Research Activities– Synchrotron Radiation Experiments –BL-1
BL-01

17AG055

Unveiling orbital characteristics of surface states in HfSiS
X. X. Wanga, J. H. Chena, M. T. Zhenga, M. Novakb, F. Orbanićb,
E. F. Schwierc, K. Sumidaa, T. Yoshikawaa, M. Nurmamata, K. Shimadac,and A. KimuraaaGraduate School of Science, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima 7398526, Japanb
Department of Physics, Faculty of Science, University of Zagreb, Bijenička 32, 10000 Zagreb,
Croatiac
Hiroshima Synchrotron Radiation Center, Hiroshima University, 2-313 Kagamiyama, HigashiHiroshima 739-0046, Japan

Keywords: orbital characteristics, Rashba-type surface state.

Recently the non-symmorphic Dirac nodal-line semimetals WHM (W = Zr, Hf, La; H = Si, Ge, Sn,
and Sb; M = S, Se, Te, O) have been theoretically and experimentally investigated. HfSiS is the mostrepresentative compound because of the emergence of spin-split surface states (SSs) due to a strong
spin-orbit coupling at the 𝑋𝑋� point with C2v symmetry near the Fermi level [1, 2]. Thus, HfSiS could bea good candidate to realize exotic orbital and spin texture due to spin orbital fields on the surfaces.
However, until now, there has been no systematic investigations on the orbital configuration of thesesurface states.
We have directly investigated the orbital configurations of the surface-derived states in HfSiS bymeans of angle-resolved photoelectron spectroscopy (ARPES) employing variable linear-polarized
light at HiSOR BL-1 as well as theoretical calculations within density functional theory. To observe
electronic states in detail we study the Fermi surface map (FSM) of HfSiS using the p- and s-polarizationgeometries. The prominent feature in the FSM is the olive-shaped electron pocket around the 𝑋𝑋� point[marked by the green rectangle in Fig. 1(i)]. Considering the experimental geometry and the matrix
elements for the dipole transition, one can observe initial-state orbitals having even symmetry withrespect to the kx-kz mirror plane (𝑝𝑝� , 𝑝𝑝� , 𝑑𝑑�� , 𝑑𝑑� � �� � , 𝑑𝑑�� ) in the p-polarization geometry, whereas
only initial-states having odd symmetry are excited (𝑝𝑝� , 𝑑𝑑�� , 𝑑𝑑�� ) in the s-polarization geometry. As
seen from Fig. 1, the shape and distribution of the spectral weight in FSM around the 𝑋𝑋�-point are
significantly different for the p- and s-polarization geometries indicating clearly directional alternationof the orbital symmetry [3].
The calculated Fermi surface spectral function for the p- and s-polarization geometries [Fig. 1 (ii)and (iv)] matches very well the experimental results. Our analysis shows that the Hf-derived 𝑑𝑑� � �� � ,𝑑𝑑�� , and 𝑑𝑑�� orbitals contribute to the spectral weight mainly along the 𝛤𝛤� 𝑋𝑋�-line when the p-polarized
geometry is used. In contrast, the data acquired with the s-polarization geometry [Fig. 1 (iii)] clearlyhighlights additional bands being part of the SSs with the odd 𝑝𝑝� , 𝑑𝑑�� and 𝑑𝑑�� orbitals [3].

－ 13 －

REFERENCES[1] D. Takane, Z. W. Wang, S. Souma, K. Nakayama, C. X. Trang, T. Sato, T. Takahashi, and Y. Ando, Phys.
Rev. B 94, 121108(R) (2016).
[2] R. Lou, J.Z. Ma, Q.N. Xu, B.B. Fu, L.Y. Kong, Y.G. Shi, P. Richard, H.M. Weng, Z. Fang, S.S. Sun, Q. Wang,
H.C. Lei, T. Qian, H. Ding, and S.C. Wang, Phys. Rev. B 93, 241104(R) (2016).
[3] X. X. Wang, J. H. Chen, M. T. Zheng, T. V. Menshchikova, I. P. Rusinov, E. F. Schwier, F. Orbanić, S. L.
Wu, K. Sumida, T. Yoshikawa, K. Miyamoto, M. Nurmamat, T. Okuda, K. Shimada, M. Novak, E. V. Chulkov
and A. Kimura, Phys. Rev. B 100, 205140 (2019).

FIG. 1 Experimental Fermi surface measured at hν=60 eV in p-polarization geometry (i) and s-polarizationgeometry (iii). (ii) and (iv) Corresponding calculated Fermi surfaces for the p- and s-polarization geometries,
respectively.

－ 14 －

BL-1
BL-01

17AG056

Effect of spin-orbit coupling on bulk bandsof the non-symmorphic semimetal Hf1-xZrxSiS
X. X. Wanga, J. H. Chena, M. T. Zhenga, M. Novakb, F. Orbanićb,
E. F. Schwierc, K. Sumidaa, T. Yoshikawaa, M. Nurmamata, K. Shimadac,and A. KimuraaaGraduate School of Science, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima 7398526, Japanb
Department of Physics, Faculty of Science, University of Zagreb, Bijenička 32, 10000 Zagreb,
Croatiac
Hiroshima Synchrotron Radiation Center, Hiroshima University, 2-313 Kagamiyama, HigashiHiroshima 739-0046, Japan

Keywords: non-symmorphic semimetals, spin-orbit coupling (SOC), SOC induced energy gap

In recent years, non-symmorphic semimetals MSiS (M=Zr, Hf) has flourished because of the
discovery of many materials that exhibit highly exotic physical properties that dominate the transport
properties with extremely high carrier mobility [1, 2]. They share the same crystal structure but with adifferent spin-orbit coupling (SOC) strength. It is noted that the exotic state is governed by the electronic
structures originated from the crystal symmetry with variable SOC strength [3-6]. However, thesystematic studies of the electronic structures affected by the effect of SOC have yet to be reported forMSiS systems.
In this study, we systematically studied the bulk electronic states of Hf1-xZrxSiS using high-resolution
ARPES. VUV-ARPES measurement with p-polarized light with a photon energy (h) of 60 eV was
performed at BL-1 of HiSOR. The typical energy and momentum resolutions were set at 10 meV and
0.1°, respectively. To understand the effect of SOC on the electronic structures, we begin thesystematical analysis of the band structures with changing the x value in Hf1-xZrxSiS. The VUV-ARPES� 𝛤𝛤� 𝑀𝑀� -line are shown in Figs. 1(a)-1(c). A
spectra with p-polarized light of Hf1-xZrxSiS along the 𝑀𝑀� 𝛤𝛤� 𝑀𝑀� -line, where the gap size is the
hybridization gap opens at the band crossing point halfway of the 𝑀𝑀largest in HfSiS [~90 meV, see Fig. 1 (a)] than those in Hf0.6Zr0.4SiS [~70meV, see Fig. 1(b)] and ZrSiS,[~30 meV, see Fig. 1 (c)], indicating that we can quantitatively adjust the strength of the spin-orbit
coupling by controlling the x composition in Hf1-xZrxSiS.
REFERENCES[1] R. Singha, A. K. Pariari, B. Satpati, and P. Mandal, Proceedings of the National Academy of Sciences 114,
2468 (2017).
[2] M. Novak, S. N. Zhang, F. Orbanic, N. Biliskov, G. Eguchi, S. Paschen, A. Kimura, X. X. Wang, T. Osada,
K. Uchida,7 M. Sato, Q. S. Wu, O. V. Yazyev, and I. Kokanovic, Phys. Rev. B 100, 085137 (2019).
[3] M. Neupane, I. Belopolski, M. M. Hosen, D. S. Sanchez, R. Sankar, M. Szlawska, S.Y. Xu, K. Dimitri, N.
Dhakal, P. Maldonado, P. M. Oppeneer, D. Kaczorowski, F. Chou, M. Z. Hasan, and T. Durakiewicz, Phys.
Rev. B 93, 201104 (2016).

－ 15 －

[4] L. M. Schoop, M. N. Ali, C. Straßer, A. Topp, A. Varykhalov, D. Marchenko, V. Duppel, S. S. P. Parkin, B.
V. Lotsch, and C. R. Ast, Nat. Commun.7, 11696 (2016).
[5] Q. D. Gibson, L. M. Schoop, L. Muechler, L. S. Xie, M. Hirschberger, N. P. Ong, R. Car, and R. J. Cava, Phys.
Rev. B 91, 205128 (2015).
[6] I. Zeljkovic, Y. Okada, C. Y. Huang, R. Sankar, D. Walkup, W. W. Zhou, M. Serbyn, F. C. Chou, W. F. Tsai,
H. Lin, A. Bansil , L. Fu, M. Z. Hasan and V. Madhavan, Nat. Phys. 10, 572 (2014).

� 𝛤𝛤�𝑀𝑀� -line acquired with p-polarized light at h=60 eV
FIG. 1 VUV-ARPES dispersion intensity maps along the 𝑀𝑀of (a) HfSiS, (b) Hf0.6Zr0.4SiS, and (c) ZrSiS.

－ 16 －

BL-1
BL-01

17BG029

Effect of spin-orbit coupling on surface statesof the non-symmorphic semimetal Hf1-xZrxSiS
X. X. Wanga, J. H. Chena, M. T. Zhenga, M. Novakb, F. Orbanićb,
E. F. Schwierc, K. Sumidaa, T. Yoshikawaa, M. Nurmamata, K. Shimadac,and A. KimuraaaGraduate School of Science, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima 7398526, Japanb
Department of Physics, Faculty of Science, University of Zagreb, Bijenička 32, 10000 Zagreb,
Croatiac
Hiroshima Synchrotron Radiation Center, Hiroshima University, 2-313 Kagamiyama, HigashiHiroshima 739-0046, Japan

Keywords: non-symmorphic semimetals, spin-orbit coupling (SOC), Rashba-type spin splitting

In recent years, non-symmorphic semimetals MSiS (M=Zr, Hf) has flourished because of the
discovery of many materials that exhibit highly exotic physical properties which dominate the transport
properties with extremely high carrier mobility [1, 2]. They share the same crystal structure but with adifferent spin-orbit coupling (SOC) strength. It is noted that the exotic state is governed by the electronic
structures originated from the crystal symmetry with variable SOC strength [3-6]. However, thesystematic studies of the electronic structures affected by the effect of SOC have yet to be reported forMSiS systems. We systematically studied the surface electronic states of Hf1-xZrxSiS using highresolution ARPES.
VUV-ARPES measurement with p-polarized light with a photon energy (h) of 60 eV was
performed at BL-1 of HiSOR. The typical energy and momentum resolutions were set at 10 meV and
0.1°, respectively. The SOC also plays an important role in causing surface states splitting. The VUV� 𝑋𝑋�𝑀𝑀� -line show a ‘V’ shaped band dispersion at a photon
ARPES band intensity maps along the 𝑀𝑀energy of 60 eV in Fig. 1(a). These bands are assigned to the surface states (SSs) since they are locatedinside the gap of the projected bulk-band. The SSs around the 𝑋𝑋� points are formed by distinctly
separated bands in HfSiS as well as in Hf0.6Zr0.4SiS [see Fig. 1(a)]. To further evaluate the size of the
splitting, we extracted the momentum distribution curves (MDCs). The ARPES energy band dispersions
� 𝑋𝑋�𝑀𝑀� -line exhibits a much smaller or no splitting as shown in Fig. 1(a) inof ZrSiS along the 𝑀𝑀comparison with those of HfSiS and Hf0.6Zr0.4SiS. The corresponding MDC shows almost no splitting[Fig. 1(b)]. This certainly stems from the lighter atomic mass and thus weaker SOC of Zr compared tothat of Hf.
REFERENCES[1] R. Singha, A. K. Pariari, B. Satpati, and P. Mandal, Proceedings of the National Academy of Sciences 114,
2468 (2017).

－ 17 －

[2] M. Novak, S. N. Zhang, F. Orbanic, N. Biliskov, G. Eguchi, S. Paschen, A. Kimura, X. X. Wang, T. Osada,
K. Uchida,7 M. Sato, Q. S. Wu, O. V. Yazyev, and I. Kokanovic, Phys. Rev. B 100, 085137 (2019).
[3] M. Neupane, I. Belopolski, M. M. Hosen, D. S. Sanchez, R. Sankar, M. Szlawska, S.Y. Xu, K. Dimitri, N.
Dhakal, P. Maldonado, P. M. Oppeneer, D. Kaczorowski, F. Chou, M. Z. Hasan, and T. Durakiewicz, Phys.
Rev. B 93, 201104 (2016).
[4] L. M. Schoop, M. N. Ali, C. Straßer, A. Topp, A. Varykhalov, D. Marchenko, V. Duppel, S. S. P. Parkin, B.
V. Lotsch, and C. R. Ast, Nat. Commun.7, 11696 (2016).
[5] Q. D. Gibson, L. M. Schoop, L. Muechler, L. S. Xie, M. Hirschberger, N. P. Ong, R. Car, and R. J. Cava, Phys.
Rev. B 91, 205128 (2015).
[6] I. Zeljkovic, Y. Okada, C. Y. Huang, R. Sankar, D. Walkup, W. W. Zhou, M. Serbyn, F. C. Chou, W. F. Tsai,
H. Lin, A. Bansil , L. Fu, M. Z. Hasan and V. Madhavan, Nat. Phys. 10, 572 (2014).

