimetals [10,11]. The latest
development is the theoretical prediction and experimental verification on the “new Fermion” topologicalsemimetal with quite interesting physical properties [12-15], which are characterized by three-, six- oreight-fold degenerate band crossing points. Many candidate materials hosting such “new Fermions” havebeen proposed through ab-initio calculation. Among such materials, only the MoP and WC have been
preliminarily confirmed by recent ARPES measurements to own a triply degenerate point in the band
structure [16, 17]. ZrTe, one of the WC-structure family of materials, was reported as an intriguingmaterial that hosts both the “three-fold degenerate new Fermion” state and Weyl Fermion state near Fermi
energy [14,18]. It calls for a systematic research on the electronic structure to study the topological natureon the new fermion in ZrTe.
Transition metal dichalcogenides (TMDCs) are currently hot-spot materials for their rich physical andoptical properties. Among various MX2, MoTe2 comes into notice for its structural phase transition from
2H to 1T’ by an electrostatic-doping in monolayer[19]. However, there’s almost no band structure study on
2D MoTe2 with 2H-type lattice [20], which calls for a systematic ARPES study on them.
The approved beam time in HiSOR were used to do Angle Resolved photoemission Spectroscopy(ARPES) measurements on new fermion semimetal ZrTe and monolayer MoTe2.
First, we measured ZrTe. We changed the photon energy from 30eV to 100eV, and found the band
structure is clear at 52eV. Then we fixed the photon energy at 52eV and got good mapping. In Fig.1 (c), we
can see that the projected brillouin zone on (001) surface is six-fold symmetrical. And in Fig.1 (d) to (g),we got a six fold symmetrical electronic structure. This indicates that the cleaved surface is (001) plane. As
the sample size is too small, and the photon beam size is much bigger than sample, the signal is weak andthe background is high. But we can still recognize hole pockets around Γ and K poins.
In Fig.2, we show band structures on the high symmetry cuts along Γ M and Γ K direction. In Fig.2(b)we can only resolve one hole-like band at Γ point. While Fig.2(c) along Cut2 shows more hole like bands
at K point but not being clearly resolved. They look very similar to the calculated band structures in
Fig.2(d). Fig.2(e) and (f) show the detailed band structure crossing Γ point measured with 7eV laser, we
can see clear band splitting of the hole like band and a very flat band bottom of an electron pocket lying at
Fermi level.
In summary, we got good band mapping along high symmetry cuts in ZrTe at (001) surface. The band
structure in the whole Brillouin zone can roughly fit the band calculation. But we miss some critical details
due to sample size and instrument resolution issue, even 7eV laser data can be a supplement. To pin down
the topological nature of ZrTe, we need to measure other cleaved surfaces and do photon energy- 49 -

dependence mapping.

FIGURE 1. The electronic structure of ZrTe. (a) The crystal structure of WC-type ZrTe and (b) the top view [14]. (c) The
bulk Brillouin Zone and its projection onto the (100) and (001) surface. The high symmetrical crystal momenta are indicatedwith letters[14]. (d)-(g) The Fermi surface and constant energy contours of ZrTe on (001) surface measured with photon
energy at 52eV.

Figure 2. (a) Fermi surface of ZrTe with guideline of cuts. Band dispersion on Cut 1 along Γ M direction(b) and on Cut 2
along Γ K direction (c). (d) Calculated band structure along high symmetry direction [14]. (e) The measured band along a cut
at Γ point measured with 7eV laser in our home lab and (f) The EDC second derivative image for (e).

For monolayer MoTe2 sample, we first annealed the sample in the preparation chamber, and thentransferred to analysis chamber. In Fig.3, it gives the crystal structure and energy contour of MoTe2 film.
Fig.3(a) and (b) show the crystal structure [21] and Brillouin zone (BZ). We got energy contour by ARPES
in Fig.3(c). Because MoTe2 is a typical semiconductor, there’s no electron density of states between Fermi
energy and -0.7 eV binding energy. At -1.1eV binding energy, the energy contour clearly show a splitting
triangle at K point, which is corresponding with the SOC splitting at K.
Fig.4 shows the electronic structure along high symmetry direction. We can see the classical SOC
splitting at K. Compared with the reported data, our ARPES result is consistent with band calculation ofbi-layer MoTe2. By Lorentz fitting of EDC, we get a band splitting of ~ 200meV at K point.

- 50 -FIGURE 3. Crystal structure and energy contour of MoTe2. (a) The crystal structure of 2H-MoTe2 [21]. Red and blue balls
stand for Mo and Te atoms, respectively. (b) The hexagonal Brillouin zone (BZ) of MoTe2. Green line and Red line
constitute MoTe2 BZ. Blue lines are momentum position in Fig.2. (c) Energy contour of bi-layer MoTe2 film.

FIGURE 4. Band structure of MoTe2 film along high symmetry directions.(a) The Brillouin zone of MoTe2 and cuts inmeasurements. (b) Electronic band structures along high symmetry direction. The corresponding momentum positions areshown in (a) by blue lines. (c) The calculated band structure for double layer MoTe2 [22].

In conclusion, we get some good experimental results about ZrTe and bilayer MoTe2 which have notbeen reported in literature. The band structures are basically in accord with band calculations. To publish
the obtained results, more experiments need to be done on other cleaved surfaces of ZrTe and mono layer
MoTe2.

REFERENCES
1. Hasan MZ and Kane CL. , Rev Mod Phys 82, 3045 (2010).
2. Qi XL and Zhang SC , Rev Mod Phys 83, 1057(2011).
3. Ando Y. , J Phys Soc Jpn 82, 102001 (2013).
4. Liu ZK, Zhou B, Zhang Y, et al., Science 343, 864 (2014).
5. Xu SY, Liu C, Kushwaha SK, et al., Science 347, 294 (2015).
6. Liu ZK, Jiang J, Zhou B, et al., Nat Mater 13, 677-681 (2014).
7. Xu SY, Belopolski I, Alidoust N, et al., Science 349, 613-617 (2015).
8. Lv BQ, Weng HM, Fu BB, et al., Phys Rev X 5, 031013 (2015);5:031013.
9. Yang LX, Liu ZK, Sun Y, et al., Nat Phys 11, 728-732 (2015).
10. Neupane M, Belopolski I, Hosen MM, et al., Phys Rev B 93, 201104 (2016).
11. Schoop LM, Ali MN, Strasser C, et al., Nat Commun 7, 11696 (2016).
12. B. Bradlyn, J. Cano, Z. Wang, et al., , Science 353, aaf5037 (2016).
13. Zhu, Z., Winkler, G. W., Wu, Q. S., et al., Phys. Rev. X 6, 031003 (2016).
14. Weng, H., Fang, C., Fang, Z. & Dai, X., Phys. Rev. B 94, 165201 (2016).
15. Chang, G. et al,. Scientific Reports 7, 1688 (2017).
16. B. Lv, Z.-L. Feng, Q.-N. Xu, et al., Nature 546, 627 (2017).
17. J.-Z. Ma, J.-B. He, Y.-F. Xu, et al., Nature Physics 14, 349-354 (2018).
18. W. L. Zhu, J. B. He, S. Zhang, et al., arXiv:1707.00942.
19. Ying Wang, Jun Xiao, et al., Nature 550, 487-491 (2017).
20. Th. Boker, A. Muller, et al, Phys. Rev. B 60, 4675 (1999).
21. YJ Zhang, et al., 2D Mater 2, 044004 (2015).
22. C. Robert, et al., Phys. Rev. B, 94, 155425 (2016).

- 51 -

17AU010

BL-5

XPS study of boron-doped diamond films covered by V2O5
Y. Muraokaa, T. Wakitaa, T. YokoyaaaResearch Institute for Interdisciplinary Science, Okayama University Okayama 700-8530, Japan

Keywords: XPS, Boron-doped diamond(100) films, V2O5

X-ray photoemission spectroscopy (XPS) study of the boron-doped diamond films covered by V2O5 thinfilms is performed. It is reported that in H-terminated diamond (100) films covered by V2O5 the efficient surface
transfer doping of H-diamond by V2O5 takes place due to the nature of negative electron affinity in H-diamond(100) surface [1, 2]. It would be interesting to see whether the boron-doped diamond(100) superconducting
films are available instead of the diamond(100) films. In this work, we investigate the valence state of carbon
and vanadium in boron-doped diamond films without and with the cover of V2O5 films to clarify the possibilityof surface transfer doping of the boron-doped diamond by V2O5 films.
Boron-doped diamond(100) films are prepared by a hot-filament chemical vapor deposition method on
diamond(100) substrates [3]. The thickness of the boron-doped films is about 0.5 micrometer. V2O5 thin films
are fabricated on the boron-doped diamond(100) films by a pulsed laser deposition (PLD) technique. The
thickness of V2O5 films is approximately 10 nm. XPS measurements were carried out on the beamline BL-5, atthe Hiroshima Synchrotron Radiation Center in Hiroshima University. Photon energy was set at 350 and 660 eV.
Fermi level position was determined by measuring the molybdenum spectra. All measurements are carried out atroom temperature.
Figure 1 shows the C 1s core-level spectra of the boron-doped diamond(100) films without and with thecover of V2O5 thin films. The C 1s signal is clearly observed for both films. The peak position for the films
without the cover of V2O5 is 284.0 eV which is the same as that for the films with V2O5 cover, while the peakwidth of the former films is narrower compared to the latter. In the presentation, the results of vanadium 2p
core-level spectra will be shown.

