Spin- and angle-resolved photoemission study of MAX phase compound
V2AlC
Takahiro Ito1,2, Masashi Ikemoto2, Damir Pinek3, Koichiro Yaji4, Masashi Nakatake5,
Shik Shin4, and Thierry Ouisse3

1

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

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

4

5

Grenoble Alpes, CNRS, Grenoble INP, LMGP, F-38000 Grenoble, FranceInstitute for Solid State Physics, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba
277-8581, Japan

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

MAX phases (Mn+1AXn, where M is an early transition metal, A belongs to groups 13-15
and X is either C or N, n = 1 - 3) have recently attracted much attention due to their possible
application to the production of a new class of two-dimensional (2D) systems called MXenes[1]. However, the bulk electronic structure of MAX phases has been studied mostly throughab initio, DFT calculations, mainly due to a lack of single crystalline samples. We have
performed angle-resolved photoemission spectroscopy (ARPES) on several MAX phasesingle crystals to directly investigate the electronic structure of these systems [2,3]. Amongthe MAX phases, V2AlC has been expected to be categorized as a high-symmetry pointsemi-metal with crossing point with some Dirac-like properties referred as “Dirac point (DP)”at the Fermi level (EF) along ΓM line together with “nodal-line (NL)” around 0.2 eV from
DFT calculation [3, 4]. To elucidate the electronic structure, especially, the existence of DP
and spin-polarized states, we have performed spin - and angle - resolved ARPES (SARPES)
with using a quasi - continuous - wave laser [5].
As a result, we have found the formation of the Dirac cone is consistent with DFT
calculation (Fig.1). Furthermore, the SARPES clearly demonstrates spin-polarized branches(S+ and S-) in the larger electron pocket (eL), where the spin directions are opposite to eachother and tangential to the Fermi surface along the ΓM line (Fig.2). Furthermore, missing of
the out-of-plane spin polarization as well as inversion symmetry breaking at the observed

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

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

spin-polarization with respect to the Γ point have been elucidated. There are at least two typesof mechanism to understand the origin of the observed spin-textures. One is the existence ofhelical spin texture around the M point with anti-parallel spin splitting due to Rashba effect[6]. The other is the formation of spin-polarized “drum-head surface states” connecting NL[7]. We expect that the present observation of the spin-textures on V2AlC might suggest thepotential of layered MAX phases as new series of topological materials.
REFERENCES[1] M. Barsoum, MAX phases (Wiley, Weinheim 2013).
[2] T. Ito et al., Phys. Rev. B 96, 195168 (2017).
[3] D. Pinek et al., Phys. Rev. B 98, 035120 (2018).
[4] T. Zhang et al., Nature 566, 475 (2019).
[5] K. Yaji et al., Rev. Sci. Instrum. 87, 053111 (2016).
[6] V. Sunko et al., Nature 549, 492 (2017).
[7] G. Bian et al., Phys. Rev. B 93, 121113(R) (2016).

SPIN-ORBITAL TEXTURE IN QUASI-ONE-DIMENSIONAL GIANTRASHBA SYSTEM ON BI/INAS(110)-(2×1)
Takuto Nakamura1, Yoshiyuki Ohtsubo2,1, Ayumi Harasawa3, Koichiro Yaji3,
Shik Shin3, Fumio Komori3, Shin-ichi Kimura2,1
1

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

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

(b)

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

(a) Sy (p-pol.)(b) Sy (s-pol.)(c) Sx (p-pol.)Figure 2. Spin-resolved energy distribution curves of Bi/InAs(110)-(2×1)
along Γ X (parallel to the surface Bi chains). The solid (open) triangles
correspond to spin polarization parallel (antiparallel) to (a-b) [001] and (c)[1̅10] direction. θe = 10° corresponds to k// = 0.14 Å-1.
Figure 2 (a) and (b) show the spin-resolved energy distribution curves (EDCs) taken with pand s-polarized lights, respectively. The peak positions of the EDCs agree well with the surface
band dispersions shown in Fig. 1. The spin polarizations of the peaks in Figs. 2 (a) and (b)invert together with the sign inversion of the emission angles (θe), indicating that these spinpolarizations obey time-inversion symmetry. The polarization orientations in Fig. 2 (b) are thesame for one side of the emission angles. These spin-polarization is consistent with a typical,
helical spin texture expected from the Rashba-type SOI. In contrast, the polarizations in Fig. 2(a) inverts within the same parabola (e.g. see -5.4° and -11.4°). Based on our density-functionaltheory (DFT) calculation, it would be because the fractional ratio of the orbital componentschanges depending on the k// positions; the pz orbital of surface Bi is entangled with the opposite
spin polarizations to the other orbitals. Figure 2 (c) shows spin-resolved EDCs for spinpolarization along [1̅10], parallel to k//. One can find clear polarizations whose sign do notchange with the sign of emission angle. Apparently, such behaviour breaks the time-inversion
symmetry. Since this sample has no magnetic order, this spin polarization should not from thespin polarization of the initial states but from photoexcitation process, so called final-state effect.
In summary, we have found many unconventional spin textures on Bi/InAs(110), whichcannot be understood by a simple helical spin texture expected for Rashba effect. Detailed
analysis including DFT calculations and spin-dependent photoexcitation processes are ongoing.
REFERENCES[1] É. I. Rashba, Sov. Phys.-Solid State 2, 1109 (1960).
[2] M. Z. Hasan and C. L. Kane, Rev. Mod. Phys. 82, 3045 (2010).
[3] A. Manchon et al., Nature Mat. 14, 871 (2015).
[4] K. Yaji et al., Nat. Commun. 8 (2017) 14588.
[5] R. Noguchi et al., Phys. Rev. B 95, 041111(R).
[6] K. Kuroda et al., Phys. Rev. B 94 (2016) 165162.
[7] T. Nakamura et al., Phys. Rev. B 98, 075431 (2018).