FIG. 1 (a) ARPES dispersion intensity maps for HfSiS, Hf0.6Zr0.4SiS and ZrSiS with p-polarized light along the� 𝑋𝑋�𝑀𝑀� -line of the surface BZ. (b) The corresponding momentum distribution curve (MDC).
𝑀𝑀－ 18 －

BL-1
BL-01

17BG030

ARPES study of the electronic structure of HfSiS withvariable polarized light
X. X. Wanga, J. H. Chena, M. T. Zhenga, M. Novakb, F. Orbanićb,
E. F. Schwierc, K. Sumidaa, T. Yoshikawaa, M. Nurmamata, K. Shimadac,and A. KimuraaaGraduate School of Science, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima 7398526, Japanb
Department of Physics, Faculty of Science, University of Zagreb, Bijenička 32, 10000 Zagreb,
Croatiac
Hiroshima Synchrotron Radiation Center, Hiroshima University, 2-313 Kagamiyama, HigashiHiroshima 739-0046, Japan

Keywords: non-symmorphic semimetals, spin-orbit coupling (SOC), Rashba-type spin splitting, orbital configuration

VUV-ARPES measurement with s and p-polarized light with a photon energy (h) of 60 eV was
performed at BL-1 of HiSOR for one of the non-symmorphic semimetals HfSiS. To understand theelectron orbital configuration, we focus on the high symmetry lines of HfSiS as shown in Figs.1(a) and� 𝑋𝑋�1(b). We find that the Rashba-like surface states (SSs) with a quasi-linear dispersion along the 𝑀𝑀
line indicated as SS1 [Fig.1 (a)(i)], which is visible with the p-polarization geometry. Its degeneracy
point is located at 320 meV below EF at the 𝑋𝑋�-point. We should note that the Rashba-type split SSs canbe reproduced by ab initio calculations only when the SOC is included (not shown).
Assuming the Rashba term to be the source of this state, we can extract the Rashba coefficient α as~3.125 eV·Å, which is in accordance with previous reports [1]. Based on the quasi-linear Rashba-like
dispersion, the derived Fermi velocity vF can be evaluated as 4.55 eV·Å (6.9×105 m/s), which iscomparable to previous reports on Rashba-type spin-split surface states [1,2]. Under the s-polarizationgeometry, the Rashba-like surface spectral weight is strongly suppressed as shown in Fig.1 (a)(ii),indicating the even orbital character. The p-polarized band dispersion along the 𝛤𝛤� 𝑋𝑋�-line [Fig.1 (b)]shows a quasi-linear Dirac-like band dispersion, with the crossing point located around 500 meV being
protected by the non-symmorphic symmetry. In addition, the M-shaped states along the 𝛤𝛤� 𝑋𝑋�-line around� 𝑋𝑋�-line, which emerge from the bulk states at lower binding
300 meV are connected to SS1 along the 𝑀𝑀energies. In contrast, ARPES result for the s-polarization geometry in Fig.1 (b)(ii) exhibits surface state(SS2) having a steep dispersion forming the terminal arc-like Fermi surface piece of the electron pocket
around the 𝑋𝑋� point (not shown), which shows a discernible spectral weight from the bulk statescompared to the s-polarization, demonstrating that the SS2 is dominated by odd symmetry orbitals.
Meanwhile, the intensity of the bulk states close to Fermi level is well suppressed under the spolarization geometry. We deduce that the 𝑑𝑑𝑑𝑑2−𝑦𝑦2, 𝑑𝑑𝑑𝑑2 and dxz orbitals of Hf make the main
contribution to the spectrum intensity of the body region with p-polarized light. Instead, the dataacquired with s-polarized light [Fig.1 (b)(ii)] clearly indicates the ARPES appreciable intensity at the－ 19 －

two terminals along the (𝛤𝛤� 𝑋𝑋�) line of the olive-shaped electron pocket, while the corresponding intensity
is strongly suppressed for the p-polarized light.
As is expected, the M-shaped SS1 with the even initial states also show almost vanishing spectral
weight under the s-polarized light, which is in accordance with the behavior of the Rashba-like surface
� 𝑋𝑋� line, revealing the real symmetry characteristic of the initial states.
states along the 𝑀𝑀
REFERENCES[1] D. Takane, Z. W. Wang, S. Souma, K. Nakayama, C. X. Trang, T. Sato, T. Takahashi, and Yoichi Ando, Phys.
Rev. B 94, 121108(R) (2016).
[2] L. Jiao, Q. N. Xu, Y. P. Qi, S.-C. Wu, Y. Sun, C. Felser, and S. Wirth, Phys. Rev. B 97, 195137 (2018).

FIG. 1 (a) and (b) Experimental electronic structures of HfSiS recorded by ARPES at hν=60 eV with p- (i) and� 𝑋𝑋� line and 𝛤𝛤�𝑋𝑋� line of the surface BZ, respectively. The black and green dot liness-polarized light (ii) along 𝑀𝑀
are guided for the eyes to trace the bulk and surface bands. Notations c1, c2 and c3 correspond to the measured
ARPES band dispersions.

－ 20 －

18AG017, 18BG010

BL-1

Superconductivity and Fermi-surface nesting in thecandidate Dirac semimetal NbC
D. Ya,1,4,5 D. Geng,1,4 Q. Gao,1,4 Z. Cui,1,4 C. Yi,1,4,5 Y. Feng,2 C. Song,1,4 H.
Luo,1,4 M. Yang,1,4,5 M. Arita,3 S. Kumar,3 E. F. Schwier,3 K. Shimada,3 L.
Zhao,1,4 K. Wu,1,4,5,6 H. Weng,1 L. Chen,1,4,5,6 X. J. Zhou,1,4 Z. Wang,1,4 Y.
Shi,1,4,5,6 and B. Feng,1,4
1Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China
Beijing Academy of Quantum Information Sciences, Beijing 100193, China
3
Hiroshima Synchrotron Radiation Center, Hiroshima University, 2-313 Kagamiyama, Higashi-Hiroshima
739-0046, Japan
4
School of Physical Sciences, University of Chinese Academy of Sciences, Beijing 100049, China
5Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of
Sciences, Beijing 100049, China
6
Songshan Lake Materials Laboratory, Dongguan, Guangdong 523808, China
2

Keywords: superconductivity, Fermi surface nesting, NbC, ARPES, first-principles calculations

Transition metal carbides (TMCs) are a large family of materials that have held industrial research interest
for more than 100 years. In TMCs, carbon atoms are incorporated into the interstitial sites of the parent
metals and form strong metal-carbon bonds [1–3]. This unique structure gives TMCs coexisting ionic,
covalent, and metallic bonds, leading to metallic conductivity combined with excellent mechanicalproperties, including extreme hardness, high melting points, excellent corrosion resistance, and interestingcatalysis properties [1–3]. Therefore, the TMC family of materials has been widely used in a variety of areas,
including cutting tools, energy storage devices, and supercapacitors.
One prototypical TMC material is niobium monocarbide (NbC), which crystallizes into a NaCl-type cubic
structure. NbC has various outstanding aspects: its melting point is one of the highest among all solidmaterials, while its hardness (in the 9–10 range on the Mohs scale) is comparable to that of diamond. Moreinterestingly, NbC is a superconductor and has a Tc of approximately 11 K, which is higher than that of
niobium. Recent theoretical studies have predicted Fermi-surface nesting in NbC, which will enhance theelectron-phonon interactions in the material. Fermi-surface nesting is a subject of intense interest in studies
of high-temperature superconductivity and charge/spin density waves. However, the biggest obstacle to thestudy of NbC is the challenge of synthesizing single-crystal samples. To date, only NbC polycrystals havebeen synthesized and this has strongly limited further experimental studies. Therefore, only a few
experimental works on NbC and NbCx (0 < x < 1) have been performed, although the material was discovered
more than half a century ago.
In this work, we have successfully synthesized single-crystal NbC. Transport and magnetic susceptibility
measurements confirmed that single-crystal NbC is a type-II superconductor that has a Tc of 11.5 K. Angleresolved photoemission spectroscopy (ARPES) measurements provided evidence of Fermi-surface nesting,analogous to some iron-based superconductors and charge density wave systems. First-principles
calculations showed that NbC hosts type-II Dirac cones near the Fermi level, which qualitatively agrees with
our ARPES measurements. We also studied the superconducting gap using high-resolution laser ARPES,
and, interestingly, we found that the behavior of the gap deviates from that of conventional Bardeen-CooperSchrieffer (BCS) theory.

－ 21 －

The crystal structure, Brillouin zone, and XRD characterization of NbC single crystals are shown in Fig.
1(a)-(c), which indicates the high quality of the crystals. From the DFT calculation results in Fig. 1(d) and(e), we found that there is a Type-II Dirac point in NbC, as indicated by the blue cycle.
We then performed ARPES measurements to study the electronic structures of NbC, as shown in Fig. 2.
First, we observed strong Fermi surface nesting effects, as indicated by the red arrows in Fig. 2(a). The
experimental Fermi surface agree well with the DFT calculation results. Second, we confirmed the existenceof Dirac cones and topological surface states. These results show that NbC is an ideal material for exploring
topological superconductivity and Majorana fermions.
This work has been published in Phys. Rev. B 102, 155109 (2020).

FIGURE 1. Growth of single-crystal NbC and calculations on its
electronic structures. (a),(b) Schematic illustrations of the atomic
structure and (001)-projected BZ of NbC, respectively. (c) X-ray
diffraction pattern of the flat surface of the NbC single crystal. The
inset shows a photograph of a typical NbC single crystal. (d)Calculated band structure of NbC, showing that a gap (gray shaded)
exists between the black-colored bands and the red-colored bands,
with the exception of a gapless Dirac point along X. (e) Magnified
view of the band structure in the green shaded region of (d), whichindicates the existence of a type-II Dirac cone. This cone is formedby irreducible representations of G6 and G7 under the double groupof the C4v symmetry on the line.

FIGURE 2. Synchrotron ARPES measurements and theoretical calculations of NbC. (a) ARPES intensity map of
the Fermi surface of NbC measured using 120 eV photons. Solid and dashed lines indicate the bulk and surface
BZs, respectively. Red arrows indicate the possible nesting vectors. (b) ARPES intensity map of the constant energy
contour (CEC) at EB = 0.6 eV measured using 120 eV photons. (c),(d) ARPES intensity plots of band structures
along K and X measured using 120 eV photons. (e),(j) ARPES intensity plots of band structures along K measured
using 20 eV and 60 eV photons, respectively. (f),(g) Calculated bulk bands on the Fermi surface and CEC at EB =0.6 eV. (h),(i) Purple-yellow colored images are semi-infinite calculation results using the Green’s function method
along directions that are parallel to K and X, respectively. The surface states can be determined as sharp yellow
lines. White lines are the calculated bulk bands along K and X. “SS1” and “SS2” indicate two surface states of
NbC.

－ 22 －

REFERENCES[1] S. T. Oyama, The Chemistry of Transition Metal Carbides and Nitrides (Springer, Berlin, 1996).
[2] H. H. Hwu and J. G. Chen, Surface chemistry of transition metal carbides, Chem. Rev. 105, 185 (2005).
[3] Y. Xiao, J.-Y. Hwang, and Y.-K. Sun, Transition metal carbidebased materials: synthesis and applications in electrochemicalenergy storage, J. Mater. Chem. A 4, 10379 (2016).
[4] D. Yan, et al. Superconductivity and Fermi-surface nesting in the candidate Dirac semimetal NbC. Phys. Rev. B 102,
205117 (2020).

－ 23 －

BL-1,
BL-1
and Laser-ARPES

18AG029 and
18BG031
18AG029,
18BG031

Investigations on Angle-resolved Photoemission
Spectroscopy and Pressure-induced Transport Properties
of BiSbTe3 Topological Insulator
Shiv Kumara#, Vinod K. Gangwarb#, Mahima Singhb, Labanya Ghoshb, Yufeng
Zhangc, Prashant Shahid, Matthias Muntwilere, Swapnil Patilb, Kenya Shimadaa,
Yoshiya Uwatokoc, Sandip Chatterjeeb*a

Hiroshima Synchrotron Radiation Center, Hiroshima University, 739-0046 Japan
Department of Physics, Indian Institute of Technology (BHU) Varanasi 221005, IndiacInstitute for Solid State Physics, University of Tokyo, Kashiwa, Chiba 277-8581, Japand
Department of Physics, D.D.U. Gorakhpur University, Gorakhpur 273009, IndiaePaul Scherrer Institut, Villigen, Switzerlandb

Keywords: TopologicalSuperconductivity

Insulator,

Angle-resolved

Photoemission

Spectroscopy(ARPES),

Pressure-induced

Structural, pressure-dependent resistivity and angle resolved photoemission spectroscopy (ARPES) havebeen investigated for BiSbTe3. It has been demonstrated that the Dirac point of the topological surface state(TSS) located exactly at the Fermi level. Additionally, superconductivity emerges under pressure of 8 GPawith a critical temperature of ~2.5 K. With further increase of pressure, the superconducting transition
temperature (Tc) increases and at 14 GPa it shows the maximum Tc (~3.3 K). The investigation indicatesthat the BiSbTe3 has robust surface states and becomes superconductor under pressure.
In case of n-type Bi2Te3, the Fermi level (EF) lies in the bulk conduction band (BCB). On the other hand,the EF of the Sb2Te3 TI lies in the bulk valence band (BVB) locating the Dirac point (DP) above EF. The
position of the DP and EF may be tuned simultaneously by mixing of these two compounds. This may leadto an ideal TI in which EF and the DP coincides in the insulating bulk band gap, which paves the way toapply topological surface states for future devices. The reported stoichiometric of Bi/Sb for the transition
from n to p-type in (Bi1−xSbx)2Te3 is debatable [1–5].
In this work, we have successfully synthesized a single crystal of BiSbTe3, and found emergence of thesuperconducting states under pressure. We have revealed that the DP of topological surface state (TSS) is
located at EF and observed the hole-like bulk conductivity due to crossing of BVB to the EF.
Experimental
FittedYobs-YCal(c)
11.5 GPa
Phase II

10

20

30

40

50

30

40

50

60

70

80

9.4 GPa
Phase I+ II
7.24 GPa
Phase I+II

10

15

20

25

(3 1 5)(2 0 15)(2 0 10)(2 0 5)

Ambient
Pressure
Phase I

(1 0 10)

1 GPa
Phase I
(1 1 0)(0 0 15)

Intensity (a.u.)(1115)(0021)(0120)(125)(1118)(2110)(0123)(0219)(0210)(205)(0018)(1013)(0015)(0111)(110)(1010)

20

(018)

(101)
(009)

Intensity(a.u.)(0012)

(003)

(009)

(006)

Intensity(a.u)

Bragg positions(1 0 5)

(015)

(b)

(0015)(a)30

2(deg.)2(deg.)2(deg.)
FIGURE 1. (a) Single crystal X-ray diffraction of BiSbTe3 cleaved along (00l) direction, Inset: Laue diffraction pattern (b)Le Bail refinement of powder XRD of as grown BiSbTe3 single crystal sample using Full Prof software. (c) X-ray diffractionpatterns of BiSbTe3 at various pressures with wavelength 0.7101 Å. The peaks indicated with solid green square are thereflections from high pressure phase.