Intensity (arb. unit)

BDD4
BDD4 + V2O5 10 nm

288

286

284

282

280

Binding energy (eV)FIGURE 1. C1s core-level spectra for boron-doped diamond(100) films without and with the coverof V2O5 thin films. Photon energy of 350 eV was used.

REFERENCES
1. K. G. Crawford, L. Cao, D. Qi, A. Tallaire, E. Limiti, C. Verona, A. T. S. Wee, and D. A. J. Moran, Appl. Phys. Lett. 108,042103 (2016).
2. C. Verona, W. Ciccognani, S. Colangeil, E. Limiti, M. Marinelli, and G. V. Rinati, J. Appl. Phys. 120, 025104 (2016).
3. T. Doi, T. Fukaishi, C. Hiramatsu, T. Wakita, M. Hirai, Y. Muraoka, T. Yokoya, Y. Kato, Y. Izumi, T. Muro, and Y.
Tamenori, Diam. Relat. Mater. 25, 5 (2012).

- 52 -

(A)(16AU003,
16AU003, 17AU010)
17AU010BL
BL-5Photoelectron Spectra of Lutetium Encapsulated Fullerenes
T. MIYAZAKIa, T. Wakitaa, T. YOKOYAa, H. SHINOHARAb and S. HINOca

Research Laboratory for Surface Science, Okayama University, Okayama 700-8530,
700 8530, Japan
Graduate School of Science, Nagoya University, Chikusa
Chikusa-ku,
ku, Nagoya, 464-8602,
464 8602, Japanc
Graduate School of Science and Engineering
Engineering,, Ehime University, Matsuyama, 790
790-8577,
8577, Japanb

Keywords: Lutetium encapsulated
encapsulated fullerenes, Electronic structure, Photoelectron spectroscopy, Electron transfer

Fullerene cages often encapsulate metal atom
atom(s). The encapsulate
ncapsulated atom(s)atom donate electrons to fullerene
cages which induces the change of their electronic structure. Ultraviolet photoelectron spectroscopy is adirect method to determine the electronic structure. Recently, Lutetium
ium atoms encapsulated C 82 fullerenes
have been isolated. In this article,
article, ultraviolet photoelectphotoelectronron spectra (UPS) of Lu2@C 82 are presented with
hv = 30 ~ 200 eV incident photon energy and are discussed on the electronic structure.
The UPS of the endohedral fullerenes were measured at the beam line 5 of HISOR of Hiroshima
University. Specimens for the measurement of the photoelectron spectra were prepared by vacuum
deposition of the fullerenes onto a gold deposited metal disk using a resistive heating quartz crucible in asample preparation chamber directly attached to the measurement chamber. The measured UPS werereferenced
nced against Fermi level (EF) of gold and were plotted as a function of binding energy relative to E F.
The valence band UPS of Lu2@C82 obtained with hv = 30−30
206 eV incident photon energy are shown in Figur
Figure 1. The
he peak
intensity in the upper than 9 eV regions
region monotonously decreasein accordance with the increase of the incident photon energy.
This depends on the decrease in excitation cross section of C2p(2s). Their eelectronic
lectronic structures in the UPS of fullerenes andendohedral fullerenes appeared in the deeper than 5 eV region
are strongly due to σ–electrons
σ electrons that constituteconstitutes the C82 skeletal
C–C
C bonds and those between 0−0−5 eV are due to π–electrons.
electrons.
Remarkable differendifference
ce among these UPS is observed at 9.6.6 and
11.1 eV.. Two structures are only observed in the UPS of Lu
entrapped fullerenes. These structures are not observed when theincident photon energy is less than 35 eV, but their intensity
becomes stronger in accordance with the increase of the incident
photon energy. The ionization energies of Lu 4f 7/2 and 4f5/2
levels of Lu metal are 7.5 eV and 8.9 eV, respect
respectively
ively [1]. But
their binding energies are much higher than
than those of Lu metal
atoms by 2.1 eV. Because of this donation entrapped metal
atoms become ele
electronctron deficit, which brings the ionization
zationenergy of entrapped atom
atoms large. These results suggest that two
FIGURE 1. The UPS of Lu 2@C82 at
structures are not derived
rived from thethe electrons of the carbon cage
hv =3030-206 eV incident photon energy.
but derived from Lu atom
atom. Lu4f7/2 and Lu4f5/2 peak are observed
and their binding energies deviate, which could be a good indicator of the electron configuration of theseendohedral fullerenes. Thus, the encapsulated metal atoms donate electrons to the cage.
REFERENCE
ENCE
1. M. Cardona, L. Ley
Ley,, Photoemission in Solids. I: General Principles, Springer-Verlag,
Springer Verlag, Berlin, 1978.

- 53 -

16AU003, 17AU010

BL-5

Current activities of research and education on BL-5
(FY2017)
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
2017. 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 electronic states of the boron-doped diamond films coveredby the V2O5 thin films by the core-level photoemission spectroscopy, and the charge states of Lu atoms in
Lu2@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.

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).

- 54 -

17AU018

BL-06

The Electronic States of D/L Amino Acid Absorbed on
WO3
Yoshihiro Nakaoa, Hiroki Itob, Yui Ohnob, Hiroaki Yoshidac, d Kenta Adachia andYuka HorikawaaaGraduate School of Sciences and Technology for Innovation, Yamaguchi University
1677-1 Yoshida, Yamaguchi, Yamaguchi 753-8512 Japanb
Physics and Informatics, Faculty of Science, Yamaguchi University
1677-1 Yoshida, Yamaguchi, Yamaguchi 753-8512 Japanc
Department of Physical Science, Graduate School of Science, Hiroshima University
1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8526 Japand
Hiroshima Synchrotron Radiation Center, Hiroshima University
2-313 Kagamiyam , Higashi-Hiroshima, Hiroshima 739-0046 Japan
Keywords: Soft X-ray absorption spectroscopy, Helium path technique, Phenylalanine, Cyclodextrin, WO3

A tungsten(VI) oxide (WO3) is the most typical photochromic metal oxide semiconductors. It is known
that a color change (colorless → blue) corresponding to the reduction of W6+ into W5+ states in WO3
nanoparticles occurs under UV illumination. It is also known that the photochromism is enhanced whensome molecules are adsorbed on the WO3 nanoparticles. For example, when phenylalanine molecules areadsorbed on the WO3 nanoparticles, a color of the WO3 aqueous solution shows darker blue under UV
illumination. Phenylalanine is an -amino acid and has D/L enantiomer. An adsorption ratio of phenylalanine
to the WO3 particles is same for D/L phenylalanine in a solution of phenylalanine and WO3. However, whena cyclodextrin(CD) is added in the solution, L-phenylalanine/CD/WO3 solution shows darker blue, that is,
the adsorption ratio becomes higher in the L-phenylalanine/CD/WO3 solution. It is proposed that a
cyclodextrin inclusion complex with L-phenylalanine is selectively formed and the complex has a highaffinity for the WO3 particle.
We have performed soft X-ray absorption spectroscopyof L-phenylalanine/CD/WO3 powder made by solvent
drying of the aqueous solution under helium environment.
We have confirmed that phenylalanine has a * peak at
532.6 eV and WO3 shows a strong resonant peak at 530.8
eV. Figure 1 shows absorption spectra of D (upper) and Lphenylalanine/CD/WO3 (bottom). Graph legends are thecomponent molar fraction of D/L-phenylalanine/CD/WO3.
Resonant peak shows characteristic changes for D and Lphenylalanine mixture. When the fraction is 1:1:1, acomponent fraction of phenylalanine is the highest among
all samples and absorption spectra of this sample shows
strong resonant peak at 532.6 eV. The spectral shape of
1:1:1 sample is same for D and L-phenylalanine contained
powder. A resonance peak at 530.8 eV of upper graph isenhanced when the fraction of phenylalanine decreases(0.1:0.1:1 and 0.1:0.01:1) because a molar fraction of WO3increases. However, the enhancement of the peak is weakin the bottom graph. We consider it as a result of increaseof the adsorption fraction of L-phenylalanine on the WO3particles; WO3 is less irradiated when the particles are
FIGURE 1. O K-edge X-ray absorption spectraof Phenylalanine/-cyclodextrin/WO3.
covered with phenylalanine.