SPIN-POLARIZED QUASI-ONE-DIMENSIONAL METALLIC
SURFACE ELECTRONIC STATE OF BI/GASB(110)-(2×1)
Takuto Nakamura1, Yoshiyuki Ohtsubo2,1, Kenta Kuroda3, Ayumi Harasawa3, Koichiro Yaji3,
Shik Shin3, Fumio Komori3, Shin-ichi Kimura2,1
1

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

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

(b)

(c)

ഥ
Y

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

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

It would be due to the qualitatively common surface atomic structures of Bi/GaSb(110)-(2×1)
and Bi/InAs(110)-(2×1), the “missing – row” model with the zig-zag Bi chains [7, 8].
Figures 2 shows the spin-resoled energy distribution curves (EDCs) at the Fermi wavevector(kF) along Y-M taken by SARPES. The spin-polarized peaks in Fig. 2 were clearly observed
around EF. These peak positions are consistent with the band dispersions in Fig. 1 (b). The spinpolarization of these peaks inverts together with the signs of the wavenumbers, indicating thetime-reversal symmetry, consistent with the Rashba-type spin-splitting.
These results suggest that Bi/GaSb(110)-(2×1) surface is the good Q1D candidate forfollowing researches of spin-dependent phenomena, hosting a metallic and widely spin split
surface states in Q1D.
REFERENCES[1] É. I. Rashba, Sov. Phys.-Solid State 2, 1109 (1960).
[2] M. Z. Hasan and C. L. Kane, Rev. Mod. Phys. 82, 3045 (2010).
[3] A. Manchon et al., Nature Mat. 14, 871 (2015).
[4] B. Braunecker et al., Phys. Rev. B 82, 045127 (2010).
[5] V. Mourik et al., Science 336, 1003 (2012).
[6] T. Nakamura et al., Phys. Rev. B 98, 075431 (2018).
[7] M. G. Betti et al., Phys. Rev. B 59, 15760 (1999).
[8] S. C. A. Gay and G. P. Srivastava, Phys. Rev. B 61, 2688 (2000)

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

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

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

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

INVESTIGATION OF TOPOLOGICAL PROPERTIES IN
SUPERCONDUCTING TASE3
Chun Lin1, Ryo Noguchi1, Kenta Kuroda1, Peng Zhang1, Shunsuke Sakuragi1, Jiadi Xu1,
Koichiro Yaji1, Ayumi Harasawa1, Shik Shin1, Atsushi Nomura2, Masahito Sakoda3,
Masakatsu Tsubota4, Satoshi Tanda3, and Takeshi Kondo1
1

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

Department of Physics, Tokyo University of Science3

Department of Applied Physics, Hokkaido University
4

Department. of Physics, Gakushuin University

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

Fig. 1 (a) Crystal structure and unit cell of TaSe3 [7]. (b) Superconductivity in pure and Cu-doped TaSe3 (left)[4], CDW signature in Cu-doped TaSe3 (middle) [5], and direct observation of CDW in mesowire TaSe3 (right)[6]. (c) Strain-induced topological phase diagram of TaSe3 including strong topological insulator (TI), weak TI,
Dirac semimetal, and normal insulator phases [7].