In order to see the pressure dependency on the structure of the investigated system we have performed
XRD experiment with pressure variation using diamond anvil cells. Fig. 1 (c) illustrates the diffraction－ 24 －

pattern of BiSbTe3 polycrystalline sample under various pressure at room temperature. It is clear from Fig.
1 (c) that evaluation of secondary phase has been started from 7.24 GPa and persisted up to 11.5 GPa. But
at 11.5 GP monoclinic phase (secondary Phase) is dominating over rhombohedral phase which is consistent
with reported results [6]. Therefore, rhombohedral R3̅m phase is robust up to 9.5 GP as observed frompressure dependent XRD.
In Fig. 2 (a), we can see the variation of the electrical resistance (R) at various pressures, where R isdecreased between 2 GPa to 6 GPa. However, no sharp transition in the resistance was observed up to thelowest measured temperature (~2 K) (inset: Fig. 2(a)). When pressure was further increased to 8 GPa, a sharp
decrease in resistance value is observed with Tonset = 2.47K. We assume that this sharp decrease in theresistance at a particular temperature above 8 GPa can be assigned for the superconducting transition. The
superconducting transition is getting more pronounced with increasing pressure and Tonset is also shifting
toward higher value with applied pressure. The highest Tonset (3.36 K) was observed at 14 GPa, above which
it becomes almost constant up to 15 GPa.
(b)

(c)

FIGURE 2. (a) Temperature dependent of resistance at different pressures with a superconducting transition around 8 GPa.
Inset: Variation of resistance with respect to temperature at 2, 4 and 6 GPa. Band structure of the BiSbTe3 single crystal
measured at (b) T=20 K and (c) 60 K.

Figure 2(b) and (c) represent the ARPES spectra taken at 20 K and 60 K, respectively. These images show
the BVB of the BiSbTe3 forming a V-shaped valley like bulk band gap region around the Г-point in the
Brilliouin zone and the topological surface state (TSS) residing inside it as observed earlier [7-8]. The
sharpness of the linear dispersion of the TSS reveals good quality of the sample as well as the high energy
and momentum resolution of the laser ARPES spectra. Note that one can discern the unoccupied part of the
Dirac cone (DC) related to the TSS above EF at 60K due to the thermal broadening of the Fermi distributionfunction.
In conclusion, we have investigated the external pressure effect on the resistance and the ARPES on
BiSbTe3. It is observed that with increase of pressure resistance decreases and at a value of 8 GPa a sharp
drop in resistance is observed which indicates the occurrence of superconductivity. With further increase ofpressure, the superconducting transition temperature (Tc) increases and at 14 GPa it shows the maximum Tc(~3.3 K). This may indicate that the superconducting transition might be due to the change in the bulk band
structure in BiSbTe3. Furthermore, the ARPES study clearly indicates the Dirac point is located at the Fermi
level. This is an ideal condition for the technological exploitation of the topological properties of the TSS.
(#These authors contributed equally to this work. This work has been published in Physica Scripta 96,055802 (2021))

REFERENCES
1. M.Z. Hasan and C.L. Kane, Reviews of Modern Physics 82, 3045 (2010).
2. C.-Z. Chang, J. Zhang, X. Feng, J. Shen, Z. Zhang, M. Guo, K. Li, Y. Ou, P. Wei, L.-L. Wang, Z.-Q. Ji, Y. Feng, S. Ji, X.
Chen, J. Jia, X. Dai, Z. Fang, S.-C. Zhang, K. He, Y. Wang, L. Lu, X.-C. Ma, and Q.-K. Xue, Science 340, 167 (2013).
3. J.E. Moore, Nature 464, 194 (2010).
4. Y. Xia, D. Qian, D. Hsieh, L. Wray, A. Pal, H. Lin, A. Bansil, D. Grauer, Y.S. Hor, R.J. Cava, and M.Z. Hasan, Nature
Physics 5, 398 (2009).
5. A. Nakayama, M. Einaga, Y. Tanabe, S. Nakano, F. Ishikawa, and Y. Yamada, High Pressure Research 29, 245 (2009).
6. Zhao, H. Liu, L. Ehm, Z. Chen, S. Sinogeikin, Y. Zhao, and G. Gu, Inorganic Chemistry 50, 11291 (2011).
7. J. Zhang, C.-Z. Chang, Z. Zhang, J. Wen, X. Feng, K. Li, M. Liu, K. He, L. Wang, X. Chen, Q.-K. Xue, X. Ma, and Y.
Wang, Nature Communications 2, (2011).
8. X. He, T. Guan, X. Wang, B. Feng, P. Cheng, L. Chen, Y. Li, and K. Wu, Applied Physics Letters 101, 123111 (2012).

－ 25 －

BL-1
BL-01

18AG034

Study of the band structure of LaAgSb2 by angle-resolved
photoemission spectroscopy
Tomoki Yoshikawaa, Yuto Kitayamaa, Hiroshi Tanidab, E. F. Schwierc,
K. Shimadac, and A. KimuraaaGraduate School of Science, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima 7398526, Japanb
Liberal Arts and Sciences, Toyama Prefectural University, Imizu, Toyama 939-0398, Japanc
Hiroshima Synchrotron Radiation Center, Hiroshima University, 2-313 Kagamiyama, HigashiHiroshima 739-0046, Japan

Keywords: a square lattice, Dirac electrons, linear magnetoresistance

Dirac electrons form a cone-shaped band structure (Dirac cone) near the Fermi level, which appearson the surface of topological insulators and graphene [1-2], and is known to cause various interesting
quantum phenomena. Recently, first-principles calculations have shown that LaAgSb2, a type of layered
rare-earth silver antimonide with a square lattice, forms a Dirac cone due to the intersection of linear
bands derived from Sb 5p orbitals. LaAgSb2 exhibits interesting magnetotransport properties such asvery large linear magnetoresistance due to its characteristic electronic state [3].
To explore the band structures of LaAgSb2, we performed an angle resolved photoelectron
spectroscopy (ARPES) at BL1 of HiSOR. Figure 1 shows the experimental band structure along the𝛤𝛤� 𝑋𝑋� line of LaAgSb2 acquired with p-polarized at a photon energy (h) of 60 eV and a temperature of
30 K. Here, the vertical axis is the energy (E) from the Fermi level (EF) and the horizontal axis is thewavenumber (Å-1). As indicated by the color bar in the upper right corner, the darker color shows thestronger photoelectron intensity. Focusing on the vicinity of the 𝛤𝛤� point at wavenumber 0 Å-1, we
observe an upwardly convex linear band crossing EF and a downwardly convex band with apex at E-EF
= -0.8 eV. The two bands intersect at E-EF = -0.6 eV, indicating that they are linearly symmetric aroundwavenumber 0 Å-1. Focusing on the vicinity of the second Γ point at wavenumber ±1.4 Å-1, we can seethat the upwardly convex band observed near the first 𝛤𝛤� point at wavenumber 0 Å-1 appears repeatedly.
Focusing on wavenumber -0.9 Å-1 and E-EF = -0.2 eV, we can see that there is a small upward convex
band that is differentiated from the upward convex band at the second 𝛤𝛤� point at -1.4 Å-1. This bandis linear from -0.9 to -0.7 Å-1. This linear band around the X point is also observed at wavenumbers
from 0.7 to 0.9 Å-1. The upwardly convex band near the 𝛤𝛤� point is observed with greater intensity atlower wavenumbers, and the band is clearly dense. This can be attributed to the effect of the transition
probability of the photoelectron intensity.
The upward convex band dispersions around the first and second 𝛤𝛤� points observed in theexperiment are well reproduced by the first-principles calculation. The downward convex band
dispersion of the first 𝛤𝛤� point, the small upwardly convex band dispersion observed at wavenumbers-0.9 and -0.5 Å-1 and E-EF = -0.2 eV are also consistent with the first-principles calculation.

－ 26 －

REFERENCES[1] M. Z. Hasan and C. L. Kane, Rev. Mod. Phys. 82, 3045 (2010).
[2] A. H. Castro Neto et al , Rev. Mod. Phys. 81, 109 (2009).
[3] K. Wang and C. Petrovic, Phys. Rev. B 86, 155213 (2012).
[4] K. D. Myers et al , Phys. Rev. B 60, 13371 (1999).

FIG. 1 ARPES band dispersions of LaAgSb2 acquired with p-polarized at a photon energy of 60 eV.

－ 27 －

BL-1,
BL-9A
BL-1/BL-9A

18BG035, 19AG042, 19BG018

Self-energy Analysis of High-Tc Cuprate Superconductor
La2-xSrxCuO4
H. Odaa, H. Iwasawab,c, S. Ozawaa, A. Kimuraa, R. Yanod, S. Kashiwayad,
T. Sasagawae, E. F. Schwierc, K. ShimadacaGraduate School of Science, Hiroshima University, Japan
National Institutes for Quantum and Radiological Science and Technology, Japanc
Hiroshima Synchrotron Radiation Center, Hiroshima University, Japand
Department of Applied Physics, Graduate School of Engineering, Nagoya University, Japane
Materials and Structures Laboratory, Tokyo Institute of Technology, Japanb

Keywords: cuprate, ARPES.

For uncovering high-temperature superconductivity in cuprates, many-body interactions have been
investigated by angle-resolved photoemission spectroscopy (ARPES) [1]. Yet, it had been challenging toobtain high-quality ARPES spectra from three-dimensional crystal systems such as La2-xSrxCuO4 (LSCO)
due to the roughness of a cleavage plane. On the other hand, in recent years, spectral quality has been
improved as developing experimental equipment and performance, enabling us to obtain fine electronic
states and study many-body interactions even in three-dimensional systems. Indeed, a recent ARPES study
on an overdoped LSCO (x=0.23) revealed quasiparticle scattering rate Γ�𝜔𝜔�=2ImΣ�𝜔𝜔� as expressed byusing the imaginary part of the self-energy and claimed its energy dependence deviates from the Fermi liquidbehavior [2]. However, since the analysis of the scattering rate is based on only electron-electron interaction,
it is necessary to consider other many-body interactions, such as electron-boson interaction (boson: phonon
or magnon, etc.) dominating the quasiparticle scattering in the vicinity of the Fermi level. In this work, wehave performed a high-resolution ARPES study on an optimally doped LSCO to understand the role of manybody interactions in this system.
High-quality single crystals of optimally doped LSCO (x=0.155, Tc ~ 39 K) were prepared by thetraveling-solvent floating-zone technique [3]. ARPES experiments were measured at the linear undulator
beamline (BL-1) of Hiroshima Synchrotron Radiation Center (HiSOR) using the R4000 electron analyzer(Scienta Omicron) [4]. Clean surfaces of the samples were obtained by cleaving in situ under ultra-highvacuum conditions (4×10-9 Pa) at a low temperature (~27 K). Present ARPES data were taken with thephoton energy of 70 eV at 27 K in the s-polarization geometry.
Figure 1(a) shows the ARPES image of the optimally doped LSCO taken along the nodal direction, where
the circles represent the band dispersions determined by fitting momentum distribution curves (MDCs). To
evaluate the many-body interactions, we compared the ARPES dispersion with the calculated dispersion by
the tight-binding (TB) model [5], as shown in Fig. 1(a). At first, the TB model dispersion (dashed line) was
calculated using TB parameters were first determined to reproduce the band-structure calculations within thelocal density approximation (LDA). However, we found that the extracted real and imaginary parts of theself-energy do not satisfy the causality, i.e., Kramers-Kronig relation. Thus, we adjusted the TB parameters
to satisfy the causality, and the resulting TB model dispersion (solid line) was employed as a non-interacting
bare band dispersion in this work. Compared with the TB model dispersion, we found that the experimental
dispersions show a slightly larger effective mass in the vicinity of the Fermi level while a smaller one in thehigher energy region. Notably, this two-distinct behavior can be characterized by the crossing point between
the ARPES and TB model dispersion. At this point, the real and imaginary parts of the self-energy become
zero and minimum, respectively. This characteristic behavior is a signature of the high-energy anomaly,
widely reported in strongly correlated electron systems, including cuprates [6].
The renormalization effects can be more clearly seen in the extracted real and imaginary parts of the selfenergy, as shown in Figs. 1(c) and 1(d), respectively. We found that the imaginary part of the self-energy－ 28 －FIGURE 1. (a) APRES image of optimally doped LSCO along the nodal direction. The red and blue circles represent the
MDC-derived band dispersion. The dashed and solid lines are the tight-binding (TB) model dispersions based on the
different parameters: the parameters were set to reproduce the LDA calculation for the former dispersion, while they were
adjusted for the latter dispersion to satisfy the self-consistency between the extracted real and imaginary parts of the selfenergy. (b) and (c) Real and imaginary parts of the self-energy, respectively. The imaginary part of the self-energy was
fitted with the model self-energy including the contributions from the electron-electron interaction (el-el), the electronphonon interaction (el-ph), and the impurity scattering as well as the final state effects (imp+fin). Each contribution isindicated by a shaded area with a different color. Note that the TB model dispersion using the adjusted parameters was
employed as the bare band for extracting the self-energy.

was well reproduced by the model self-energy assuming the electron-electron interaction (ImΣel-el) [6],electron-phonon interaction (ImΣel-ph) [7], and impurity scattering as well as final state effects (ImΣimp+fin)[8]. Notably, the real part of the model self-energy, obtained via Kramers-Kronig transformation, agrees wellwith the experimental self-energy. The result thus is validating the assumption of the bare band andquantitative strength of the extracted self-energy. As clearly seen in Fig. 1(b), the imaginary part of the selfenergy is dominated mainly by two scattering channels, ImΣel-el and ImΣimp+fin. The significant contributionof the final state effects, giving the energy dependence of ImΣimp+fin, would be related to a relatively strong
three-dimensionality in LSCO. In contrast, the real part of the self-energy is composed of ReΣel-el and ReΣelph, as shown in Fig. 1(c), because ReΣimp+fin gives zero contribution. The peak maximum of the real part ofthe self-energy is located at around 0.3 eV, which agrees well with one of the antiferromagnetic spin
fluctuations reported by resonant inelastic X-ray scattering (RIXS) measurements [9]. Our result thus
indicates that the antiferromagnetic spin fluctuations play an important role in the electronic band
renormalization effects in LSCO. Our analysis presented here also allows evaluating the coupling strengthof the electron-electron interaction and electron-phonon interaction as λel-el=1.2 and λel-ph=0.6, respectively.
It should be emphasized here that they are underestimated or overestimated in the previous works [2, 10]:λel-el<0.3 (experiment), λel-ph=1.0-1.3 (experiment), and λel-ph=0.1-0.2 (theory). The discrepancy between ourand previous results must be originated from that only one interaction (el-el or el-ph) was only consideredin earlier works. Consequently, our results indicate the importance of considering all the interactions toevaluate the many-body interactions quantitatively.