- 55 -

17AU018, 17BG010

BL-6

Development of an apparatus for soft X-ray absorption
experiments of solid and liquid samples underatmospheric helium gas environment
Yoshihiro Nakaoa, Hiroki Itob, Yui Ohnob, Hiroaki Yoshidac, d,Takashi Tokushimae, f and Yuka HorikawaaaGraduate School of Sciences and Technology for Innovation, Yamaguchi University
1677-1 Yoshida, Yamaguchi, Yamaguchi 753-8512 Japanb
Physics and Informatics, Faculty of Science, Yamaguchi University
1677-1 Yoshida, Yamaguchi, Yamaguchi 753-8512 Japanc
Department of Physical Science, Graduate School of Science, Hiroshima University
1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8526 Japand
Hiroshima Synchrotron Radiation Center, Hiroshima University
2-313 Kagamiyam , Higashi-Hiroshima, Hiroshima 739-0046 Japane
SANKA High Technology Co. Ltd.,90-1 Kurimachi, Shingu-cho, Tatsuno-shi, Hyogo 678-5148 Japanf
Laboratory of Advanced Science and Technology for Industry, University of Hyogo
3-1-2 Koto, Kamigori-cho, Hyogo 678-1205 Japan
Keywords: Soft X-ray absorption spectroscopy, XAS, helium path technique, liquid samples

1. Introduction
Soft X-ray absorption experiments are normally performed in a vacuum environment to avoid
attenuation by air. It was generally considered that measurements of liquids by soft X-ray is difficult due tothe attenuation. Recently, thin films with a thickness around 100 nm made of silicon nitride (Si3N4) orsilicon carbide (SiC), which can be used as a window for soft X-ray transmission, are commerciallyavailable [1-2]. For example, we have developed an experimental apparatus in which a vacuum chamber
and a liquid sample under atmospheric pressure were separated by the thin film window [1]. However,since thin films of window is fragile, there is a danger that the liquid samples flow into the vacuum
chamber when the membrane window breaks. It is difficult to install liquid flow cells using thin film
window in a normal soft X-ray beam line without cares for window break accidents. Therefore, we havedeveloped a new apparatus for soft X-ray absorption experiments for liquids and solid with helium
environment. In the helium gas environment, it is possible to arrange the sample away from the window
surface since the attenuation length of the soft X-ray is longer by about 10 to 100 times than in the air [3].
Even if the window material breaks, only environment gas could flow into the vacuum chamber, andsamples are stay on sample plate. Therefore newly designed apparatus requires much less cares forconnecting it to the beamline.
2. Development of an apparatus
Fig. 1 is a schematic illustration of a apparatus for soft X-ray absorption experiments of solid and liquidsamples under atmospheric helium gas environment. The left panel shows the apparatus for solid samples,
and the right shows the apparatus for liquid samples. In this apparatus, we have used 100nm thickmembrane window with 0.5mm×0.5mm opening made of silicon nitride on φ3mm silicon frame (NT050C,
Norcade, Canada). In order to shorten the distance between the sample and window for reducing
attenuation of soft X-rays, the membrane window was glued directly on the tip of a 3 mm diameter tube
which was connected to the chamber of the end-station. For solids, the sample was mounted on the copper
plate slope by a double sided adhesive carbon tape. For liquid samples, the sample was dropped on a pit ofa copper plate and kept by surface tension. A photodiode sensitive in the soft X-ray region was also placednear sample to detect soft X-ray emission (fluorescence) caused by soft X-ray irradiation. Then finally, the- 56 -

whole apparatus including a sample and the detector was covered with a vacuum piping so as to replace
the sample environment by helium gas with atmospheric pressure.

Fig. 1 Schematic illustration of sample apparatus including a membrane window, XY stage to adjust optical axis , aphotodiode to detect fluorescence yield and a sample plate. The left panel is apparatus for solid samples and the right isapparatus for liquid samples.

3. Experiments
The developed apparatus was installed in the end station of the at HiSOR BL6 [4]. Soft X-ray absorption
experiments were performed in oxygen K-edge of L-phenylalanine (powder) and H2O under He gas
environment. For the evaluation of this apparatus, we compared the spectra obtained at HiSOR BL6 withthe spectra of same sample measured at BL17SU in SPring-8. We have also compare previously reported
spectrum of oxygen and carbon dioxide gas to confirm whether the sample environment is sufficiently
replaced with helium or not.
4. Results
Fig. 2 shows obtained spectra of L-phenylalanine (left panel) and H2O (right panel) in O K-edge.

Fig. 2 The left panel shows absorption spectra of L-phenylalanine in O K-edge taken at HiSOR BL6 and SPring-8 B17SU.
In lower part of the left panel, reference spectra of gas phase oxygen [5] and carbon dioxid [6] in O K-edge were plotted.
The right panel shows absorption spectra of H2O in O K-edge at HiSOR BL6 (top) and at SPring-8 B17SU (bottom).

Spectral shapes of the absorption spectra obtained at HiSOR BL6 are consistent with that obtained at
SPring-8 BL17SU as shown in left panel of Fig. 2. In addition, there is no peaks assignable to peaks ofoxygen and carbon dioxide in spectra of L-phenylalanine (top). Therefore, we conclude that the sample
environment was successfully replaced with helium gas. The absorption spectra of liquid H2O obtained at
HiSOR BL6 is also consistent with the spectrum of SPring-8 BL17SU. According to these results, weconclude that the developed apparatus works fine for x-ray absorption measurements under He gas withatmospheric pressure.

REFERENCES
1. T. Tokushima et. al., Phys. Chem. Chem. Phys., 11(2009), 1679-1682.
2. M. Nagasaka and N. Kosugi, J. Vac. Soc. Jpn., Vol. 59, No. 11, 2016, 301-306.
3. Y. Tamenori, J. Vac. Soc. Jpn., Vol. 59, No. 12, 2016, 333-340.
4. H. Yoshida, Y. Senba, T. Goya et. al., J. Elec. Spectrosc. Relat. Phenom., 144-147 (2005) 1105.
5. Hitchcook Research Group http://unicorn.mcmaster.ca/
6. B.X. Yang, et. al., Phys. Rev. A 36 (1987) 4298

- 57 -

17BU018

BL-6

Soft x-ray absorption measurement of hybrid organicinorganic perovskite, a material for dye-sensitized solar
panels, under atmospheric pressure He environment
Takashi Tokushimaa,b, Masahito Niibea, Sho Yokouchia, Yuka Horikawac,
Yoshihi Nakaoc and Hiroaki Yoshidad,eaLaboratory of advanced science and technology for industry, University of Hyogo, 3-1-2 Koto, Kamigoricho, Ako-gun, Hyogo, JAPANb
SANKA high technology Co. Ltd., 90-1 Kurimachi, Shingu-cho, Tatsuno-shi, HyogocPhysics and Informatics, Faculty of Science, Yamaguchi University, 1677-1 Yoshida, Yamaguchi,
YamaguchidDepartment of Physical Science, Graduate School of Science, Hiroshima University, 1-3-1 Kagamiyama,
Higashi-Hiroshima, Hiroshimae
Hiroshima Synchrotron Radiation Center, Hiroshima University, 2-313 Kagamiyama, Higashi-Hiroshima,
Hiroshima
Keywords: Soft x-rays, X-ray absorption, He environment.

Hybrid organic-inorganic perovskite is a candidate for amaterial for dye-sensitized solar panels [1]. However, thesematerials show degradation when exposed to humid
atmospheres. Therefore, it is important to know electronic
structure at realistic condition rather than under vacuum [2].
We have recently been developing helium path apparatus
where air is replaced by He gas to improve transmission of soft
x-rays. Because of strong K-edge absorption of nitrogen andoxygen, 5 mm of light path in air is enough to decay less than
FIGURE 1. FY intensity and sample
1% in intensity for soft x-rays from N 1s absorption (aroundcurrent (EY) plot vs incident energy for
400eV) above O 1s absorption (around 530eV). Owing to He
C1s edge of (CH3NH3)PbI3 (red lines)
path, transmission is expected to recover over 90%. Thisand PbI3 (solid lines).
technique is well known and frequently used in experiments in
hard x-ray region. However, applications using helium environments for soft x-ray region (around 1001000eV) are still small in number. In our new apparatus, a commercially available silicon nitride membrane(Norcada Inc., Canada) made from Si wafer, which is around 100nm in thickness and have high transmission
for soft x-rays, was used as a window to separate vacuum and He gas at atmospheric pressure.
Figure 1 shows fluorescence intensity and sample current plot vs incident energy for C1s edge underatmospheric He gas. One can observe small differences between (CH3NH3)PbI3 and PbI3, however thedifference is not enough to clarify electronic structure. It is required to improve signal to noise ratio.
According to obtained energy profile of incident beam intensity, mirrors and gratings of the beamline might
be contaminated by carbon. Hence cleaning of the optics is required for further experiments using C1s edge.

REFERENCES
1.
2.

H Zhu, K.Miyata et al., Science 353 (6306), pp.1409-1413(2016).
J. Chun-Ren Ke, A. Walton et al., Chem. Commun. 53 (37), pp. 5231–5234 (2017).

- 58 -

16AG012, 17AU004, 17AU005, and 17BG035

BL-7, RIPES

Electronic Structures and Impurity Cluster Features in
Mg-Zn-Y Alloys with a Synchronized Long-Period
Stacking Ordered Phase
Shinya Hosokawaa, Kenji Maruyamab, Kentaro Kobayashib,Jens Rüdiger Stellhorna, Benedict Paulusa, Akihide Kouraa, Fuyuki Shimojoa,
Michiaki Yamasakic,d, Yoshihito Kawamurac,d, Satoru Yoshiokae, Hitoshi Satofa

Department of Physics, Kumamoto University, Kumamoto 860-8555, Japanb
Department of Chemistry, Niigata University, Niigata 950-2181, Japanc
Department of Materials Science, Kumamoto University, Kumamoto 860-8555, Japand
Magnesium Research Center, Kumamoto University, Kumamoto 860-8555, Japane
Department of Applied Quantum Physics and Nuclear Engineering, Kyushu University, Fukuoka
819-0395, Japanf
Hiroshima Synchrotron Radiation Center, Hiroshima University, Higashi-Hiroshima 739-0046, Japan
Keywords: Structural metal, LPSO structure, Impurities, Electronic structure, Chemical states.