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

REFERENCES[1] X.-L. Qi, S.-C. Zhang, Rev. Mod. Phys. 83, 1057–1110 (2011).
[2] L. Fu and C. L. Kane, Phys. Rev. Lett. 100, 096407 (2008).
[3] P. Monceau, Adv. Phys. 61, 325–581 (2012).
[4] A. Nomura, et al., EPL. 124, 67001 (2019).
[5] A. Nomura et al., EPL. 119, 17005 (2017).
[6] J. Yang et al., ArXiv190609527 Cond-Mat (2019).
[7] S. Nie et al., Phys. Rev. B. 98, 125143 (2018).
[8] A. Perucchi et al., Eur. Phys. J. B. 39, 433–440 (2004).
[9] K. Yaji et al., Rev. Sci. Instrum. 87, 053111 (2016).
[10] P. Zhang et al., Rev. Sci. Instrum. 82, 043712 (2011).

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

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

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

(a)

Bi 5d

(c)

G

G

0

G

Ga 3d

30

25

20

15

10

5

0

Binding Energy (eV)

G

(b)

X

0.1

0.1

up

0

0.2

0.2

y spinBinding Energy (eV)

(b)

(a)

Bi/MnGaHe II

Binding Energy (eV)Intensity (arb. units)[1] N. H. D. Khang et al., Nat. Mater. 17, 808 (2018).
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0.3

0.3
-0.20

-0.10

0

0.10-0.20

Momentum (1/Å)
0.5

-0.4

-0.2

0

0.2

-0.10

0

0.10

Momentum (1/Å)

-0.05

0

Momentum (1/Å)

0.05

down

0.4

0.4

Momentum (1/Å)

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

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

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

(a)

G

(b)

0 (c)

Z

49

s-pol
p-pol

b (ky)

eanalyzer

q

36

y

Chain direction

q (deg)50°hn

E-EF (eV)

72

G

(d)

G

q =72°
High

-0.2
-0.4

28

-0.1

0

0.1

-0.1

0

0.1

Low

-1

20

ky (Å )0 (e)p-pol.

E-EF (eV)100q =72°

Zq =49°(f)Z

q =49°-0.2-0.4

chain (ky)
-0.1

0

0.1

-0.1

0

0.1

-1

ky (Å )

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

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

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

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

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

2

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

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

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

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

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

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

Summary(b)

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

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

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

2

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

ANGLE-RESOLVED PHOTOEMISSION SPECROSCOPY STUDY OF
TYPE-II WEYL SEMIMETAL WTe2
Yuxuan Wan1,2, Lihai Wang3, Keisuke Koshiishi1, Masahiro Suzuki1, Jaewook Kim4, Kenta
Kuroda2, Koichiro Yaji2, Ryo Noguchi2, Ayumi Harasawa2, Shik Shin2, Sang-Wook Cheong3,
Takeshi Kondo2, Atsushi Fujimori1
1

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

The Institute for Solid State Physics, The University of Tokyo

The Center for Quantum Materials Synthesis, Rutgers University

Introduction
So far, no example of a Weyl fermion has been observed in particle physics research. But
some topological materials can host electrons behaving as Weyl fermions, which are called
Weyl semimetals. In Weyl semimetals, there are bands crossing linearly near Weyl points,
which means that electrons near Weyl points have zero-mass [1-3].
In some Weyl semimetals, the hole- and electron-like pockets touch each other at several
Weyl points, and these kinds of Weyl semimetals are called type-II Weyl semimetals. Weyl
points are topologically protected, and topological surface states (Fermi arcs) connected a pairof Weyl points with opposite chirality. WTe2 is a kind of quasi-2D type-II Weyl semimetal. The
space inversion symmetry along c-axis is broken, and the top and bottom surfaces are different.
According to theoretical calculation, the surface electronic states of the two surfaces aredifferent [4]. However, so far there is no experimental research showing the differences between
the two surface states. On the other hand, some of Weyl points in WTe2 locate above the Fermi
level (EF), which cannot be observed by angle-resolved photoemission spectroscopy directly,
and neither the Fermi arcs above EF have been observed. On the other hand, it is difficult to say
whether the band structure is topological surface state or not by ARPES. Spin-ARPES canprovide more evidence.
Experiment
The single crystals of WTe2 with large domains were grown via the iodine vapor transport
method [4]. We cut one piece of WTe2 sample into two halves and turn one of them upside down.
We cleaved them in situ.
The experiment was
carried out with the spinresolved laser ARPES(SARPES) at ISSP, theUniversity of Tokyo. The
light source is 7 eV laserfrom KBBF crystal. We
measured two kinds ofsurfaces of WTe2: surface A
and B. We used the 7 eV
laser ARPES to map theenergycontours
andmeasuredtheband
structures. To observe thetopological Fermi arcs are
Fig.1 Band dispersion on surface A and B of WTe2 measured by laser
surfaces
stateswhich ARPES. The surface states connect the bulk electron- and hole-like
connect hole and electron pocket.

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

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

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

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