REFERENCES
1. A. Damascelli, Z. Hussain, and Z.-X. Shen, Rev. Mod. Phys. 75, 473 (2003).
2. J. Chang et al., Nat. Commun. 4, 2559 (2013).
3. T. Sasagawa et al., Phys. Rev. Lett. 80, 4297–4300 (1998).
4. H. Iwasawa et al., J. Synchrotron Rad. 24, 836-841 (2017).
5. R. S. Markiewicz et al., Phys. Rev. B 72, 054519 (2005).
6. H. Iwasawa et al., Phys. Rev. Lett. 109, 066404 (2012).
7. Very High Resolution Photoelectron Spectroscopy, edited by S. Hüfner (Springer-Verlag, Berlin, 2007).
8. H. Hayashi et al., Phys. Rev. B 87, 035140 (2013).
9. M. P. M. Dean et al., Nat. Mater. 12, 1019 (2013).
10. F. Giustino et al., Nature 454, 975 (2008).

－ 29 －

18BG036

BL-1, BL-9A

Strain Induced Metal-Insulator Transition
in Mott Insulator Ca2RuO4
T. Miyashitaa, H. Iwasawab,c, H. Odaa, S. Ozawaa, A. Inod, F. Nakamurad,
A. Kimuraa, M. Aritac, E. F. Schwierc, K. ShimadacaGraduate School of Science, Hiroshima University, Japan
National Institutes for Quantum and Radiological Science and Technology, Japanc
Hiroshima Synchrotron Radiation Center, Hiroshima University, Japand
Kurume Institute of Technology, 2228-66 Kamitsu, Kurume, Fukuoka 830-0052, Japanb

Keywords: Mot insulator, Metal-insulator transition, ARPES

One of the representative Mott-insulators, ruthenate Ca2RuO4, exhibits a dramatic change in its physicalproperties due to external fields such as temperature, pressure, electric or magnetic field, and element
substitution [1-2]. Recently, it has been reported that a weak electric field corresponding to a battery can
induce metal-insulator transition and giant anti-magnetism in Ca2RuO4 [3], attracted much interest in termsof low electric switching devices. To understand such intriguing physical properties, it is essential to study
the electronic structure of Ca2RuO4. In particular, it is most straightforward to observe the electric-fieldinduced change of the electronic structures using high-resolution ARPES. However, since the electric field
may bring unexpected electronic behavior, we have decided to examine the temperature-induced metalinsulator transition of Ca2RuO4 at high temperature (~350 K) using high-resolution ARPES in this proposal.
High-quality single crystals of Ca2RuO4 were prepared by traveling-solvent floating-zone method with
RuO2 self-flux [3]. ARPES experiments were measured at BL-1 and BL-9A of Hiroshima Synchrotron
Radiation Center (HiSOR) using an R4000 electron analyzer (Scienta Omicron). Clean surfaces of thesamples were obtained by cleaving in situ under ultra-high-vacuum conditions (~10-10 Torr) at a hightemperature (~360 K).
Unfortunately, we could not observe clear band dispersion of Ca2RuO4 under various experimental
conditions (photon energies and polarizations) despite many trials (cleaves). Since the core levels observedin angle-integrated spectra can be well explained by the constitute of Ca2RuO4, we speculate that we wereunable to observe the band dispersion due to the small size of samples or surface quality. We will improve
the size and/or quality of samples for future studies on this material.

REFERENCES
1. S. Nakatsuji and Y. Maeno, Phys. Rev. Lett. 84, 2666 (2000).
2. L. Miao et al,. App. Phys. Lett. 100, 052401 (2012).
3. F. Nakamura et al., Sci. Rep. 3, 2536 (2013).

－ 30 －

19AG006,and
19AG028
19AG006
19AG028

BL-1
and BL-9A
BL-1,

Experimental observation of Dirac cones in artificial
graphene lattices
S. Yuea,c, H. Zhoua,c, D. Genga,c, Z. Suna,c, M. Aritab, K. Shimadab, P. Chenga,c,L. Chena,c,d, S. Menga,c, K. Wua,c,d and B. Fenga,cabInstitute of Physics, Chinese Academy of Sciences, Beijing 100190, China
Hiroshima Synchrotron Radiation Center, Hiroshima University, 2-313 Kagamiyama, Higashi-Hiroshima
739-0046, Japanc
School of Physical Sciences, University of Chiese Academy of Sciences, Beijing 100049, ChinadSongshan Lake Materials Laboratory, Dongguan, Guangdong 523808, China

Keywords: artificial graphene lattice, ARPES, Dirac cone, Shockley surface state

Graphene is a single layer of carbon atoms with a honeycomb lattice, and it has been intensively studied
in the past decade [1,2]. In proximity to the Fermi level, the electrons in graphene behave as massless Diracfermions. This is the origin of graphene’s various exotic properties, such as half-integer quantum Hall effects
and the Klein paradox. An alternative way to realize the exotic properties of graphene is by confining theelectrons of a two-dimensional electron gas (2DEG) to an equivalent honeycomb lattice, called artificial
graphene (AG) [3-5]. Because of the confinement, there exists a Dirac cone at each K point of the Brillouinzone (BZ). As a result, electrons of the otherwise 2DEG behave as massless Dirac fermions, which isanalogous to the case of natural graphene. In addition to possessing the novel properties of graphene,
artificial lattices possess various tunable parameters, thereby providing an ideal platform for the simulationof quantum behaviors in two-dimensional (2D) Dirac materials [6-9].
Some molecules, such as carbon monoxide and coronene, can serve as potential barriers through which
the electrons of the 2DEG are forbidden to travel. Therefore, a hexagonally patterned molecular lattice canconfine the 2DEG electrons into an equivalent honeycomb lattice and lead to the realization of AG [3,4], as
illustrated in Figure 1. The high energy and momentum resolution of angle-resolved photoemission
spectroscopy (ARPES) makes it a powerful technique to directly study the electronic structures of materials.
However, a direct experimental observation of the Dirac cones in AG using ARPES has yet not to be reported.
Here, we constructed macroscopic AG systems by growing monolayer C60 molecules on the (111)terminated surfaces of noble metals, which enabled ARPES studies of the electronic band structure. Our lowenergy electron diffraction (LEED) and ARPES measurements show that these AG systems arehomogeneous. Therefore, their size is only limited by the scale of the substrates. The Dirac cones at the 𝛫𝛫( 𝛫𝛫′ ) points were directly observed by our ARPES measurements. In addition, we performed model
calculations on these AG systems, whose results fully supported our experimental observations.
First, we studied C60 monolayers on Cu(111). The C60 molecules form a hexagonal structure with a 4×4
superstructure with respect to the 1×1 lattice of Cu(111) as shown in Fig. S1[10]. Figure 2(b) shows the
ARPES intensity of the Fermi surface. The Shockley surface state of Cu(111) almost disappears because of
the coverage of the C60 molecules. Instead, a dotlike spectral weight can be seen at each 𝛫𝛫(𝛫𝛫′) point. Suchfeatures do not exist on pristine Cu(111) because pristine Cu(111) only exhibits Shockley surface states at
the BZ center and bulk sp bands at the BZ boundary. At deeper binding energies, the dots become circles
and their size increases as the binding energies increase, as shown in Figs. 2(c)-2(e).
Figure 2(f) shows the ARPES spectra along the 𝛤𝛤 -𝛫𝛫 direction. One can observe linearly dispersing－ 31 －

bands at the 𝛫𝛫 point, as indicated by the red dashed lines. The fitted crossing point is located approximately
0.1 eV above the Fermi level. Along the 𝛫𝛫-𝛭𝛭-𝛫𝛫 direction, the dispersion of the bands is also linear, as
shown in Fig. 2(g). Combined with the evolution of the constant energy contours, we can conclude that there
is a Dirac cone at each K point. In addition, the Dirac bands do not disperse with different photon energies,
as shown in Figs. 2(h)-2(j), which agrees with their 2D character. Therefore, the ARPES results confirm that
the C60/Cu(111) system is an AG with Dirac cones at the 𝛫𝛫(𝛫𝛫𝛫) points of the BZ.

Another prototypical 2DEG system is Au(111), on which C60 molecules form a 2 √3 × 2 √3superstructure with respect to the 1×1 lattice of Au(111) as shown in Fig. S1[10]. This different
superstructure provides further evidence for the universality of this method. As expected, our ARPES results
show the existence of Dirac cones at each 𝛫𝛫(𝛫𝛫𝛫) point of the BZ, as shown in Figs. 4(b) and 4(c).
In summary, we demonstrate the synthesis of macroscopic AG by the self-assembly of C60 molecules on
metal surfaces. Our theoretical calculations and ARPES measurements directly confirm the existence ofDirac cones at the 𝛫𝛫(𝛫𝛫𝛫) points of the Brillouin zone.
This work has been published in Physical Review B 102, 201401(R) (2020).

FIGURE 1. Schematic of artiﬁcial graphene (AG). Molecular AG system,
including molecules that act as potential barriers for electrons (red balls) and theelectron density profile (blue surfaces). Band structure of AG. A Dirac cone
exists at the K (K′) point of the Brillouin zone (BZ), as an analogy to graphene.

FIGURE 2. ARPES measurements of the C60 monolayer on Cu(111). (a) Schematic of the BZs of the C60 monolayer (red)and Cu(111) (blue) (b)–(e) ARPES intensity maps of the constant energy contours at different binding energies. (f)-(j) ARPES
intensity plots along high-symmetry directions with different photon energy. From Ref. 10.

FIGURE 3. ARPES measurements of the C60 monolayer on Au(111). (a) Schematic of the BZs of the C60 monolayer (red)and Au(111) (blue). (b), (c) ARPES intensity plots along the K-M-K direction. From Ref. 10.

－ 32 －

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

A. K. Geim and K. S. Novoselov, Nat. Mater. 6, 183 (2007).
A. H. Castro Neto, F. Guinea, N. M. R. Peres, K. S. Novoselov, and A. K. Geim, Rev. Mod. Phys. 81, 109 (2009).
K. K. Gomes,W. Mar,W. Ko, F. Guinea, and H. C. Manoharan, Nature (London) 483, 306 (2012).
S.Wang, L. Z. Tan,W.Wang, S. G. Louie, and N. Lin, Phys. Rev. Lett. 113, 196803 (2014).
M. Nantoh, K. Takashima, T. Yamamoto, and K. Ishibashi, Phys. Rev. B 96, 035424 (2017).
A. Singha, M. Gibertini, B. Karmakar, S. Yuan, M. Polini, G. Vignale, M. I. Katsnelson, A. Pinczuk, L. N. Pfeiffer, K.
W. West, and V. Pellegrini, Science 332, 1176 (2011).
7. L. Tarruell, D. Greif, T. Uehlinger, G. Jotzu, and T. Esslinger, Creating, Nature (London) 483, 302 (2012).
8. M. Polini, F. Guinea, M. Lewenstein, H. C. Manoharan, and V. Pellegrini, Nat. Nanotechnol. 8, 625 (2013).
9. S. N. Kempkes, M. R. Slot, J. J. van den Broeke, P. Capiod, W. A. Benalcazar, D. Vanmaekelbergh, D. Bercioux, I. Swart,
and C. M. Smith, Nat. Mater. 18, 1292 (2019).
10. S. Yue, H. Zhou, D. Geng, Z. Sun, M. Arita, K. Shimada, P. Cheng, L. Chen, S. Meng, K. Wu, and B. Feng, Phys. Rev.
B 102, 201401 (2020).

－ 33 －

19AG007, 19BG007

BL-1

Topological electronic structure in the antiferromagnet
HoSbTeS. Yuea,c, Y. Qiana,c, M. Yanga,c,d, D. Genga,c, C. Yia,c,d, S. Kumarb, K. Shimadab,P. Chenga,c, L. Chena,c,e, Z. Wanga,c, H. Wenga,c,e , Y. Shia,c,d,e , K. Wua,c,e and B.
Fenga,cabInstitute of Physics, Chinese Academy of Sciences, Beijing 100190, China
Hiroshima Synchrotron Radiation Center, Hiroshima University, 2-313 Kagamiyama, Higashi-Hiroshima
739-0046, Japanc
School of Physical Sciences, University of Chiese Academy of Sciences, Beijing 100049, ChinadCenter of Materials Science and Optoelectronics Engineering, University of Chinese Academy of
Sciences, Beijing 100049,ChinaeSongshan Lake Materials Laboratory, Dongguan, Guangdong 523808, China

Keywords: magnetic topological material, nodal line, weak topological insulator, ARPES

Topological quantum materials, such as topological insulators and semimetals, have attracted muchattention in the last decade. These materials host topologically nontrivial electronic structures that can becharacterized by symmetry protected topological invariants [1-3]. Among the topological quantum materials,
Dirac nodal line (DNL) semimetals are of particular interest because they are neighbors of various quantum
states. In a DNL semimetal, the conduction and valence bands touch at extended lines in the threedimensional Brillouin zone (BZ), and the degeneracy of the line nodes is protected by certain symmetries,
such as mirror reflection, time-reversal, and spin-rotation symmetries. The breaking of these symmetries canfully or partially gap out the nodal lines, giving rise to rich topological quantum states, such as Diracsemimetals, Weyl semimetals, and topological insulators.
Magnetic topological materials, in which the time-reversal symmetry is broken, host various exotic
quantum phenomena, including the quantum anomalous Hall effect, axion insulator states, and Majorana
fermions. The study of magnetic topological materials is at the forefront of condensed matter physics.
Recently, a variety of magnetic topological materials have been reported, such as Mn3Sn, Co3Sn2S2, and
MnBi2Te4 [4-6]. Recently, single-crystal HoSbTe, a new member of the ZrSiS (or LnSbTe) family, wassynthesized [7]. Because of the unpaired 4f electrons of Ho atoms, HoSbTe is an antiferromagnetic (AFM)
material with a TN of approximately 4 K.
Here, we performed angle-resolved photoemission spectroscopy (ARPES) and first-principles
calculations to study the electronic structure and topological properties of HoSbTe. Firstly, we performed
theoretical calculation. A DNL exists at the kz = 0 and π planes, and the degeneracy of the line nodes isprotected by the glide mirror symmetry of the crystal. The degenerate points along the high-symmetry
directions are indicated by the red dotted circles in Fig. 1(c). When SOC is taken into account, the DNLs of
HoSbTe are fully gapped, as shown in Fig. 1(d). Figures 1(e)-1(h) show the calculated SOC gaps of HoSbTe.
One can see that the gap reaches 200 meV along the Z-R direction, which is much larger than the thermal
excitation energy. Such a large gap makes the realization of the WTI states in HoSbTe possible in theexperimentally accessible regime.
To confirm the intriguing electronic structure of HoSbTe, we performed high-resolution ARPES
measurements on the (001) surface of HoSbTe. Figures 2(a)-2(d) and 2(e)-2(h) show the CECs of HoSbTe
measured with 55 and 120 eV photons, which corresponds to kz = 12.3 and 17.2 π/c, respectively. Overall,－ 34 －

the data at 55 and 120 eV are quite similar, despite the subtle differences such as the positions of the corners
� direction, a parabolic band occurs that originates fromand intensities at the BZ boundary. Along the 𝛤𝛤�-𝛭𝛭
the gapped DNL, as shown in Figs. 3(a)-3(c).
In summary, we report the observation of topological electronic structures in the antiferromagnetic
material HoSbTe. When SOC is neglected, HoSbTe is a DNL semimetal, and its electronic structure isanalogous to that of ZrSiS. However, the intrinsic strong SOC fully gaps out the nodal lines and drives thesystem to be a WTI. Each layer of HoSbTe is a 2D TI, and the bulk crystal can be viewed as a stacking of
2D TIs. The intriguing electronic structure of HoSbTe has been directly confirmed by ARPES measurements.
Notably, the SOC gap of HoSbTe is as large as hundreds of meV, which is accessible for most experimental
techniques.
This work has been published in Physical Review B 102, 155109 (2020).