In the recent fifteen years, novel Mg-based alloys with microstructures forming a long-period stacking
ordered (LPSO) structure, the so-called KUMADAI Magnesium [1], have achieved much attention due tothe potential for widespread applications as structural materials because these alloys are light-weighted
rather than Al. By adding a small amount of Zn and rare-earth (Y or Gd) impurities, the soft and flammable
Mg metal turns to much strong and non-flammable with high thermal stability. Because of such excellent
properties together with the ease of recycling, these Mg alloys are expected as next-generation structural
materials for, e.g., bodies of subways or even aircrafts.
According to an atomic-resolution angle annular dark field scanning transmission electron microscopeobservations by Abe et al. [2], it was found that the Zn and Y impurities are enriched around the stacking
faults, and thus, this curious structure is referred to as the synchronized LPSO phase. The existence ofL12-type clusters formed by the impurities in the Mg-Zn-Y alloys was proposed by Egusa and Abe [3],which were composed of Zn6Y8 and embedded in the stacking faults.
From the structural point of view, the overall features of the L12 clusters are well described as cited inRef. [3]. However, the electronic structures should be clarified for the understandings of the electronic
properties and the chemical natures of the Zn/Y impurities, which may be highly influenced for thestability of the L12 clusters in the Mg LPSO alloys. We have recently investigated the valence- andconduction-band electronic structures of Mg97Zn1Y2, Mg85Zn6Y9, and Mg75Zn10Y15 polycrystalline alloys
together with pure -Mg by measuring photoemission and inverse-photoemission spectroscopies (PES andIPES), respectively. Here, the Mg97Zn1Y2 and Mg85Zn6Y9 alloys have 18R type LPSO structures and the
Mg75Zn10Y15 alloy has a 10H type after the Ramsdell notation with the volume fractions of the LPSO
phase up to ∼24%, ∼100%, and ∼100%, respectively. The core levels of the constituent elements were alsostudied using core-level PES to examine the chemical natures of the elements. From these results, wefound clear heterogeneous indications concerning the L12 clusters.
The cast ingots of the samples were prepared using high frequency induction melting of pure Mg (99.99
wt.%), Zn (99.9 wt.%), and Y (99.9 wt.%) metals in a cylindrical carbon crucible in a pure Ar atmosphere.
The ingots were cut to be a rectangle plate with a size of 3  3  1 mm3. The PES spectra were measured
using a spectrometer installed at BL-7 of HSRC. The IPES spectra were obtained using the RIPES
spectrometer at HSRC. All of the measurements were performed at room temperature.
Figure 1 shows the PES and IPES spectra of (a) pure Mg, (b) Mg97Zn1Y2, (c) Mg85Zn6Y9, and (d)Mg75Zn10Y15. The magnitudes of the PES spectra are normalized to the maximum values at about -6 eV,
and the IPES spectra were normalized so that the intensities show 0.5 at 5 eV. With increasing the impurity- 59 -

fraction, the Zn 3d core-level peaks appear, and show high incident photon energy, h, dependence. The
impurity fraction dependence in the magnitude of the Zn 3d peak is unusual, indicating that this signal may
originate from the Zn atoms in the Mg host, and not in the Zn6Y8 clusters or their fragments. Small spectralchanges are observed in the PES spectra, a small shoulder at about -7.8 eV and a large peak at about -12.0
eV. The IPES spectra show large impurity effects in the slope at about 3 eV and the tail beyond 5 eV.

FIGURE 1. The valence-band PES and conduction-band IPES spectra of (a) pure α-Mg, (b) Mg97Zn1Y2, (c) Mg85Zn6Y9, and(d) Mg75Zn10Y15 polycrystals indicated by the solid and dashed curves, respectively.

The thick solid curves in Fig. 2 show the Y 3d core-level PES spectra of the Mg97Zn1Y2, Mg85Zn6Y9, and
Mg75Zn10Y15 alloys from top to bottom measured at h = 230 eV with the energy resolution of 0.23 eV.
The intensities are reduced by the Y 3d5/2 peak height with respect to the background at the shallower
energy of -153 eV. For the Mg85Zn6Y9, and Mg75Zn10Y15 alloys, mostly one doublet is observed, which
may originate from the Y atoms in the Zn6Y8 clusters. On the other hand, the spectrum of Mg97Zn1Y2 has
at least three doublets, suggesting that the Y atoms in this alloy exhibit different chemical natures. The
spectrum was fitted using three doublets, and the results are given by the dashed, dotted, and chain curves.
The largest one is located at the same energy of others, confirming that mostly a half of the Y atoms may
belong to the Zn6Y8 clusters, and other two may indicate that the Y atoms are located alone in the host Mg
or in fragments of the L12 clusters. Detailed analysis is now in progress.

FIGURE 2. Y 3d core-level PES spectra measured at hν = 230 eV. The spectra are fitted with at most three pairs of the Y
3d5/2 and 3d3/2 core electrons (the dashed, dotted, and chain curves) and the background (the two-dot chain curve).

REFERENCES
1. Y. Kawamura et al., Mater. Trans. 42, 1172-1176 (2001).
2. E. Abe et al., Philos. Mag. Lett. 91, 690-696 (2011).
3. D. Egusa and E. Abe, Acta Mater. 60, 166-178 (2012).

- 60 -

16AG012, 17AU004, 17AU005, and 17BG035

BL-7, RIPES

The seeds of Zn6Y8 L12-type clusters in amorphous
Mg85Zn6Y9 alloy investigated by photoemission
spectroscopy
Shinya Hosokawaa, Jens Rüdiger Stellhorna, Benedict Paulusa,
Kenji Maruyamab, Kentaro Kobayashib, Hiroshi Okudac, Michiaki Yamasakid,e,Yoshihito Kawamurad,e, Hitoshi Satofa

Department of Physics, Kumamoto University, Kumamoto 860-8555, Japanb
Department of Chemistry, Niigata University, Niigata 950-2181, Japanc
Department of Materials Science and Engineering, Kyoto University, Kyoto 606-8501, Japand
Department of Materials Science, Kumamoto University, Kumamoto 860-8555, Japane
Magnesium Research Center, Kumamoto University, Kumamoto 860-8555, Japanf
Hiroshima Synchrotron Radiation Center, Hiroshima University, Higashi-Hiroshima 739-0046, Japan
Keywords: Amorphous metal, LPSO structure, Impurities, Electronic structure, Chemical states

In the recent fifteen years, novel Mg-based alloys with microstructures forming a long-period stacking
ordered (LPSO) structure, the so-called KUMADAI Magnesium [1], have achieved much attention due tothe potential for widespread applications as structural materials because these alloys are light-weighted
rather than Al. By adding a small amount of Zn and rare-earth (Y or Gd) impurities, the soft and flammable
Mg metal turns to much strong and non-flammable with high thermal stability. Because of such excellent
properties together with the ease of recycling, these Mg alloys are expected as next-generation structural
materials for, e.g., bodies of subways or even aircrafts.
According to an atomic-resolution angle annular dark field scanning transmission electron microscopeobservations by Abe et al. [2], it was found that the Zn and Y impurities are enriched around the stacking
faults, and thus, this curious structure is referred to as the synchronized LPSO phase. The existence ofL12-type clusters formed by the impurities in the Mg-Zn-Y alloys was proposed by Egusa and Abe [3],which were composed of Zn6Y8 and embedded in the stacking faults.
Recently, Okuda et al. [4] investigated the transformation process of amorphous-to-crystal in
Mg85Zn6Y9 LPSO alloy using small angle x-ray scattering. They found a novel hierarchical phasetransformation that the clustering of the impurity atoms occurs first, and the spatial rearrangement of theclusters induces a secondary transformation leading to two-dimensional ordering of the clusters. From theirresults, we expect that the seeds of impurity clusters are already implanted even in the amorphous phase.
The electronic structures will give valuable hints for the understandings of the electronic properties andchemical natures of the Zn/Y impurities in the amorphous and crystalline Mg LPSO alloys, which may behighly influenced for the stability of the L12 clusters. We have recently investigated the valence- andconduction-band electronic structures of amorphous and crystalline Mg85Zn6Y9 alloys by measuring
photoemission and inverse-photoemission spectroscopies (PES and IPES), respectively. The core-levels
were also studied using core-level PES to examine the chemical natures of the constituent elements. Fromthese results, we found clear heterogeneous indications concerning the Zn6Y8 L12 clusters.
An amorphous sample was manufactured at Magnesium Research Center, Kumamoto University by anormal melt-spinning technique. The amorphous phase of the obtained ribbons was examined by x-ray
diffraction and transmission electron microscopy. The composition of the sample was confirmed to be
nominal value within 1 at.% by an electron-probe micro-analysis equipment. The PES spectra were
measured using a spectrometer installed at BL-7 of HSRC. The IPES spectra were obtained using the
RIPES spectrometer at HSRC. All of the measurements were performed at room temperature.
Figure 1 shows the PES and IPES spectra of (a) crystalline and (b) amorphous Mg85Zn6Y9 alloys. The
magnitudes of the PES spectra are normalized to the maximum values at about -6 eV, and the IPES spectra- 61 -

were normalized so that the intensities show 0.5 at 5 eV. The spectral features of the both phases look very
similar to each other.

FIGURE 1. The valence-band PES and conduction-band IPES spectra of (a) crystalline and (b) amorphous Mg85Zn6Y9
indicated by the solid and dashed curves, respectively.

The thick solid curves in Fig. 2 show the Y 3d core-level PES spectra of amorphous (upper) andcrystalline (lower) Mg85Zn6Y9 alloys measured at the incident photon energy h = 230 eV with the energy
resolution of 0.23 eV. The intensities are reduced by the Y 3d5/2 peak height with respect to the backgroundat the shallower energy of -153 eV. For the crystalline phase, mostly one doublet is observed, which may
originate from the Y atoms in the Zn6Y8 clusters. On the other hand, the spectrum of the amorphous phasehas at least three doublets, suggesting that the Y atoms in this amorphous alloy exhibit different chemical
natures.

FIGURE 2. Y 3d core-level PES spectra measured at hν = 230 eV. The spectra are fitted with at most three pairs of the Y
3d5/2 and 3d3/2 core electrons (the dashed, dotted, and chain curves) and the background (the two-dot chain curve).

The spectrum was fitted using three doublets, and the results are given by dashed, dotted, and chain
curves. The largest contribution is located at the same energy of that of the crystalline phase, confirmingthat mostly a half of the Y atoms may belong to the Zn6Y8 clusters, and other two indicate that the Y atoms
are located alone in the host Mg or in fragments of the L12 clusters. Detailed analysis is now in progress.

REFERENCES
1.
2.
3.
4.

Y. Kawamura et al., Mater. Trans. 42, 1172-1176 (2001).
E. Abe et al., Philos. Mag. Lett. 91, 690-696 (2011).
D. Egusa and E. Abe, Acta Mater. 60, 166-178 (2012).
H. Okuda et al., Sci. Rep. 5, 14186-1-6 (2015).

- 62 -

17AG012

BL-07

Photoemission Study of Sr-Cu-O Compounds With Multi
CuO2 Layer
S. Tsudaa, H. Yamaokab, H. Sakuraic, H. Satod, and K. ShimadadaNational Institute for Materials Science 3-13 Sakura, Tsukuba 305-0003, Japanb
RIKEN SPring-8 Center, Sayo, Hyogo 679-5148, Japanc
National Institute for Materials Science 1-1 Namiki, Tsukuba 305-0044, Japand
Hiroshima Synchrotron Radiation Center, Hiroshima University, Higashi-Hiroshima, Hiroshima 739-0046,
Japan
Keywords: Photoemission spectroscopy, transition metal oxides

After the pioneer work by Dr. Matsuno[1], there had been no report on the systematic study ofelectronic structure of layered Perovskite compounds of 3d transition metal oxides. While the former
study is on thin films, recently high quality bulk samples are available [2,3]. In this study, we performed
photoemission measurements of bulk Sr2MO4 with M = V(3d1), Cr(3d2), Mn(3d3) and Srn+1CrnO3n+1 with
n= 1, 2, 3 at BL-07 in HiSOR.
Figure 1 shows valence band photoemission spectra of Sr2MO4 (M = V, Cr, Mn) obtained by h = 60 eV.
Those spectra contains mainly two components ; (A) huge peak around -5 eV mostly due to O 2p (B)relatively small but finite humps below -3 eV probably due to 3d (and lower Hubbard) bands. With
increasing the 3d electron number, the intensity (B) increases.

FIGURE 1. Valence band photoemission spectra of Sr2MO4 (M = V, Cr, Mn) obtained by h = 60 eV.

- 63 -FIGURE 2. Valence band photoemission spectra of Srn+1CrnO3n+1 (n= 1~3) obtained by h = 60 eV.

Figure 2 shows valence band spectra of Srn+1CrnO3n+1 (n= 1~3) obtained by h = 60 eV. With
increasing the n (the number of the CrO2 layer), the humps around -1.2 eV are broadened and the intensity
at Fermi level is increased. The increase of the n corresponds to the increase of the dimensionality (from2 dimensional to 3 dimensional). In the limit of n -> ∞, Srn+1CrnO3n+1 becomes SrVO3, which is known
as a correlated metal. This trend is consistent with our results
A first principle calculation [4] predicts that Sr2CrO4 shows negative charge transfer (). Now weare trying to evaluate physical parameters such as U and  within cluster model.

REFERENCES
1.
2.
3.
4.

J. Matsuno et al., Phys. Rev. Lett. 95, 176404 (2005).
H. Sakurai, Physics Procedia 75, 829 (2015).
H. Sakurai, J. Phys. Soc. Jpn. 83, 123701 (2014).
T. Ishikawa et al., J. Phys. Soc. Jpn. 86, 033701 (2017).

- 64 -

17AG035

BL-7

Elucidation of Electronic Band Structure of Monolayer
Germanium Film
Osamu Kuboa, Satoshi Endoa, 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. We have recently reported that germanene
is formed on Al(111) surface with a periodicity of Al(111)√7×√7 [6], which is different from thepreviously reported 3×3 phase [4]. However, the electronic band structures of both √7×√7 and 3×3 phases
have not yet been reported. Thus, we have studied on the electronic band structure of the germanene with
√7×√7 phase measured using angle-resolved photo emission spectroscopy.

FIGURE 1. Band dispersions for clean Al(111) along Γ-Κ direction of Al(111) brillouin zone obtained by ARPES.

Figure 1(a) shows the band dispersion for a clean Al(111) obtained along Γ-Κ direction of Al(111)brillouin zone. The band dispersion after germanene formation is show in Fig. 1(b), in which two linear
bands crossing at Γ point are seen. We then calculated the band dispersions along the same direction as
the spectra shown in Fig. 1 by using DFT. The atomic structure of germanene reported in ref. 6 is

- 65 -

adopted in the calculation. From the DFT calculation, it is suggested that the newly formed bands arecaused by subsurface Al layers rather than by top germanene layer.

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

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).

- 66 -

17BU015

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

Tungsten (W) is a refractory metal ranging in applications such as armor material in magnetic
fusion devices or as replacement of copper metallic interconnects in microelectronics. In suchapplications, the bcc cubic crystal structure α-W is used. An alternative A15 structure, β-W, ismeta-stable and is considered for use in spintronic devices. Spintronic devices utilize both the charge
and the spin of electrons and have the potential to overcome issues facing conventional semiconductor
electronics. The transfer efficiency of the charge-to-spin current is defined by the spin-Hall angle
which is ~0.3 for β-W in comparison to <0.07 for α-W at 300 K. Many studies have examined themagnitude 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.
The reason for the large magnitude of the spin Hall angle in β-W is presently not known. The spin
Hall effect can arise: (1) extrinsically, such as asymmetric scattering for spin-up and spin-down
electrons from impurities, or (2) intrinsically, where spin imbalance occur as a result of the band
structure even in the absence of scattering. Recently, first principle calculations have shown that the
spin Hall angle of the transition metals like W depends on the electron filling of the d-orbitals (i.e.
band structure related) suggesting spin Hall effect arises from intrinsic properties [1]. Clarifying
whether the spin Hall effect arises from extrinsic or intrinsic properties is the first step inunderstanding and thus controlling the spin Hall effect in β-W. The experimental goal was thus tocharacterize the electron density of states of deposited β-W films by measuring high-resolution
valence band spectra near the Fermi level and making comparison to reference α-W.
Thin (~50 nm) α-W and β-W films on silicon substrates were produced using physical vapor
deposition technique. The mechanical and structural properties were characterized using a
combination of X-ray diffraction and picosecond ultrasonic measurements [2]. 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.
Figure 1(a) shows the photoelectron spectroscopy (PES) results of α-W film as function of
in-vacuo Ar sputtering time. The tungsten oxide layer formed due to air transport is sputtered away
following approximately 25 minutes of Ar sputtering (1 keV). Figure 1(b) compares the photoelectron
spectrum between α-W and β-W samples. Fine difference in the structure between -2 and -4 eV isobserved. Specifically, α-W clearly shows a double peak structure, which is less prominent in β-W.
Figure 2 shows preliminary results of calculated valence bands for α-W and β-W, respectively. The
calculations are qualitatively similar to the experimental results with the α-W exhibiting a sharp
two-peak structure between -2 and -4 eV, which is less prominent in β-W. However, suchcomparisons in peak magnitudes are complicated by the fact that there is an additional broad peakobserved between -4 and -6 eV in experiments. This signal continuously decreases with increasing
sputtering time as illustrated in both Figures 1(a) and (b) - suggesting partial contribution fromoxygen contamination. While calculations also show a peak between -4 and -6 eV, it is not possible atthis stage to associate such structure entirely to W due to the possible oxygen contamination. Future
work performed on bulk high purity W samples are planned to clarify this effect. These experiments
lay the groundwork in performing PES on thin magnetic multilayered β-W films (e.g. W-Mn) andfurther analysis of the data should provide insight into the spin orbit interaction difference in α-W and- 67 -

β-W which is responsible for the spin hall and inverse spin hall effect.

FIGURE 1. Photoemission spectra for (a) α-W as function of Ar sputtering time, and (b) comparison of α-W and β-Wsamples showing the difference in peak structure between -2 and -4 eV.
3

W tot

12

DOS(States/eV)

DOS(States/eV)

2.5
2
1.5
1
0.5
0
-10

total DOS

14

10
8
6
4
2

-8

-6

-4
-2
Energy (eV)

0

2

0
-10

4

-8

-6

-4
-2
Energy (eV)

0

2

4

FIGURE 2. Calculated valence band spectra of α-W and β-W samples, respectively.