FIGURE 1. (a) Crystal structure of HoSbTe. (b) Brillouin zone of HoSbTe. (c), (d) Calculated bulk bands of HoSbTe alonghigh-symmetry directions without and with SOC, respectively. Red and green lines in (c) highlight the highest valence andlowest conduction bands that form Dirac nodal lines. The red dotted circles in (c) indicate the degenerate points that form
DNLs. (e)–(h) Calculated SOC-induced band gaps at the line nodes along high-symmetry directions: -X, -M, Z-R, and Z-A.
The sizes of the gaps are indicated in the figure. From Ref. 8.

FIGURE 2. ARPES intensity plots of the constant energy contours (CECs) of HoSbTe(001). (a)–(d) CECs measured with 55
eV photons. (e)–(h) CECs measured with 120 eV photons. From Ref. 8.

－ 35 －

� direction measured with different
FIGURE 3. (a)–(c) ARPES intensity plots of the band structure of HoSbTe along the 𝛤𝛤�-𝛭𝛭
� -𝛸𝛸�-𝛭𝛭� direction. (e), (f) Slab calculation results
photon energies. (d) ARPES intensity plots of the band structures along the 𝛭𝛭� and 𝛭𝛭� -𝛸𝛸�-𝛭𝛭� , respectively. (g) Momentum distribution curve plots of (b). (h)of the band structure of HoSbTe along 𝛤𝛤�-𝛭𝛭
Energy distribution curves along the red dashed lines in (b) and (c). From Ref. 8.

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

M. Z. Hasan and C. L. Kane, Colloquium: Topological insulators, Rev. Mod. Phys. 82, 3045 (2010).
N. P. Armitage, E. J. Mele, and A. Vishwanath, Rev. Mod. Phys. 90, 015001 (2018).
B. Lv, T. Qian, and H. Ding, Nat. Rev. Phys. 1, 609 (2019).
N. Morali, R. Batabyal, P. K. Nag, E. Liu, Q. Xu, Y. Sun, B. Yan, C. Felser, N. Avraham, and H. Beidenkopf, Science
365, 1286 (2019).
H. Yang, Y. Sun, Y. Zhang, W.-J. Shi, S. S. P. Parkin, and B. Yan, New J. Phys. 19, 015008 (2017).
Y. Deng, Y. Yu, M. Z. Shi, Z. Guo, Z. Xu, J. Wang, X. H. Chen, and Y. Zhang, Science 367, 895 (2020).
M. Yang, Y. Qian, D. Yan, Y. Li, Y. Song, Z. Wang, C. Yi, H. L. Feng, H. Weng, and Y. Shi, Phys. Rev. Mater. 4,094203(2020).
S. Yue, Y. Qian, M. Yang, D. Geng, C. Yi, S. Kumar, K. Shimada, P. Cheng, L. Chen, Z. Wang, H. Weng, Y. Shi, K.
Wu, and B. Feng, Phys. Rev. B 102, 155109 (2020).

－ 36 －

19BG003

BL-1

Electronic structure of chiral IrGe4 studied by angleresolved photoemission spectroscopy
T. Mizokawaa, T. Shimaiwaa, Y. Akabanea, N. L. Sainib,
Y. Chibac, R. Higashinakac, T. D. Matsudac, Y. Aokic,
E. F. Schwierd, S. Kumard, K. ShimadadabDepartment 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: Kramers-Weyl Semimetal, Chiral material

In chiral materials with strong spin-orbit coupling, Bloch electrons exhibit two-fold Kramers degeneracy
at time reversal invariant momentum (TRIM) points. The Kramers degeneracy is lifted except the TRIM
points due to the spin-orbit interaction and the space inversion symmetry breaking of the chiral crystal.
Therefore, the degenerate point is viewed as a Kramers-Weyl fermion [1,2]. Degree of the degeneracy canbe larger than two when the spin-orbit interaction is negligible. CoSi with chiral crystal structure has beenstudied by means of band structure calculations and angle-resolved photoemission spectroscopy [3-5].
Although the spin-orbit interaction is relatively weak in CoSi, the observation of Kramers-Weyl fermionshas been claimed [3-5]. There are many other chiral materials which can harbor the Kramers-Weyldegeneracy at TRIM. Among them, IrGe4 has trigonal chiral structure just like α-quartz. Since IrGe4 has nowell-defined cleavage plane, it is highly difficult to perform ARPES on it. In the present proposal, we havechallenged ARPES observation of the Kramers-Weyl degeneracy and possible surface states in IrGe4.
ARPES on the single crystals of IrGe4 were performed at BL-1 of HiSOR using a SCIENTA R4000
analyzer with linearly polarized light. The total energy resolution was set to about 40 meV for hν = 58 eV.
The single crystals were oriented by ex situ XRD measurements. The crystals were cleaved at 20 K under
the ultrahigh vacuum and subsequently measured at 20 K. The obtained surfaces were roughly perpendicular
to the c-axis of IrGe4.
Figure 1 shows a Fermi surface map taken at hν = 58 eV. ARPES intensity is enhanced along the M pointsof the surface Brillouin zone. This observation is qualitatively consistent with band structure calculations inwhich Ge 4p bands cross the Fermi level around the M points [6]. However, the observed Fermi surfaces
were not clear probably due to surface roughness. Since there is no well defined cleavage plane perpendicular
to the c-axis, the obtained surface was not really flat. We need to improve surface quality in order to observe
Kramers-Weyl degeneracy in the bulk band dispersions and compare them with the prediction of the band
structure calculations [6].
In addition to the bulk Fermi surfaces, we tried to observe helicoidal Fermi arcs which are expected atsurfaces of chiral materials. However, we failed to observe surface bands and Fermi arcs in the surfaces
obtained by cleaving. It is often claimed that Fermi arcs at surfaces of Weyl semimetals are topologicallyprotected and very robust. However, such surfaces states are rather fragile against surface reconstruction,
surface roughness, etc. We need to try various surface treatments to stabilize the Fermi arcs which may haveinteresting spin polarizations.
In conclusion, we have investigated the electronic structure of chiral IrGe4 by means of ARPES. Although
the observed Fermi surfaces are very broad due to surface roughness, the overall band structure is roughly
consistent with the prediction of the band structure calculations [6]. In future, we need to improve surface
treatment in order to obtain flat surfaces which can harbor Kramers-Weyl fermions and helicoidal Fermi arcs.

－ 37 －FIGURE 1. Fermi surface map observed at h = 58 eV. The solid lines indicate the surface Brillouin zone boundaries.

REFERENCES
1. G. Chang, B. J.Wieder, F. Schindler, D. S. Sanchez, I. Belopolski, S.-M. Huang, B. Singh, D. Wu, T.-R. Chang, T. Neupert,
S.-Y. Xu, H. Lin, and M. Z. Hasan, Nature Mater. 17, 978 (2018).
2. P. Tang, Q. Zhou, and S.-C. Zhang, Phys. Rev. Lett. 119, 206402 (2017).
3. Z. Rao et al., Nature 567, 496 (2019).
4. D. S. Sanchez et al., Nature 567, 500 (2019).
5. D. Takane, Z. Wang, S. Souma, K. Nakayama, T. Nakamura, H. Oinuma, Y. Nakata, H. Iwasawa, C. Cacho, T. Kim, K.
Horiba, H. Kumigashira, T. Takahashi, Y. Ando, T. Sato, Phys. Rev. Lett. 122, 076402 (2019).
6. T. Shimaiwa, Y. Akabane, Y. Chiba, R. Higashinaka, T. D. Matsuda, Y. Aoki, S. Kumar, E. F. Schwier, K. Shimada, N.
L. Saini, T. Mizokawa, submitted to J. Phys. Soc. Jpn.

－ 38 －

BL-1, BL-9B－ 39 －

－ 40 －

－ 41 －

18AU015, 19AG041, 19BG017

BL-1

Low-energy electron-boson couplings in Sr2RuO4
S. Ozawaa, H. Iwasawab,c, H. Odaa, T. Sugiyamaa, A. Kimuraa,
S. Kumarc,Y. Yoshidad, I. Hased, Y. Aiurad, K. ShimadacabGraduate School of Science, Hiroshima University, Japan
National Institutes for Quantum and Radiological Science and Technology, Japanc
Hiroshima Synchrotron Radiation Center, Hiroshima University, Japand
National Institute of Advanced Industrial Science and Technology, Japan

Keywords: Ruthenate, ARPES.

Strontium ruthenate Sr2RuO4 is a representative unconventional superconductor (Tc~1.5 K) and a strongly
correlated two-dimensional electron system [1-2]. To understand the unconventional superconductivity of
Sr2RuO4, it is essential to evaluate electron-boson interactions and strong correlation effects quantitatively
and separately. In this regard, angle-resolved photoemission spectroscopy (ARPES) is one of the mostsuitable probes as it can obtain information on many-body interactions. Indeed, many previous ARPES
results reported the existence of electron-boson interactions fingerprinted by a “kink” structure in the γ-band
derived from Ru 4dxy orbital [3-6]. On the other hand, the latest high-resolution ARPES study on theelectronic states near the Fermi energy (EF) reported that the electrons and bosons are not strongly coupled[7]. Such a discrepancy could be attributed to the fact that the existing ARPES measurements only focusedon the near EF region and could not accurately evaluate electron correlations with an energy scale 10 to 100
times larger than that of the boson mode (<100 meV). This suggests that it is necessary to observe not only
the band structures in the vicinity of the EF but also the entire band structure for the accurate identificationof coupling components by ARPES. In this regard, to separate and evaluate multiple types of the many-bodyeffects, we have examined the energy and momentum dependence of the many-body effects via highresolution ARPES measurements in a wide energy range at various momentum locations.
High-quality single crystals of Sr2RuO4 were prepared by the floating zone method with a self-flux
technique (Tc ~ 1.36 K). High-resolution ARPES experiments were measured at the linear undulator
beamline (BL-1) of Hiroshima Synchrotron Radiation Center (HiSOR) using an R4000 electron analyzer(Scienta Omicron) [8]. Clean and flat surfaces of the samples were obtained by cleaving in situ under ultrahigh-vacuum conditions (2×10-11 Torr) at a low temperature (~20 K). Energy and angular resolution were
better than 15 meV and 0.3°, respectively
Figure 1(a) shows the ARPES images taken at different azimuthal angles (ϕ = 0, 12, 30 deg), as indicatedin the schematic Fermi surface. Then, we have determined the γ band dispersion by fitting the momentum
distribution curves (MDCs) as shown in the blue-filled circles in Fig. 1(b), where a phenomenological model
dispersion is also shown. The model dispersion was calculated by including the electron correlation effects
with the estimated strength from our soft X-ray ARPES results (not shown). One can see that theexperimental dispersion is not reproduced by the model calculations, including the electron correlation
effects. This indicates that the electron-boson coupling should be considered to explain the mass
renormalization effects in the γ band.
The electron-boson coupling could be more visualized in the real part of the self-energy shown in Fig.
1(c), induced by taking the energy difference between the experimental and model dispersion. We found thecharacteristic peak at -40 meV and shoulder structures at -60 meV and -20 meV. These structures in the realpart of the self-energy correspond to sudden changes of the group velocity, or kinks, in the band dispersion.
In our previous ARPES results [5], the structures at -40 meV and -60 meV were also observed, and they canbe attributed to the phonon modes, as reported in an inelastic neutron scattering experiment [9]. The
remaining structure at -20 meV was not observed in our previous ARPES results. On the other hand, a kinkstructure having a similar energy-scale at -15 meV was reported for the surface states in Sr2RuO4, and its－ 42 －

origin was discussed in terms of the electron-phonon coupling [6]. To uncover the origin of the kink structure,
more detailed self-energy analysis, including the imaginary part of the self-energy, is in progress.

FIGURE 1. (a) High-resolution ARPES measurements on Sr2RuO4 as a function of the azimuthal angle (ϕ = 0, 12, 30deg) taken with a photon energy of 65 eV below 20 K with p-polarization geometry. (b) ARPES dispersion determined byfitting MDCs for ϕ = 0 deg. The blue dashed line is the phenomenological model dispersion including the electron
correlation effects and employed as the bare band to extract the electron self-energy dominated by the electron-boson
coupling. (c) Real part of the self-energy as a function of the azimuthal angle (ϕ = 0, 12, 30 deg). The arrows indicates thecharacteristic peak/shoulder structures at -20, -40, and -60 meV.

REFERENCES
1.
2.
3.
4.
5.
6.
7.
8.
9.

A. P. Mackenzie and Y. Maeno, Rev. Mod. Phys. 75, 657 (2003).
Y. Maeno et al., J. Phys. Soc. Jpn. 81, 011009 (2012).
Y. Aiura et al., Phys. Rev. Lett. 93, 117005 (2004).
H. Iwasawa et al., Phys. Rev. B 72, 104514 (2005).
H. Iwasawa et al., Phys. Rev. Lett. 105, 226406 (2010).
Y. Akebi et al., Phys. Rev. B 99, 081108(R) (2019).
A. Tamai et al., Phys. Rev. X 9, 021048 (2019).
H. Iwasawa et al., J. Synchrotron Rad. 24, 836-841 (2017).
M. Braden et al., Phys. Rev. B 76, 014505 (2007).

－ 43 －

19BG026

BL-1

Ultrathin Ferromagnetic Films on Spin-Orbit-Influenced
Metals: Interplay between Exchange and Spin-Orbit
Interaction
Marcel Holtmanna, Pascal J. Grenza, Shiv Kumarb, Eike F. Schwierb, c , Taichi
Okudab, Markus DonathaaPhysikalisches Institut, University of Münster, Wilhelm-Klemm-Straße 10, 48149 Münster, Germany
Hiroshima Synchrotron Radiation Center, University of Hiroshima, Higashi-Hiroshima 739-0047, JapancPresent address: Physikalisches Institut, Julius-Maximilians-Universität Würzburg, Am Hubland, 97074
Würzburgb

Keywords: ARPES, Iron, Tungsten, Electronic Structure, Dirac-cone-like surface state

The W(110) surface exhibits a Dirac-cone-like surface state (DSS). In recent years, this state has beenstudied extensively [1-3], showing its anisotropic shape due to the two-fold symmetry of the W(110) surface
and its topologically nontrivial character [4]. Upon deposition of ultrathin Fe layers the “Dirac-fermion mass”
of the DSS was found to be manipulable by varying the film thickness (and thereby the magnetic propertiesof the Fe film) [5].
Theoretical studies of W(110) predict that the energetic position of the DSS is related to the interlayer
distance of the first W layers. For the case of Fe/W(110) the DSS is expected to shift towards higher binding
energies, contrary to the findings of Honma et al. This has lead us to re-investigate the Fe/W(110) systemby means of angle-resolved photoelectron spectroscopy (ARPES) at the BL-1.
Our ARPES results on W(110) and Fe/W(110) are shown in figure 1. We have labeled a parabolic band(“W”) and the DSS and sketched the dispersion of the latter in the bottom panel. For the pristine W(110)
surface, the Dirac Point – the band crossing at the Brillouin zone center Γ – appears at ED = -1.25 eV whichis in good agreement with previous experimental and theoretical results. For the Fe/W(110) case, the samefeatures can be recognized with an additional broad feature at about -0.3 eV. When comparing the positionof the DSS between the two figures, it appears to have shifted downward in energy by ED = -75 meV. The
same trend was measured for the Co/W(110) case, where the DSS energy shift amounts to ED = -107 meV.
These findings are consistent with our theoretical calculations.
Coming back to our initial motivation on the sample system, we do not find evidence for an opening ofa DSS gap. Our results suggest, that the upper part of the DSS gap reported in [5] might be the parabolic
band W persisting upon Fe adsorption. Our results are published in [6].

－ 44 －

Figure 1. ARPES measurements for the 𝐻𝐻 − 𝛤𝛤 − 𝐻𝐻 line of W(110) (a) and 0.5 ML Fe/W(110) (b). Below, the dispersion
and energetic position of the DSS is depicted. Figure extracted from [6].

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

K. Miyamoto et al., Phys. Rev. Lett. 108, 066808 (2012).
K. Miyamoto et al., Phys. Rev. B 86, 161411 (2012).
H. Mirhosseini et al., New J. Phys. 15, 033019 (2013).
D. Thonig et al., Phys. Rev. B 94, 1155132 (2016).
K. Honma et al., Phys. Rev. Lett. 115, 266401 (2015).
P. J. Grenz et al., J. Phys.: Condens. Matter 33, 285504 (2021).

－ 45 －

19BG044, 19BU002, 19BU005, 19BU012

BL-1, Laser-ARPES

Half-Magnetic Topological Insulator with MagnetizationInduced Dirac Gap at a Selected Surface
Ruie Lua#, Shiv Kumarb#, Yuan Wanga#, Meng Zenga, Yu-Jie Haoa, Xiao-Ming
Maa, Zhanyang Haoa, Ke Zhangb, Wumiti Mansuerb, Cai Liua, Ke Denga, Kenya
Shimadab, Eike F. Schwierb*, Chang Liua*, Qihang Liua*, Chaoyu Chena*aShenzhen Institute for Quantum Science and Engineering (SIQSE) and Department of Physics, Southern
University of Science and Technology (SUSTech), Shenzhen 518055, China.
b
Hiroshima Synchrotron Radiation Centre, Hiroshima University, Higashi-Hiroshima, Hiroshima 7390046, Japan

Keywords: axion-insulator, MnBi8Te13, angle-resolved photoemission spectroscopy, magnetic topological materials.

The recent discovery of the intrinsic magnetic topological insulator (TI) MnBi2Te4 and
MnBi2Te4/(Bi2Te3)n (n=1,2,3,…) have boosted exciting possibilities of producing exotic quantum
phenomena by engineering topology and magnetism at the atomic scale [1-4]. Such magnetic TIs arepromised to host two exotic quantum phenomena: the “quantum anomalous Hall effect” and the “axion
insulating state” [5, 6]. The former has integer-quantized surface Hall resistivity, while the later might present
Half-quantized surface resistivity. However, partially because of the lack of a suitable material, it is still not
clear if the Half-quantized surface resistivity can be observed in transport measurement, lacking the direct
proof of the “axion insulating state”.
In this study, we performed a µ-Laser-angle-resolved photoemission spectroscopy (µ-Laser-ARPES) on
an intrinsic ferromagnetic TI, MnBi2Te4-(Bi2Te3)3 (MnBi8Te13), and demonstrated that MnBi8Te13 can be an
ideal platform to explore and manipulate the exotic half-quantized surface resistivity. The energy bands showa Dirac gap at the magnetic MnBi2Te4 layer but gapless Dirac cone at its nonmagnetic Bi2Te3 layers in the
opposite side. Remarkably, the magnetic Dirac gap (~ 28 meV) decreases monotonically with increasing
temperature and closes right at the Curie temperature (as shown in Figure 1), smoking-gun evidence ofmagnetization-induced topological surface gap among all known magnetic topological materials.

－ 46 －FIGURE 1. ARPES results of the Dirac gap evolution with temperature at the magnetic MnBi2Te4 layer of MnBi8Te13. Spectra
are shown in the form of original ARPES spectra (top row) and 2D curvature spectra (bottom row). Black dashed line indicatesthe position of gapless Dirac point.

(#These authors contributed equally to this work. This work has been published in Physical Review X 11,011039 (2021))

REFERENCES
1.
2.
3.
4.
5.
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Otrokov, M. M. et al. Nature 576, 416-422 (2019).
Rienks, E. D. L. et al. Nature 576, 423-428 (2019).
Deng, Y. et al. Science 367, 895-900 (2020).
Liu, C. et al. Nat Mater 19, 522-527 (2020).
Essin, A. M., Moore, J. E. & Vanderbilt, Phys. Rev. Lett. 102, 146805 (2009).
Qi, X.-L., Li, R., Zang, J. & Zhang, S.-C. Science 323, 1184-1187 (2009).

－ 47 －

BL-1
BL1

狤狢'-狢狢狦

ARPES Study on low dimensional chiral Dirac material
CoNb3S6
Ke Denga, Shiv Kumarb, Yuan Wanga, Cai Liua, Kenya Shimadab, Chaoyu ChenaaShenzhen Institute for Quantum Science and Engineering (SIQSE), Southern University of Science andTechnology, Shenzhen 518055, Chinab
Hiroshima Synchrotron Radiation Center, Hiroshima University, Higashihiroshima, Hiroshima 739-0046,
Japan
Keywords: chiral lattice, Dirac semimetal, CoNb3S6, antiferromagnetic order (AFM), ARPESThe properties two-dimensional materials with layered structure, can be artificially tuned to new orderingby doping, temperature, pressure, and intercalation [1-6]. One significant progress in the 2D material is therealization of two-dimensional magnetism, which can be achieve not only in the systems with intrinsic
magnetic order [7-14], but intercalation of magnetic layers in nonmagnetic materials as well [15-19]. As themagnetism can be introduced to layered TMDCs by magnetically element intercalated into the van de Waals
gap of TMDCs. Unlike the valley degree of freedom arise from the breaking of inversion symmetry in
TMDCs, intercalation of 3d metal can not only breaking the time reversal symmetry and leading to exotic
quantum phase of matter, including magnetic topological insulator, QAHE and topological semimetal [1518], but also triggering crystalline chirality and chiral Dirac fermion as well[19-21].

FIGURE 1. (a)Structure of 3d-metal intercalated transition metal dichalcogenides (TMDCs). While red balls represent for
3-d element Co as intercalated layers, yellow and green balls stand for the host TMDCs. Figure (a) is reproduced from [18].
(b) Chirality associated with a specific configuration of octahedra filled in a hexagonal crystal. Figure (a) is reproduced from[20]. (c) Hall conductivity of CoNb3S6 as a function of magnetic field. (d) Calculated band structure of CoNb 3S6 withinclusion of SOC. Figure (c-d) are reproduced from [19]. (e) Distribution of in-plane componenet of Berry curvature on
𝑘𝑘𝑧𝑧 = 0 plane, N1, N2 and P1, P2 denote the source and sink, respectively. (f) Chiral edge states of 2D slice with Chern
number of 2. Doubly degenerate edge bands locate at SU(2)-preserved edge, while the nondegenerate ones on SU(2)-borkenedge. Figure (e, f) are reproduced from [21]－ 48 －

CoNb3S6 is an intercalated transition metal dichalcogenides compound with chiral lattice [18-21]. Theintercalated Co atom layers with 3-d electrons sandwiches between layers of 5d transition metal
dichalcogenides van de Waals gap, form interlayer antiferromagnet order [18,19,21], and therefore leading
to a 1/3 fractional intercalation of NbS2 layers with chiral lattice (P6322) [21]. Along with its crystalline
chirality, the intercalated Co layers contributes magnetic order below 26 K, gives rise to a complex
susceptibility for the magnetic field applied along c-axis, large anomalous Hall effect [19] and predicted tohost novel state of electrons e.g. chiral fermion (FIG.1) [21]. Although CoNb3S6 has been studied via DFT
calculation and transport measurements, the experimental studies of its electronic structure still lacking, it isthus crucial to study its electronic structure experimentally [18,19,21].
We performed ARPES measurement on the chiral-lattice antiferromagnet CoNb3S6. Our results revealed
the hexagonal Fermi surface, agrees well with existed band calculation [18,19]. Two linear hole-like bands
dominate the density of state around the center of Brillouin zone and forms concentric circles centered at Γ
point. With increasing the binding energy, the centered hole-like dispersions reach its bottom at both 𝐾𝐾 and
𝑀𝑀 points, in good agreement with DFT calculation. According to the theoretical calculation, CoNb3S6 canbe a Weyl semimetal with several pairs of Weyl points circled around the Γ point, and the Fermi arcs
connect these Weyl points with opposite chirality surrounding Γ point can form a nearly circled feature on
its FS. Our observation of the concentered circles feature might partially originate from such multiple Fermi
arcs around the Γ point.

FIGURE 2. Our ARPES intensity map of CoNb3S6 single crystal taken @ BL1 with different linear polarized light. The uppanels shows constant energy maps with LH. And the panels downside shows electronic structure along 𝐾𝐾 − 𝛤𝛤 − 𝐾𝐾 (left)
and 𝑀𝑀 − 𝛤𝛤 − 𝑀𝑀 (right) direction, respectively

FIGURE 3. ARPES measured electronic structure along 𝑀𝑀 − 𝛤𝛤 − 𝑀𝑀 direction at different temperature with LH linear
polarized light.

By taking advantage of changeable measurement configuration of BL1 end-station of HiSOR, it isstraightforward to get ARPES measurement with different linear polarized light. While p-polarized lightclear revealed the dispersion feature, especially the electron pocket resides along Γ − K direction, the s－ 49 －polarized light resolved the hole band centered at Γ point with its band maximum locate around 200 meV
below Fermi level (FIG. 2). By combining the different polarization of incident light, we obtained thecomplete information CoNb3S6 electronic structure. In addition, transport measurement indicates a magnetictransition occurs at ~ 25 K [19]. To uncover its electronic structure evolution upon temperature variation, we
performed systematic ARPES measurement upon different temperature (15 K-35 K), and our ARPES data
indicates no obvious change in its electronic structure along high symmetry direction (FIG. 3). The
anticipated magnetic order transition might lead to a more delicate changes such as the spin polarization or
Fermi arc length, rather than electronic band structure. Further spin-ARPES and CD-ARPES experiments
will be able to resolve such magnetic transition associated spin evolution.