REFERENCES
1. T. Tanaka, H. Kontani, M. Naito, T. Naito, D. S. Hirashima, K. Yamada, and J. Inoue, Phys. Rev. B 77, 165117 (2008).
2. A. Nagakubo et al., “Elastic constant of alpha and beta tungsten films studied by picosecond ultrasonics” USE2017
https://www.use-jp.org/proceedings/USE17/pdf/1P1-9.pdf- 68 -

16AG044

BL-9A

Angle-resolved photoemission study of WTe2
R. Matsumotoa, T. Sugimotoa, T. Mizokawaa, N. L. Sainib, M. Aritac,
H. Namatamec, M. Taniguchic, R. Jhad, R. Higashinakad, T. D. Matsudad,
and Y. Aokida

Department of Applied Physics, Waseda University, Shinjuku, Tokyo 169-8555, Japan
Department of Physics, University of Roma "La Sapienza", 00185 Roma, Italyc
HiSOR, Hiroshima University, Higashi-hiroshima, Hiroshima 739-0046, Japand
Department of Physics, Tokyo Metropolitan University, Hachioji 192-0397, Japanb

Keywords: d-Electron Systems/transition metal compound/photoemission spectroscopy/topological

INTRODUCTION
Various narrow gap semiconductors and semimetals possessing band structure with non-trivial
topological number have been attracting great interest due to their spin textured surfaces states which canbe applied for the future spintronics devices [1-8]. In addition, the coexistence of electron and hole carriers
can provide dramatic transport properties such as non-saturating magnetoresistance [9-16]. Among thesemimetals with non-saturating magnetoresistance, electronic structure of WTe2 has been studied in detailby means of angle-resolved photoemission spectroscopy (ARPES) [17-19]. A pair of electron pockets andhole pockets were reported in refs. 17 and 18 whereas five hole pockets and four electron pockets wereidentified in ref. 19. It is difficult to reconcile the three different ARPES results by chemical potential shift
alone. The authors of ref. 19 pointed out that the number of Fermi surfaces is very sensitive to atomic
position and the observed nine Fermi surfaces would be explained by calculation with some atomic
arrangement. However, the band dispersions near the Fermi level reported in the three ARPES works
largely deviate from band structure calculations which cannot be explained by adjusting atomic position. In
this content, it is interesting and important to examine the band dispersion considering their orbital
character and to reconsider the assignment of Fermi surfaces.

EXPERIMENTAL
ARPES measurements were performed at beamline 9A of Hiroshima Synchrotron Radiation
Center(HiSOR). The base pressure of the spectrometer of HiSOR BL-9A was in the 10-9 Pa range. The
crystals were cleaved at 300 K under the ultrahigh vacuum and then cooled to 20 K for the ARPES
measurements. The ARPES data were obtained within 12 hours after the cleavage. The total energy
resolution was set to about 15 meV for excitation energies of 22.9-33.9 eV in order to observe the valenceband top near the zone cernter ( or Z point). Binding energies were calibrated using the Fermi edge ofgold reference samples.

RESULTS & DISCUSSION
Figure 1 shows band maps near the zone center constructed from second derivatives of energy
distribution curves. The observed band dispersions are compared with the band-structure calculation
indicated in the right panel of Fig. 1. At photon energy of 22.9 eV, one hole pocket at the zone center andtwo electron pockets around k ~ -0.3 and 0.3 Å-1 are observed as predicted by the calculation. Theirobserved positions are much closer to the zone center than the calculated positions. In addition, the holeband at the zone center is very flat compared to the calculation. The photon energy dependence of the holeband excludes possibility of surface states. Therefore, the poor agreement between the experiment andtheory can be assigned to correlation effect between W 5d holes and electrons near the Fermi level. The
present results suggest that electron-hole correlation effect or excitonic correlation should be taken into
account for understanding of transport properties of WTe2.

- 69 -FIGURE 1. Band maps near the zone center of WTe2 taken at photon energies of 22.9 eV (left) and 25.9 eV (middle).
The right panel shows band structure calculation around  pioint.

CONCLUSION
In conclusion, we have studied the electronic structure of WTe2 by means of angle-resolved
photoemission spectroscopy and band structure calculation. The valence band top reaches the Fermi leveland forms a complicated hole pocket around the zone center. In addition, two electron pockets areobserved besides the hole pocket which are modified from the prediction of band calculation. The
disagreement between the experiment and theory indicates importance of electron-hole correlation in
WTe2.

REFERENCES
1 D. Hsieh, Y. Xia, L. Wray, D. Qian, A. Pal, J. H. Dil, J. Osterwalder, F. Meier, G. Bihlmayer, C. L. Kane, Y. S. Hor, R. J.
Cava, and M. Z. Hasan, Science 323, 919 (2009).
2 D. Hsieh, Y. Xia, D. Qian, L. Wray, J. H. Dil, F. Meier, J. Osterwalder, L. Patthey, J. G. Checkelsky, N. P. Ong, A. V.
Fedorov, H. Lin, A. Bansil, D. Grauer, Y. S. Hor, R. J. Cava, and M. Z. Hasan, Nature 460, 1101 (2009).
3 C. Jozwiak, Y. L. Chen, A. V. Fedorov, J. G. Analytis, C. R. Rotundu, A. K. Schmid, J. D. Denlinger, Y.-D. Chuang, D.-H.
Lee, I. R. Fisher, R. J. Birgeneau, Z.-X. Shen, Z. Hussain, and A. Lanzara, Phys. Rev. B 84, 165113 (2011).
4 Z.-H. Pan, E. Vescovo, A. V. Fedorov, G. D. Gu, and T. Valla, Phys. Rev. B 88, 041101(R) (2013).
5 B. Yan, B. Stadtmuller, N. Haag, S. Jakobs, J. Seidel, D. Jungkenn, S. Mathias, M. Cinchetti, M. Aeschlimann, and C.
Felser, Nat. Commun. 6, 10167 (2015).
6 Peng Zhang, J.-Z. Ma, Y. Ishida, L.-X. Zhao, Q.-N. Xu, B.-Q. Lv, K. Yaji, G.-F. Chen, H.-M. Weng, X. Dai, Z. Fang,
X.-Q. Chen, L. Fu, T. Qian, H. Ding, and S. Shin, Phys. Rev. Lett. 118, 046802 (2017).
7 B. Q. Lv et al., Phys. Rev. Lett. 115, 217601 (2015).
8 S. Y. Xu et al., Phys. Rev. Lett. 116, 096801 (2016).
9 R. Xu, A. Husmann, T. F. Rosenbaum, M.-L. Saboungi, J. E. Enderby, and P. B. Littlewood, Nature 390, 57 (1997).
10 M. Lee, T. F. Rosenbaum, M.-L. Saboungi, and H. S. Schnyders,Phys. Rev. Lett.88, 066602 (2002).
11 F. Y. Yang, K. Liu, K. Hong, D. H. Reich, P. C. Searson, and C. L. Chein, Science 284, 1335 (1999).
12 F. Y. Yang, K. Liu, K. Hong, D. H. Reich, P. C. Searson, C. L. Chien, Y. Leprince-Wang, K. Yu-Zhang, and K. Han,
Phys. Rev. B 61, 6631 (2000).
13 D.-X. Qu, Y. S. Hor, J. Xiong, R. J. Cava, and N. P. Ong, Science 329, 821 (2010).
14 T. Liang, Q. Gibson, M. N. Ali, M. Liu, R. J. Cava, and N. P. Ong, Nat. Mater.14, 280 (2015).
15 M. N. Ali, J. Xiong, S. Flynn, J. Tao, Q. D. Gibson, L. M. Schoop, T. Liang, N. Haldolaarachchige, M. Hirschberger, N.
P. Ong, and R. J. Cava, Nature 514, 205 (2014).
16 K. Wang, D. Graf, L. Li, L. Wang, and C. Petrovic, Sci. Rep. 4, 7328 (2014).
17 I. Pletikosić, Mazhar N. Ali, A. V. Fedorov, R. J. Cava, and T. Valla, Phys. Rev. Lett. 113,216601 (2014).
18 J. Jiang, F. Tang, X. C. Pan, H. M. Liu, X. H. Niu, Y. X. Wang, D. F. Xu, H. F. Yang, B. P. Xie, F. Q. Song, P. Dudin, T.
K. Kim, M. Hoesch, P. Kumar Das, I. Vobornik, X. G. Wan, and D. L. Feng, Phys. Rev. Lett. 115,166601 (2015).
19 Y. Wu , N. H. Jo, M. Ochi, L. Huang, D. Mou, S. L. Bud’ko, P. C. Canfield, N. Trivedi, R. Arita, and A. Kaminski, Phys.
Rev. Lett. 115,166602 (2015).

- 70 -

16AG044

BL-9A

Angle-Resolved Photoemission Studyof SnSe and Na-doped SnSeM. Maedaa, S. Suzukia, K. Yamamotoa, T. Mizokawaa, N. L. Sainib, M. Aritac,
H. Namatamec, M. Taniguchic, G. Tand, L. D. Zhaod, and M. G. KanatzidisdaDepartment of Applied Physics, Waseda University, Shinjuku, Tokyo 169-8555, Japanb
Department of Physics, University of Roma "La Sapienza", 00185 Roma, Italyc
Hiroshima Synchrotron Radiation Center, Hiroshima University,
Higashi-hiroshima, Hiroshima 739-0046, Japand
Department of Chemistry, Northwestern University, Evanston, IL 60208, USA

Keywords: ARPES, doped semiconductor, thermoelectric material, Seebeck coefficient

INTRODUCTION
The recent discovery of ultrahigh thermoelectric performance in hole-doped SnSe has shown that bandengineered SnSe is one of the key materials for the future thermoelectric devices [1]. The performace isdetermined by the dimensionless figure of merit ZT:
𝑆 2𝜎
𝑍𝑇 =
𝑇
𝜅
where 𝑆, 𝜎, 𝜅 and 𝑇 are the Seebeck coefficient, the electrical conductivity, the thermal conductivity
and the temperature. SnSe has a layered structure constructed with strong bonds along the b-c plane andweak bonds along the a-axis as shown in Figure 1(a) [2]. SnSe commonly exhibit high thermoelectricperformance with the large Seebeck coefficient due to the multiple valence band maxima [4]. The Seebeck
effect of SnSe is enhanced by the smaller energy separation between the multiple valence band maxima [5].
Although band structure calculations on SnSe are consistent with the transport properties [6-9], it is rather
difficult to estimate the effect of Na doping from the theoretical calculation. The Na doping is expected to
introduce atomic and electronic disorder to the system in a complicated manner which should be studied
using spectroscopic experiments.