REFERENCES
1. Manzeli S, et al. 2D transition metal dichalcogenides. Nat. Rev. Mater. 2(8): 17033 (2017).
2. M. Zeng, et al., Exploring two-dimensional materials toward the next-generation circuits: from monomer design toassembly control, Chem. Rev. 118, 6236−6296 (2018)
3. K. S. Burch, et al., Magnetism in two-dimensional van der Waals materials. Nature 563, 47–52 (2018)
4. Clarke R, Uher C. High pressure properties of graphite and its intercalation compounds. Adv. Phys., 33(5): 469-566 (1984)
5. Dresselhaus M S, Dresselhaus G. Intercalation compounds of graphite. Adv. Phys., 30(2): 139-326 (1981).
6. Inoue M, et al., The electronic and magnetic properties of the 3d transition metal intercalates of TiS2. Adv. Phys., 38(5):565-604 (1989).
7. Huang B., et al. Layer-Dependent Ferromagnetism in a Van Der Waals Crystal Down to the Monolayer Limit. Nature 546,
270-273 (2017)
8. Gong C, et al. Discovery of Intrinsic Ferromagnetism in Two-Dimensional Van Der Waals Crystals. Nature 546, 265-269(2017)
9. Jiang S, et al., Electric-Field Switching of Two-Dimensional Van Der Waals Magnets. Nat. Mater. 17, 406-411 (2018)
10. Deng Y, et al. Gate-tunable room-temperature ferromagnetism in two-dimensional Fe3GeTe2. Nature 563(7729): 94-99(2018)
11. Burch K S, et al., Magnetism in two-dimensional van der Waals materials. Nature 563(7729): 47-52 (2018)
12. Ghazaryan D, et al. Magnon-assisted tunnelling in van der Waals heterostructures based on CrBr3. Nat. Elec. 1(6): 344349 (2018)
13. Kim M, et al. Micromagnetometry of two-dimensional ferromagnets. Nat. Elec. 2(10): 457-463 (2019)
14. Klein D R, et al. Enhancement of interlayer exchange in an ultrathin two-dimensional magnet. Nat. Phys. 15(12): 12551260 (2019)15. Otrokov M M, et al. Prediction and observation of an antiferromagnetic topological insulator. Nature, 576(7787): 416-422(2019)
16. Li J, et al. Intrinsic magnetic topological insulators in van der Waals layered MnBi2Te4-family materials. Sci. Adv., 5(6):eaaw5685 (2019)
17. Wu J, et al. Natural van der Waals heterostructural single crystals with both magnetic and topological properties. Sci. Adv.,
5(11): eaax9989 (2019)18. Inoshita T, et al. Topological semimetal phases manifested in transition metal dichalcogenides intercalated with 3d metals.
Phys. Rev. B, 100(12): 121112 (2019)19. Ghimire N J, et al. Large anomalous Hall effect in the chiral-lattice antiferromagnet CoNb3S6. Nat. Comm. 9(1): 1-6 (2018)20. Huang P, et al. Crystalline chirality and interlocked double hourglass Weyl fermion in polyhedra-intercalated transition
metal dichalcogenides. NPG Asia Mater. 13(1): 1-9 (2021)
21. Liu P, et al., Chiral Dirac fermions protected by SU (2) flavor symmetry in spin-orbit-free antiferromagnetic semimetals.
arXiv preprint arXiv:2107.09984, 2021.

－ 50 －

BL-1

20AU002

Termination-Dependent Surface States of Cr2O3 on
Graphene Covered Ni(111) Substrate
Xueyao Hou a, Shiv Kumar b, Masahiro Sawada b,

a Department of Physical Science, Graduate School of Science, Hiroshima Universityb Hiroshima Synchrotron Radiation Center, Hiroshima University
Keywords: Cr2O3, heterointerface, band structure, ARPES

Interfaces between a magnetic material and heavy metal that has a large spin-orbit coupling exhibit avariety of spin-related interfacial phenomena, such as anomalous Hall effect, interfacial DzyaloshinskiiMoriya interaction. These phenomena have been of great importance in spintronic applications.
Investigations of such interfacial properties have been extended to heavy metal/antiferromagnet systems. It
has so far been predicted that a nontrivial band gap opening in the graphene Dirac bands asymmetric between
K and K’ valleys at antiferromagnets Cr2O3/graphene interface, which is mediated by symmetry protectedboundary magnetization of Cr2O3. The successful fabrication of Cr2O3 on graphene covered Ni(111)substrate pave the way for verifying the band alignment of Cr2O3/graphene heterostructure. A narrowed band
gap of ultrathin α-Cr2O3 than that of crystal was also observed by STS. While the progresses are impressive,
it should be recognized that directly experimental observation of an interfacial band structure of
Cr2O3/graphene is still lack. Antiferromagnet Cr2O3 exhibits a spontaneous ferromagnetic termination(boundary magnetism) which is insensitive to the surface roughness. A general consensus is that the lattice
parameter c of the topmost several layers is compressed. In fact, the surface structure of Cr2O3 can be quite
complex. The variation of fabrication method, oxygen pressure could make peculiar reconstruction andoccupation states in ultrahigh vacuum environment. And it is possible that the electronic structure ofheterointerface is distinguished to their bulk and forms particular band structure specific to the its termination
structure. From this perspective, it is beneficial to direct describe band structure of ultrathin Cr2O3/graphene.
In this study, ultrathin Cr2O3 thin films are deposited on Gr/Ni(111) by O2-assisted MBE method. Here,we confirmed O-terminated and Cr-terminated Cr2O3 thin films coexistence on graphene covered Ni(111)substrate by ARPES and first principle calculation. The Cr-terminated model shows novel surface states at
BE~0.25 eV due to the uncompensated dangling bonds formed a , while O-terminated model shows
traditional bulk band characteristic. Besides, the adsorption type of graphene is depending on not onlyNi(111) substrate but the Cr2O3 termination. Graphene is freestanding type with clear Dirac cone at BE~2
eV O-terminated model, but keeps metallic characteristic in Cr-terminated model.

Figure 1 (a)-(c) LEED pattern of Ni(111), Graphene/Ni(111), and 0.5nm-Cr2O3/Graphene/Ni(111), (d) corresponding
differentiated AES spectra, (e) spot-spot distance of corresponding sample, (f) Cr L2,3 XAS spectrum. Red, green and blue
curves corresponding to Ni(111), Graphene/Ni(111), and 0.5nm-Cr2O3/Graphene/Ni(111).

－ 51 －

Figure 1 ARPES intensities along ΓK direction of (a) graphene/Ni(111) (b)0.75 nm-Cr2O3/Graphene/Ni(111) and (c) zoom-in
image of (b) 0.75 nm-Cr2O3/Graphene/Ni(111). (d)-(f) are corresponding ARPES along ΓM direction. One can find two typeof graphene coexist in Cr2O3/Graphene/Ni(111) interface, Cr2O3 3d states not only dominating at BE~1.9 eV, but also shows
a flat band around BE~0.25 eV.

REFERENCES[1]. T. Ashida, M. Oida, et al, Appl. Phys. Lett., 2015, 106,132407.
[2]. T. Kosub, M. Kopte, et al, Nat. Commun., 2017, 8, 13985.
[3]. A. Lodesani, A. Picone, et al, ACS Nano, 2019, 13, 4361.
[4]. H. Takenaka, S. Sandhoefner, et al, Phys. Rev. B, 2019, 100, 125156.

－ 52 －

BL-1, µ-LaserARPES
BL-1&μ-LaserARPES

20AU018

High-resolution ARPES of heavily overdoped Bi2201:
evaluation of coupling parameters
Y. Miyaia, K. Ishibab, Shiv Kumarc, T. Kurosawad, M. Odad, K. ShimadacaGraduate School of Science, Hiroshima University, Kagamiyama 1-3-1, Higashi-Hiroshima 739-8526,
Japanb
Factory of Science, Hiroshima University, Kagamiyama 1-3-1, Higashi-Hiroshima 739-8526, Japanc
Hiroshima Synchrotron Radiation Center, Hiroshima University, Kagamiyama 1-3-1, Higashi-Hiroshima
739-8526, Japand
Department of Physics, Hokkaido University, Sapporo 060-0809, Japan
Keywords: High-TC Superconductor, ARPES, Many-body interactions

High transition-temperature (TC) cuprate superconductors have attracted much interest since theirdiscovery in 1986. While there have been extensive studies, there still exist many unsettled physicalphenomena such as the bad metal state, density waves formation, magnetic fluctuations, and nematic phases[1-4]. These quantum states are emergent from the competing charge, spin and lattice degrees of freedoms.
To understand the origin of the physical properties of cuprates, it is desirable to disentangle these competinginteractions and quantify each contribution.
To this end, here we focus on the Bi-based high-TC cuprate, (Bi,Pb)2Sr2CuO6+δ (Bi2201). The Fermi
surface of Bi2201 is relatively simple because there exists a single CuO2 plane in the unit cell. While it has
been widely believed that the electronic state in the heavily overdoped region is the Fermi Liquid like,
Kurashima et al. recently have found a ferromagnetic fluctuation close to the vanishing TC [3]. Previously
Meevasana et al. have done angle-resolved photoemission spectroscopy (ARPES) and examined the selfenergy due to the electron-electron and electron-phonon interactions, especially for the heavily overdoped
Bi2201 with no superconducting transition [5]. However, each contribution from the electron-phonon (orelectron-boson) and electron-electron interactions as a function of hole concentration has not been
quantitatively elucidated. Our purpose is, therefore, to systematically clarify the evolution of theseinteractions as a function of the hole concentrations as well as TC.
In this study we examined the electronic structure of as grown Bi2201 single crystal with TC=6 K. We
have done linear-polarization dependent ARPES experiments on BL-1 [6] to investigate the Fermi surface
and energy band dispersion in the wide energy range. To examine the fine details of the electron-phonon
interaction near the Fermi level, we have done high-resolution ARPES using focused ultraviolet laser light[7].
Figure1(a) shows the observed Fermi surface taken at hν=90 eV at 50 K. In addition to the hole-like
Fermi surface, we have observed additional Fermi surfaces moved by (π/2,π/2), which suggests
superstructure formation. To quantitatively analyze the shape of the Fermi surfaces, we have employed thetight-binding (TB) model [8]. The red line shows the best fit result using the two-band TB model. We havealso tried the single-band TB model [5], but we could not sufficiently reproduce the shape of the Fermi
surface. It may indicate the importance of the hybridization effect.
Figure 1(b) and 1(c) show the band structure along the nodal direction (cut 1) and the anti-nodal direction(cut 2) taken at hν=40 eV. We measured cut 1 (cut2) using s-(p-)polarization geometry taking into accountthe diple selectrion rule [6,9]. One can see the reduction of the group velocity near the Fermi level comparedwith the TB model. The deviation is assumed to derive from the self-energy due to electron-electroninteraction. We also overlaid the LDA result [10] for Bi2201 in Fig. 1(b). One can see that the deviation of
the group velocity is not so strong compared with the two band TB model, which should be examined in
more detail in the future.
We have also measured fine details near the Fermi level using the μ-Laser ARPES machine. We haveobserved a clear kink structure around -80 meV. We have extracted the self-energy due to the electronphonon and electron-electron interactions referring to the two-band TB model.

－ 53 －

(a) Fermi surface of Bi22012 taken at hν = 90 eV at 50 K. The red lines show calculated Fermi
surface using the two-band TB model. ARPES image plot along (b) the nodal direction (cut 1 in (a)) and (c)the anti-nodal direction (cut 2 in (a)) respectively. Red lines in (b) and (c) show the band dispersions
calculated by the two-band TB model. The white line in (b) shows a band dispersion obtained from the LDA
calculation [10]. Blue dots in (b) and (c) show the band points evaluated by the momentum distribution curve
analyses.
FIGURE 1.

REFERENCES
1. P. Choubey et al., Proceedings of National Academy of Sciences, 117, 14805 (2020); Z. Du et al., Nature 580, 65 (2020).
2. Y.Y. Peng et al., Nature Materials 17, 697 (2018).
3. K. Kurashima et al., Physical Review Letters 121, 057002 (2018).
4. N. Auvray et al., Nature Communications 10, 5209 (2019).
5. W. Meevasana, et al., Physical Review B 75, 0174506 (2007); Physical Review B 77, 104506 (2008).
6. H. Iwasawa, K. Shimada et al., Journal of Synchrotron Radiation 24, 836 (2017).
7. H. Iwasawa et al., Ultramicroscopy, 182, 85 (2017).
8. C.E. Matt et al., Nature Communications 9, 972 (2018).
9. H. Hayashi, K.Shimada et al., Physical Review B 87, 035140 (2013).
10. J. Nokelainen et al., Physical Review B 101, 214523 (2020).

－ 54 －

20AU019

BL-1

Topological Surface State in Sb, Te, and Se Based Single CrystalsaAmit Kumara, Shiv Kumara, Gopi Govindhanb, Aravinth Kumarb,
Anandha B. Govindanb, Kenya Shimadaa*

Hiroshima Synchrotron Radiation Center, Hiroshima University, Kagamiyama 2-313, HigashiHiroshima 739-0046, Japanb
Department of Physics, SSN College of Engineering, Kalavakkam - 603110, Tamilnadu, India
Keywords: Topological Insulators, ARPES

Three-dimensional (3D) topological insulators (TIs) belong to the new quantum state of matter family,
which has attracted great attention since last decade because of their unique physical properties and potential
applications to future devices [1]. The 3D TI has an energy gap at the Fermi level (EF) in the bulk bands but
has gapless topological surface states (TSSs). The metallic property of the TSS is robust against perturbations
that conserve the topological numbers. Studies on a layered semiconductor Sb2Te3 and related compounds
have increased tremendously since Sb2Te3 has been predicted and demonstrated to behave as a 3D TI. From
the recent reports on Sb2TeSe2, however, it is not clear that whether it belongs to the TI family or not. While
Zhang et al. reported that Sb2TeSe2 belongs to the TI family by ab-initio calculations in 2010 [2],Menshchikova et al. suggested that it is a normal insulator with a 100 meV bandgap in 2013 [3]. Therefore,
it is desirable to experimentally clarify the TSSs in these families.
In this study, we examined the electronic band structure of Sb2Te3 and Sb1.9Sn0.1TeSe2 single crystals
using angle-resolved photoemission spectroscopy (ARPES). These samples behave as p-type semiconductor
and have the Dirac point inside the bulk bandgap. The latter property is favorable for dissipationless spinbased electronic devices because the surface to bulk scattering is suppressed [4,5].
We have done ARPES experiments on the linear undulator beamline BL-1 [6] with the p-polarizationgeometry. We cleaved the sample in the ultrahigh vacuum to get clean surfaces. Based on the core-level
photoemission spectra, we have confirmed all the elements included in these crystals.
(a)

(b)

(c)
(c)

(d)

(e)

Figure 1. ARPES results of Sb2Te3 and Sb1.9Sn0.1TeSe2 taken at hν = 25eV and at 22 K. (a) and (c) show ARPES image plots% directions, respectively. (b) and (d) show ARPES image plots of Sb1.9Sn0.1TeSe2 alongof Sb2Te3 along Γ" − 𝐾𝐾% and Γ" − 𝑀𝑀% and Γ" − 𝑀𝑀% directions, respectively. (e) The constant energy contours of Sb1.9Sn0.1TeSe2.
Γ" − 𝐾𝐾－ 55 －

Figures 1(a)-(d) show the ARPES image plots of Sb2Te3 and Sb1.9Sn0.1TeSe2 single crystals taken at% and Γ" − 𝑀𝑀% high symmetry directions
photon energy of hν = 25 eV at temperature of 22 K along the Γ" − 𝐾𝐾of the surface Brillouin zone. In Fig. 1(a), one can clearly see a Rashba-like band splitting in Sb2Te3 around% direction. However, the splitting is not visible alongthe Γ" point at the energy of -0.8 eV along the Γ" − 𝐾𝐾"%
the Γ − 𝑀𝑀 direction. Based on the analysis of the momentum distribution curves (MDCs), we haveevaluated the Fermi wavenumber as 𝑘𝑘! ~0.06 Å-1 and Fermi velocity as 𝑣𝑣! ~ 4.9 × 10-5 m/s for Sb2Te3,
which is consistent with previous study [4]. On the other hand, in Figs. 1(b) and 1(d), the ARPES linewidthof Sb1.9Sn0.1TeSe2 becomes much broader, and Rashba-like band splitting around the Γ" point is not observed.
We found that the evaluated Fermi wavenumber 𝑘𝑘! ~ 0.075Å-1 and the Fermi velocity 𝑣𝑣! ~ 5.6 × 10-5% direction, the bulk valence bands do not crossm/s were slightly larger than those of Sb2Te3. Along the Γ" − 𝐾𝐾"%
EF whereas they cross EF along the Γ − 𝑀𝑀 direction.
Figure 1(e) shows constant energy contour plots in the momentum space at hν = 25 eV at 22 K. A
rounded hexagonal Fermi surface derived from the surface state has been observed at EF. While the Fermi% directions.
surface derived from the bulk valence bands are weak at EF, they are discernible along the Γ" − 𝑀𝑀
The bulk valence bands become more visible below energy of -0.2 eV. While the lattice symmetry should be
six folded, the distribution of the photoemission intensity looks three-fold symmetric at energies between -0.1
eV to -0.3 eV. Note that the six-fold symmetry of the valence band is clearly visible at energies of -1.0 eV and 1.2 eV. We assume this is due to the matrix element effects.
The states crossing EF in Sb1.9Sn0.1TeSe2 look like a TSS, as in pristine Sb2Te3 in Fig. 1(a). To further
confirm the TSS, photon energy-dependent ARPES measurement on the Sn-doped and pristine Sb2TeSe2
samples are required.