EXPERIMENTE
Single crystals of SnSe and Sn0.985Na0.015Se were grown as reported in the literature [1]. ARPES
measurements were done at beamline 9A of HiSOR. The crystals were cleaved at 300 K under theultrahigh vacuum of 10-9 Pa range and then gradually cooled to 20 K for the ARPES measurements at
various temperatures. The ARPES data were obtained within 12 hours after the cleavage. The excitationenergy was set to 22.8 eV in order to observe the valence band top of the surface including  point.
Binding energies were calibrated using the Fermi edge of gold reference samples. The total energy
resolution was 22 meV.
RESULTS & DISCUSSION
In the ARPES measurement for Na-doped SnSe, multi hole pockets are observed around the Fermi
level as shown in Figure 1(b). The valence band maximum touches the Fermi level around the Z point[Figure 1(c)]. The band structure of SnSe and Na-doped SnSe is basically consistent with the band
structure calculations as reported elsewhere [10]. There are recent ARPES works on SnSe by Wang et al.
[11] and Pletikosić et al. [12]. In the present work, the energy position of the valence band maximum is ~200 meV below the Fermi level. On the other hand, it is located at the Fermi level in the work by Wang etal. [11] indicating that the SnSe crystals by Wang et al. are hole doped due to some defects. In the study by
Pletikosić et al. [12], it is located ~ 120 meV below the Fermi level which is slightly smaller than thepresent result. After confirming the band structure, we performed ARPES for SnSe and Na-doped SnSe
with changing temperature. From changing temperature, the chemical potential shift of SnSe and Na-doped- 71 -

SnSe are estimated. The large chemical potential shift of SnSe is consistent with that of undoped
semiconductors. On the other hand, the chemical potential shift of Na-doped SnSe is also large comparedto that expected for degenerate semiconductors. Although the observed band structure of Na-doped SnSe isconsistent with the electrical conductivity expected for a degenerate semiconductor, the large chemical
potential shift and the large Seebeck coefficient of Na-doped SnSe indicate that the carriers in Na-doped
SnSe have localized character.

FIGURE 1. (a) Crystal structure of SnSe created by VESTA[3]. (b) Mapping of the ARPES intensity at the valence ban
maximum in the BZ of Na-doped SnSe. (c) Energy versus momentum maps along the -Z direction (kz cut) for Na-doped
SnSe.

CONCLUSION
In conclusion, the present ARPES study has revealed that the valence band top of SnSe reaches the
Fermi level by the Na doping, and that Na-doped SnSe can be viewed as a degenerate semiconductor. Thisbehavior is consistent with the canonical hole doping in semiconductors and with the electricalconductivity of Na-doped SnSe. However, in Na-doped SnSe, the chemical potential shift with temperatureis unexpectedly large, inconsistent with the degenerate semiconductor picture. The large chemical potential
shift suggests that the holes in Na-doped SnSe cannot be described by a simple itinerant picture or by asimple impurity level picture.

REFERENCES
1. L.-D. Zhao, G. Tan, S. Hao, J. He, Y. Pei, H. Chi, H. Wang, S. Gong, H. Xu, V. P. Dravid, C. Uher, G. J. Snyder, C.
Wolverton, and M. G. Kanatzidis, Science 351, 141 (2016).
2. T. Chattopadhyay, J. Pannetier, and H. G. Von Sshnering, J. Phys. Chem. Solids 47, 879 (1986).
3. K. Momma and F. Izumi, J. Appl. Crystallogr., 44, 1272 (2011).
4. C. W. Li, J. Hong, A. F. May, D. Bansal, S. Chi, T. Hong, G. Ehlers, and O. Delaire, Nat. Phys. 11, 1063 (2015).
5. L.-D. Zhao, S.-H. Lo, Y. Zhang, H. Sun, G. Tan, C. Uher, C. Wolverton, V. P. Dravid, and M. G. Kanatzidis, Nature 508,
373 (2014).
6. R. Car, G. Ciucci, and L. Quartapelle, Phys. Sat. Sol. 86, 471 (1978).
7. L. Makinistian and E. A. Albanesi, Phys. Status Solidi B 246, 183 (2009).
8. K. Kutorasinski, B. Wiendlocha, S. Kaprzyk, and J. Tobola, Phys. Rev. B 91, 205201 (2015).
9. G. Shi and E. Kioupakis, J. App. Phys. 117, 065103 (2015).
10. M. Maeda, K. Yamamoto, T. Mizokawa, N. L. Saini, M. Arita, H. Namatame, M. Taniguchi, G. Tan, L. D. Zhao and M.
G. Kanatzidis, submiited to Phys. Rev. B.
11. Z.Wang, C. Fan, Z. Shen, C. Hua, Q. Hu, F. Sheng, Y. Lu, H. Fang, Z. Qin, J. Lu, Z.-A. Xu, D. W. Shen, and Y. Zheng,
arXiv:1706.10054v1.
12. I. Pletikosić, F. von Rohr, P. Pervan, P. K. Das, I. Vobornik, R. J. Cava, and T. Valla, arXiv:1707.04289v1.

- 72 -

16AG046

ARPES/BL-9A

Photon energy dependence of photoelectron intensity
angular distribution of topological surface states
Daiki Matsumarua, Kenta Kurodaa, Jiahua Chengb, Akio Kimurab,
Masahi Aritac, Shik Shina, and Takeshi KondoaaISSP, University of Tokyo, Kashiwa, Chiba 277-8581, Japan
Graduate School of Science, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima 7398526, Japanc
Hiroshima Synchrotron Radiation Center, Hiroshima University, Higashi-Hiroshima, 739-0046,
Japanb

Keywords: Topological insulator, Spin-orbit interaction, Surface-state
On the surface of solids, space inversion symmetry breaking
combined with strong spin-orbit (SO) coupling lifts the spindegeneracy. In the past decade, such spin-polarized states have been
extensively investigated for Rashba surface states and topologicalsurfaces states. In the conventional model to describe these states,
the eigenstates split by the SO-interaction are treated as pure spinstates, which are fully helical spin-polarized, thus protected frombackscattering. However, this assumption is generally broken in realmaterials because the SO coupling can mix different orbital states Figure 1. Sketch of the spinwith orthogonal spinors to generate an eigenstate of quasiparticles. orbit coupling state in DiracTherefore, it is essentially important to experimentally explore the cone-like topological surface
state
(in
plene
orbitalSO coupling effect not only on the lifting of spin degeneracy but also componets) [1].
on the orbital wave-functions as eigenstates. Recently, some groups
have reported, by using angle-resolved photoemission spectroscopy(ARPES) at 37-67 eV, evidence for this effect of SO coupling and its strong momentum dependence
in a topological insulator Bi2Se3 [1]. In particular, the momentum dependence of the orbital
components was deduced from the obtained photoelectron intensity angular distributions (PIAD),
which was consistent with theoretical predictions (Fig. 1) [2].
In this study, we have performed ARPES with low-energy photons ranging over 6.4-21 eV at
BL-9A of HiSOR, and directly mapped PIAD in the two topological insulators, Bi2Se3 and TlBiSe2.
We have obtained two important results: One is that the PIAD measured for the two materials at
higher photon energies share the similar fearture which shows a good agreement with theprevisouly reported orbital texture [1]. Second is that the PIAD, however, exhibits a significant
photon energy dependence when the low photon energies below 17 eV are used. This dramatic
photoen energy dependence suggests that complex matrix element effect contributes to the PIAD
pariculary for the measurements at low photon energies.
Figure 2 presents the PIAD patterns of the topological surface states in Bi2Se3 (a1)-(a3) and TlBiSe2(b1)-(b3) obtained at various photon energies of s-polarized light. In Bi2Se3, we detect large
intensties of photoelectrons around (kx, ky)=(±kF, 0) at 17 eV, while the intensity gets very weakaround (0, ±kF) [see Fig. 2 (a3)]. This PIAD pattern is consistent with that in the previous works- 73 -

Figure 2. The PIAD patterns of the topogological surface states in (a1)-(a3) Bi2Se3 and (b1)-(b3) TlBiSe2.
These patterns were obtained by various photon energies and s-polaized light indicated by arrows.

[1], which attribute it to the momentum dependence of orbital component, so-called orbital-texture[2]. We observe the similar PIAD pattern in TlBiSe2 at 17 eV [see Fig. 2 (b3)], and also no photon
energy dependence at higer photon energies. On the basis of these resutls, we conculde that the SO
coupling in these two materials establishes the smilar orbital texture.
Interestingly, we find that the PIAD pattern drastically changes in both materials when lower
photon energies are used [Fig. 2(a1)-(a2) and (b1)-(b2)]. For example, in Bi2Se3, we see thedifferent PIAD pattern at 11.5eV [see Fig. 2(a2)], in which the photoelectron itensityies get large
around (0, ±kF), but weaker around (±kF, 0) with a sharp contrast to the PIAD for 17 eV [Fig. 2(a3)].
At lower photon energies, the PIAD pattern is, however, found to show back to the same as that for
17 eV. Furtheremore, we find the similar photon energy dependence of the PIAD pattern in TlBiSe2,[Fig. 2(b1)-(b2)]. Thease resutls indicate that the PIAD pattern can be changed by the photon
energy paricularly less than 17 eV, probablly due to the matrix element effect. To unravel the originof these PIAD patterns and their photon enery dependence, further systematic investigations
combined with one-step theorey are strongly required.