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

Xiao-Liang Qi and Shou-Cheng Zhang, Rev. Mod. Phys. 83, 1057 (2011).
Wei Zhang et al., New J. Phys. 12, 1367 (2010).
T.V. Menshchikova et al., Nano Lett. 13, 6064 (2013).
C.K. Lee, C.M. Cheng, S.C. Weng et al., Sci. Rep. 6, 36538 (2016).
T.T. Yeh, C.M. Tu, W.H. Lin et al., Sci. Rep. 10, 9803 (2020).
H. Iwasawa, K. Shimada et al., J. Synchrotron Rad. 24, 836 (2017).

－ 56 －

21AG025

BL-1

Band Dispersion Unique To The Cobalt Intercalation In
The Noncentrosymmetric Antiferromagnet CoNb3S6
Hiroaki Tanaka a, Shota Okazaki b, Kenta Kuroda a, Shiv Kumar c,
Takao Sasagawa b, and Takeshi Kondo aaInstitute for Solid State Physics, The University of Tokyo, Kashiwa, Chiba 277-8581, Japanb
Materials and Structure Laboratory, Tokyo Institute of Technology, Yokohama,
Kanagawa 226-8503, Japanc
Hiroshima Synchrotron Radiation Center, Hiroshima University, Higashi-Hiroshima,
Hiroshima 739-0046, Japan

Keywords: Angle-resolved photoemission spectroscopy (ARPES), magnetic Weyl semimetals.

CoNb3S6 [Fig. 1(a)] is a transition metal dichalcogenide NbS2 with cobalt intercalation. Neutron
scattering experiments have revealed that intercalated Co atoms emerge collinear antiferromagnetic structure
along in-plane direction [1]. Furthermore, weak ferromagnetism along out-of-plane direction and anomalous
Hall effect have been observed recently [2]. However, the anomalous Hall response is giant compared to theweak ferromagnetic moments, and the mechanism of this giant anomalous Hall effect has been unclear.
We studied the electronic structure of CoNb3S6 by comparing it with that of FeNb3S6, which isintercalated with Fe instead of Cu to NbS2. Single crystals of CoNb3S6 and FeNb3S6 were synthesized bychemical vapor transport method. Angle-resolved photoemission spectroscopy (ARPES) measurements
using the vacuum ultraviolet (VUV) light were performed at BL-1 of HiSOR. We used the 120 eV light as alight source. The measurement temperature was kept lower than 20 K for CoNb3S6 and 35 K for FeNb3S6,
where both temperatures are lower than TN of these materials. All samples were cleaved in situ along the(001) direction at an ultrahigh vacuum of 5.0 x 10-9 Pa.
Figure 1(b) shows the band dispersions of CoNb3S6 and FeNb3S6. The original material NbS2 without theintercalation has two hole pockets around the Γ� point and these dispersions were also observed in theintercalated materials, as highlighted by red dashed curves. Furthermore, an additional band dispersion
appeared near the Fermi level only in CoNb3S6 [blue dashed curves in Fig. 1(b) left panel]. This dispersion
seems to come from Co atoms, because such a band dispersion does not exist in the band dispersion of NbS2,
FeNb3S6, and CrNb3S6 [3]. Therefore, we conclude that this electronic structure is unique to the cobalt
intercalation, and possibly responsible for the exceptionally large anomalous Hall effect of CoNb3S6.

FIGURE 1. Crystal structure and band dispersions of CoNb3S6 and FeNb3S6. (a) Crystal structure of CoNb3S6 and FeNb3S6.
� direction of NbS2. Band structures originating from NbS2 are(b) Band dispersions of CoNb3S6 and FeNb3S6 along the Γ� − 𝑀𝑀highlighted by red curves and an additional band in CoNb3S6 is highlighted by blue curves.

－ 57 －

REFERENCES
1. S. S. P. Parkin, et al., J. Phys. C: Solid State Phys. 16 (1983), 2765.
2. N. J. Ghimire et al., Nat. Commun. 9 (2018) 3280.
3. N. Sirica et al., Phys. Rev. B 94, 075141 (2016).

－ 58 －

19AG047

BL-06
BL-6

Development of On-site Cleaning Method of Carbon
Contamination with Atomic Hydrogen 2
Takashi Tokushimaa,b,c, Masahito Niibe,a Tomohiro Konod, Yuka Horikawad, andHiroaki YoshidaeaLaboratory of Advanced Science and Technology for Industry, University of Hyogo, Komigori, Hyogo,
JapanbSanka High Technology Co. Ltd., Tatsuno, Hyogo, Japanc
MAX IV Lab. Lund Univ., Lund, SwedendGraduate School of Sciences and Technology for Innovation, Yamaguchi University, Yamaguchi, Japane
Department of Physical Science, Graduate School of Science, Hiroshima University, Higashi-Hiroshima,
Japan
Keywords: Contamination, Cleaning, Atomic hydrogen.

Carbon contamination of optical elements in synchrotron radiation beamlines is a common and serious
problem [1-4] since accumulation of carbon on the surface makes thick carbon layer which cause drasticallydecrease of intensity even around 1KeV. There is a cleaning method which uses ozone and UV light but themethod is not applicable for easily-oxidizable-metal such as Ni, Cr, etc. since Ozone cause surface oxidation.
Therefore, the cleaning technique for this kind of mirrors is not established.
Recently, we reported off-site cleaning treatment using the atomic hydrogens (AH) is very effective inremoving contamination of mirrors [5]. In response to this, we received requests from many people todevelop AH cleaning equipment that can be used “on-site”. Therefore, we began to develop a small on-sitecleaning device that can be mounted in a mirror vacuum chamber. Here we report on the tests of the cleaning
method conducted at the HiSOR beamline BL-6 of Hiroshima University.
Fig. 1 shows the AH generator apparatus attached to the grating chamber of HiSOR BL6. Fig.2 shows

FIGURE 1. Atomic hydrogen generator setup attached to thegrating chamber. The blue cylinder is a hydrogen canister usinghydrogen storage alloy, which makes safe handling of hydrogen
without using high pressure canister.

－ 59 －FIGURE 2. Atomic hydrogen generator headin the grating chamber.
Since the filaments is too bright for taking
photos, applied current is reduced from typicalvalue when taking photo shot.

the head part of the AH generator apparatus which consists of tungsten (W) filament, current introducing
electrode, hydrogen gas introduction piping, hydrogen gas source and flow regulator. These parts can beeasily installed in the vacuum chamber of the beamline by fixing/positioning them to the short pipe with 70
mm CF flange. By using a canister with a low-pressure hydrogen storage alloy as the gas source, cleanhydrogen gas can be easily supplied into the mirror chamber at the synchrotron radiation experimental hallwhere it is difficult to introduce a high-pressure dangerous gas cylinder. Hydrogen gas was introduced in thevacuum chamber to contact with W filaments heated to about 1700 °C for generation of AHs by catalytic
reaction.
In the test experiment at the beamline in the last year, we cannot confirm cleaning effect and conclude
that amount of AH is not sufficient to remove carbons from surface within short time scale. Thus, we have
improved generator by increased number and length of filament, an expected surface area of filaments
becomes around 3 times compared to the last time. For measurement of intensity of the beam we have newly
installed gold mesh before grating which makes easier to monitor beam intensity accurately.
We have tested the atomic hydrogen cleaning of the contaminated M2 mirror of the monochromator in
BL-6 of the HiSOR facility. Fig. 3 shows the intensity profile of the monochromator before and after the AH
treatment. Overall intensity seems slightly increased, however there is no improvement after repeating AH
cleaning, therefore it might be error of measurement.
0.25

Mf PD / S1 Au current

0.20

C1s

0.15

0.10

Initial state
After AH cleaning 1
After AH cleaning 2
After AH cleaning 3

0.05

0.00

-500000-400000-300000-200000

Pulse (counts)

FIGURE 3. Intensity profiles of the monochromator before and after the atomic hydrogen treatment.

Our conclusion is the etching speed of carbon by atomic hydrogen is low and it is not enough to remove
the thick layer of carbon which is evidently seen by eye. Recently, oxygen cleaning was introduced to recoverfrom carbon contamination at MAX IV, which dosing oxygen gas around 1E-6 mbar while shining beamaround oxygen edge an quickly remove carbons around a day for whole energy region of monochromator.
This method works fine, however, weak absorption probably comes from oxidization of surface is observed.
Atomic hydrogen is effective to dioxide oxides as we observed in tungsten oxides. Therefor combination ofoxygen cleaning with atomic hydrogen cleaning will be a candidate of method to clean surface of mirrors.

REFERENCES
1.
2.
3.
4.
5.

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

－ 60 －

20AG055

BL-6

Comparison of soft X-ray absorption spectra oftransition metal sulfates
Kiminori Babaa and Hiroaki Yoshidaa,baPhysics program, Graduate School of Advanced Science and Engineering, Hiroshima University,
1-3-1 Kagamiyama, Higashi-Hiroshima, 739-8526 Japan.
b
Hiroshima Synchrotron Radiation Center, Hiroshima University,
2-313 Kagamiyama, Higashi-Hiroshima, 739-0046 Japan.

Keywords: Soft X-ray absorption spectrum, 3d transition metal sulfate, H2O, SO42-, bond length

The 3d transition metal (Fe, Co, Ni, Cu, and Zn) sulfates make an octahedral structure with 6 coordination
containing some water molecules. Those complexes have various numbers of the coordinated water such as
FeSO4_7H2O, NiSO4_6H2O, and CuSO4_5H2O. Only copper combines directly with SO42- in five elements.
Four H2O are coordinated in the same plane and two SO42- are coordinated out of plane (Figure 1). To
investigate how the electronic states of coordinated water molecules are affected by the central metal atoms,
soft X-ray absorption spectra have been measured in the energy range of 530~545 eV (O1s) and 690~1050
eV (metal 2p) of the 3d transition metals at BL-6 [1] of the HiSOR.
Figure 2 shows the total electron yield (TEY) spectra of the 3d transition metal sulfates in the O1s edge.
The lowest peaks are observed around 532.8 eV and their energies depend on the 3d transition metals. There
are two kinds of oxygen atoms (H2O and SO42-) in the present complexes. To distinguish them, the TEY
spectrum of anhydrous copper sulfate (CuSO4) is also measured and compared with CuSO4_5H2O (Figure
3, top panel). The lowest peak observed only in CuSO4_5H2O is thus ascribed to contribution of thecoordinated water molecules. The difference spectrum is calculated by normalizing the two spectra with thenumber of oxygen atoms (Figure 3, middle panel) and compared with the spectrum of a gaseous watermolecule [2] (Figure 3, bottom panel). Comparing to the 4a1 peak of gaseous water molecule, the lowest
peak of the coordinated water is shifted to lower energy side by about 1 eV and broadened. The lowest peakenergies of the 3d transition metal (Fe, Co, Ni, Cu, and Zn) sulfates at the O1s edge are summarized in Table
1.
TEY spectra are also measured at metal 2p edge. There observed spin-orbit separation peaks of 2p3/2 and2p1/2 to 3d transitions with the energy separation of 13~21 eV except Zn, which has no hole in the 3d orbit.
Comparing the experimental transition energies of 2p3/2 to 3d with the calculated differences of binding
energies between 2p and 3d electrons, the largest discrepancy for Cu is considered to be ascribed to direct
coordination of SO42- to the central metal.

REFERENCES
1. H. Yoshida et al., J. Electron Spectrosc. Relat. Phenom., 144-147(2005) 1105-1108.
2. S. Myneni et al., J. Phys. Condens. Matter 14 (2002) L213-L219.

－ 61 －FIGURE 1. Schematic diagram of CuSO4_5H2O. -

FIGURE 2. TEY spectra of 3d transition metal(Fe,Co,Ni,Cu, and Zn) sulfates at the O1s edge.

TABLE 1 The lowest peak energies of the 3d transition
metal (Fe,Co,Ni,Cu, and Zn) sulfates at the O1s edge.
Peak energy (eV)FIGURE 3. TEY spectra of(top) CuSO4_5H2O and CuSO4,(middle) calculated ligand (H2O) component, and(bottom) H2O (gas).

FeSO₄_7H₂O

532.0

CoSO₄_7H₂O

532.4

NiSO₄_6H₂O

532.0

CuSO₄_5H₂O

532.8

ZnSO₄_7H₂O

532.0－ 62 －