REFERENCES
1. Y. Cao et al., Nat. Phys. 9, 499-504 (2013).
2. H. Zhang et al., Phys. Rev. Lett. 111, 066801 (2013).

- 74 -

16BG020

BL-9A/BL-9A

Temperature dependence of the pseudogap in
Bi2Sr2Ca2Cu3O10+δ studied by angle-resolved
photoemission spectroscopy.
S. Kunisadaa, S. Adachib, M. Nakayamaa, S. Akebia, M. Aritac, T. Watanabeb,
and T. Kondoaa

The Institute for Solid State Physics, Tokyo University, Kashiwa 277-8581, Japan
Graduate School of Science and Technology, Hirosaki University, Hirosaki 036-8561, Japanc
Hiroshima Synchrotron Radiation Center, Hiroshima University, Higashi-Hiroshima 739-0046,
Japanb

Keywords: copper oxide high-Tc superconductor, ARPES
While a huge effort has been made to elucidate the mechanism of high-transition-temperature(high-Tc) superconductivity in cuprates, the consensus on it has not reached yet. These materials
significantly change physical properties with carrier doping. In particular, one of the mostfascinating features in cuprates is pseudogap state, which appears in the underdoped regime. The
pseudogap develops around (π,0), and angle-resolved photoemission spectroscopy (ARPES)
observes arc-like intensities of photoelectrons around the gap node. The understanding of thisunique phenomenon is a hot topic in this field. There are manly two notions about the role of thepseudgap state on the superconductivity: One is that it is a precursory phenomenon ofsuperconducting state, and the other is that it is rather competing against superconducting state. Itis crucial to fully understand the relationship between the psugogap and superconductivity toelucidate the mechanism of high-Tc superconductivity in cuprates.
In this study, we have performed ARPES measurements to investigate the temperaturedependence of energy gaps (superconducting gap and pseudogap) in Bi2Sr2Ca2Cu3O10+δ (Bi2223)
with triple layers of CuO2 in unit cell. Previous report on the same compound measured theantinodal spectra by using He I𝛼 (21.218 eV) in the temperature range from 40K to 170K [1].
After this report, it was revealed that Bi2223 has actually two bands with different magnitudes ofsuperconducting gap by using low photon energies (7-11eV) [2]. This anomaly in the trilayer
system has been attributed to non-uniform distribution of doped carriers over the CuO2 layers. It
is therefore significant to investigate the behavior of superconducting gap and pseudogap atelevated temperatures for each band in Bi2223 to understand the nature of these gaps.
We have carried out ARPES measurements at BL-9A of Hiroshima Synchrotron Radiation
center. The samples of single crystals were cleaved in situ to prepare a clean surface required toobtain sharp spectra. The photon energy used was ℎ𝜈 = 22 𝑒𝑉, which selectively observe theband with smaller superconducting gap. Energy resolution was set to 14 meV in our
measurements.
The physical properties we focus on is the loss of spectral weight at the Fermi level (EF) due
to a gap opening [arrows in (c)]. Before extracting these values, the original spectra obtained aresymmetrized about the Fermi level to eliminate the effect of Fermi cut-off near EF. Moreover,each curve is normalized to its area in order to investigate the intrinsic temperature variation of
the spectral weight. We found that the estimated values become constant at high temperatures,signifying that there is no gap at these temperatures. Our important finding is that a decrease of- 75 -

(1,1)

ky / π(a)0.0kx / π
(b)

(c)

-0.1
-0.2
-0.3
-0.2 0.0 0.2
k/π

6K
Tc
300K

-0.3 -0.2 -0.1 0.0 0.1 0.2 0.3
Energy (eV)(d)

EF Weight(a.u.)

Energy (eV)(0,0)

Intensity (arb. units)

the spectral weight at EF due to a gap opening starts round 250K upon cooling. This result is
consistent to the pseudogap temperature estimated from the resistivity behavior of Bi2223 [3].

T*Tc
50 100 150 200 250 300
Temperature (K)Fig. 1. (a) Schematic Fermi surfaces in Bi2223. Red and blue curves indicate two different bands.
(b) ARPES dispersions measured in the direction marked by an arrow in (a). (c) Temperaturescan of EDCs at a kF pointed by an arrow in (b) over a wide range from 6K up to 300K. The
original spectra obtained are symmetrized about EF to eliminate the Fermi cut-off. (d)Temperature variation of the spectral loss at the Fermi level [arrow in (c)].
In summary, we report ARPES studies on temperature dependence of Bi2223 around the antinode.
We have investigated the temperature evolution of pseudogap at ℎ𝜈 = 22 𝑒𝑉, and obtained a
pseudogap temperature higher than that in the previous report mostly owing to a precise
estimation of it using detailed spectral weight. For the next step, we will investigate the otherband with a higher superconducting gap by tuning photon energies especially around low
energies (8-10 eV) to fully understand the nature of pseudogap state in Bi2223.

REFERENCES
1. T. Sato et al., Phys. Rev. Lett. 89, 067005 (2002).
2. S. Ideta et al., Phys. Rev. Lett. 104, 227001 (2010).
3. S. Adachi et al., J. Phys. Soc. Jpn. 84, 024706 (2015).

- 76 -

16BG025

BL-9A

Self-energy in the superconducting states of
the high-Tc cuprates Bi2Sr2CaCu2O8+
T. Yoshidaa, D. Ootsukia, K. Yamawakia, D. Shimonakaa, D. Shibataa,
H. Eisakib, T. Sasagawac, A. Fujimorid, M. Aritae, H. Namatamee, M. TaniguchieaGraduate School of Human and Environmental Studies, Kyoto University, Sakyo-ku, Kyoto 606-8501,
Japan.
b
National Institute of Advanced Industrial Scienceand Technology (AIST), Tsukuba305-8562, Japanc
Materials and Structures Laboratory, Tokyo institute of Technology, Yokohama, Kanagawa 2268503, Japand
Department of Physics, University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japane
Hiroshima Synchrotron Radiation Center, Hiroshima University, Higashi-Hiroshima 739-0046,
Japan
Keywords: high-Tc cuprates, Cooper pair, self-energy

While the energy scales of phonons appear in the superconducting gap function () in conventional
BCS superconductors, the() of the high-Tc cuprate superconductors has not been investigated well by
experimental studies because of the complexity in the d-wave superconducting gap. In previous angleresolved photoemission (ARPES) studies, self-energy analyses have been done mostly in the nodal
direction where the superconducting gap closes. In order to approach the information of the Cooper paring,
the self-energy in the anti-nodal direction in the superconducting state is important. Several ARPES studies
have reported the self-energy for the spectra with superconducting gap [1,2] and pointed out that there isbosonic structure in the self-energy. However, the origin of the bosonic mode has not been clarified yet.
Recently, the analysis of the two-dimensional Hubbard model calculated with the cellular dynamical
mean-field theory (CDMFT) have shown that the energy scale of the pole in the normal and anomalous
self-energies does not appear in the spectral function, while they have a pole at the same energy [3,4]. Thisis because the pole in the both self-energy cancels out each other in the total self-energy. The origin of thepole has been interpreted as composite fermions.
In the present study, we have performed ARPES study of the optimally doped Bi2Sr2CaCu2O8+
(Bi2212) (Tc = 91 K) and have extracted the self-energy from the ARPES spectra. As shown in Fig. 1(a),the ARPES spectra in the off-nodal direction are symmetrized by assuming electron-hole symmetry. By
applying the Kramers-Kronig transformation, we have obtained the total self-energy along the cut direction
as shown in Figs. 1(b)(c). Here, bare electron band dispersion k is assumed as indicated by red line inpanel (a). In addition to ~70 meV kink structure, clear dispersive structure appears near the EF.
Total self-energy tot(k,) in superconducting states is given by following formulawhere nor() and ano() are normal and anomalous self-energy, respectively. In Figs. 1(b) and 1(c), thetotal self-energy shows a sharp and clear dispersive structure near the EF as indicated by light blue dotted- 77 -

lines. This structure is originated from the pole in the second term of the formula mentioned above andindicates a renormalized “hole” band near the EF. Such a strong renormalization has been pointed out inprevious self-energy analysis in the superconducting states [5] and may be related to small kink in the
nodal direction [6]. While the renormalization near EF has been interpreted as the effect of impurity
scattering [5], the origin of the small kink in the nodal direction has been interpreted as phonon mode [6].
Such a low-energy mode appears in the superconducting states has important implications for the origin of
the high-Tc superconductivity.

FIGURE 1. ARPES spectra of Bi2212 in the off-nodal direction taken at T = 10 K and total self-energy. (a) Symmetrized
ARPES spectra corresponding to spectral function A(k,). (b) (c) Real and imaginary part of the total self-energy deduced
from A(k,) by applying Kramers Kronig transformation.

REFERENCES[1] M. R. Norman et al. Phys. Rev. B 60, 7585 (1999).
[2] J. M. Bok et al., Sci. Adv. 2, e1501329 (2016).
[3] S. Sakai, M. Civelli, and M. Imada, Phys. Rev. Lett. 116, 057003 (2016).
[4] S. Sakai, M. Civelli, and M. Imada, Phys. Rev. B 94, 115130 (2016).
[5] S. H. Hong et al., Phys. Rev. Lett. 113, 057001 (2014).
[6] J. D. Rameau et al., Phys. Rev. B 80, 184513 (2009).

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