SPIN-POLARIZED SURFACE STATES ON
MAX PHASE COMPOUND V2ALC
Takahiro Ito1,2, Masashi Ikemoto2, Damir Pinek3, Koichiro Yaji4,
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

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, JapanMAX 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]. From the recent SARPES study of V2AlC with using a quasi continuous - wave laser [5], the existence of the spin-polarized states has been elucidated [6].
Since the observed spin states show the anti-parallel spin splitting along the tangential axis tothe larger electron pocket around the Γ point, Rashba effect [7] or the formation of thespin-polarized “drum-head surface states” have been expected to be possible origins of thespin states. To pursue the origin of the observed spin-states as well as the other spin-states if
exists on V2AlC, we have performed SARPES study with using He I light source, which isavailable for the spin mapping at the wide momentum space.
Figures 1(a) and (b) show the band structure near EF and the constant energy ARPES image
at 0.2 eV, comparing with the DFT band structure along the ΓM (AL) line and the DFT Fermi
surface of ΓKM (AHL) plane, respectively. While the large (small) electron pocket eL (es)seems to be in consistent with the previous ARPES studies [3,6], we have found an apparent
dispersive feature (SS) appears between eL and es bands. Since the observed SS has not been
reproduced by the bulk DFT calculation, and SS dispersive feature shows similarity with thees band with higher binding energy shift, we expect that the origin of SS to be a “surface
state”. It should be noted that the observed SS has never been observed on the previous
ARPES studies with using UV synchrotron from hν = 40 ~ 140 eV [4] or laser-source of hν ~7 eV [6]. We expect possible origin of SS might be so called surface termination effect at thecleaved surface, which could be different from the other previous studies.
In Fig. 1(c), the SARPES spectra at the momentum cut-m1 to m6 and p1 to p6 are shownwith the spin polarization along y-axis (Sy). Here, y-axis corresponds to the tangential
direction to the electron pockets around the Γ point as shown in Fig. 1(b). Around the eL
pocket, Sy− (Sy+) polarization around 50 - 200 meV following a relatively small anti-parallel
spin feature at the lower binding energy has clearly been observed around kx ~ ±0.5 Å-1. The

observed spin-states around the eL pocket show a good consistency with the previous
laser-SARPES study [6]. On the other hand, we have found a sizable spin polarization around
SS. At the cut-m1 and m2 in Fig. 1(c), Sy− polarization has been observed. Though the strong
polarization around NL (0.2 eV at the cut-m4) might be expected to affect to the polarizationin the wide momentum/energy range, appearance of Sy− polarization below 0.1 eV at the
cut-p1 and p2 should ensure the different origin of the spin-states around SS from the ones
around NL (and/or eL pocket). From the detail comparison of the relative peak position of SS
between Sy+ and Sy− SAPES spectra, Sy+ appears at the lower binding energy side at the cut
m1(m2), while it appears at the higher side at p1(p2). Therefore it has been expected that the
SS spin-polarized states might has spin-splitting which is inverted with respect to the Γ point.
This result suggests that the size of spin-splitting at SS will be much smaller than one aroundeL. To understand whether the mechanisms of the formation of spin-states around eL pocket
and SS are the same or not, further ARPES study choosing surface termination as well as
DFT calculation of spin-texture will be performed in our future work.

Fig. 1: (a) Band structure along ΓM line of V2AlC. White solid (dashed) lines are obtained
from DFT calculation along the ΓM (AL) line. (b) Constant energy ARPES image at 0.2 eV of
V2AlC. Purple solid (dashed) lines are Fermi surface on the ΓKM (AHL) plane obtained by the
DFT calculation. The definition of spin polarization axis (Sy) is indicated by black arrow. (c)SARPES spectra (Sy+:△; Sy−:▼) (left) and spin-polarization (right) of V2AlC at the momentum
cut from m1 (p1) to m8 (p6) in Fig. 1 (a).

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] T. Ito et al., ACTIVITY REPORT OF SYMCHROTRON RADIATION LABORATORY 2018,
90 (2019).
[7] V. Sunko et al., Nature 549, 492 (2017).

ABSENCE OF HYBRIDIZATION BETWEEN SPIN-POLARIZEDBANDS IN QUASI-ONE-DIMENSIONAL GIANT RASHBA SYSTEM
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

Surface electronic structure with helical-spin texture thanks to spin-orbit interaction (SOI)
have been extensively studied not only for basic physics but also for applications to spintronic
devices [1-3]. In order to describe such system, a simple spin texture, one band has only one
spin polarization orientation and orbital character, has been widely used in the earlier studies.
However, recent spin and angle-resolved photoelectron spectroscopy (SARPES) and firstprinciples calculation studies have been revealed that the spin texture of these systems are
actually not so simple, but strongly entangled with the orbital components of the surface band
structure [4-6]. Spin inversion and gap opining due to hybridization between spin-polarizedbands has also been reported about 2D Rashba system such as Bi/Ag quantum well [6, 7].
Although such studies have been carried out extensively for 2D systems, its extension to
one-dimensional (1D) or quasi-1D (Q1D) systems were not achieved yet. In this project, weobserved spin-orbital texture on a giant Rashba-type spin-splitting Q1D system
Bi/InAs(110)-(2×1) [8] by using laser-based SARPES equipment [9].
Figure 1 (a) shows spin-integrated ARPES band mapping of the Bi/InAs(110)-(2×1) surface
along Q1D chain direction with p- polarized photons. Two parabolic bands, S1 and S2, with a
top away from the center of the surface Brillouin zone (k = 0 Å-1) were observed near the
Fermi level. ARPES constant energy contour (not shown) indicated the anisotropic, Q1D
character of both S1 and S2. From the in-plane spin-resolved measurement as shown in figure
1 (b), both parabolic bands show clear spin polarization, and the signs of the spinpolarizations are the opposite to each other. These results are consistent with our previous
data [7], suggests the spin splitting due to Rashba-type SOI.
In order to clarify the hybridization effect of the two spin-polarized bands, we extracted theenergy distribution curves (EDCs) from figure 1 (a) and show the results in figure 1 (d). The
peak positions of the S1 (S2) band are indicated by a filled (open) triangles. Although the twobands are crossing each other at k = 0.21 Å-1, no energy gap was observed. Moreover,spin-resolved EDCs as shown in figure 1 (e) indicated no spin-inversion nor interference at
the crossing point (k = 0.21 Å-1). These results suggest that there is no band modulation in
Bi/InAs(110)-(2×1) surface, such as spin flipping or spin gaps opening , due to thehybridization between the spin-polarized bands. This behaviour is different from that of the
known 2D Rashba systems such as Bi/Ag quantum wells [6,7].
We are currently tracking the detailed changes in orbital components and spin polarization
at the initial state in combination with the density-functional-theory calculation, in order tounderstand this lack of hybridization between Rashba-type spin-polarized surface bands.

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, 14588 (2017).
[5] K. Kuroda et al., Phys. Rev. B 94, 165162 (2016).
[6] R. Noguchi et al., Phys. Rev. B 95, 041111(R) (2017).
[7] H. Bentmann et al., Phys. Rev. Lett. 108, 196801 (2012).
[8] T. Nakamura et al., Phys. Rev. B 98, 075431 (2018).
[9] K. Yaji et al., Rev. Sci. Instrum. 87, 053111 (2016).

Figure1. Surface electronic structure of Bi/InAs(110)-(2×1) surface.
(a) Spin-integrated and (b) spin-resolved ARPES band mapping along
Bi chain direction. Spin-direction was set to in-plane chain normal(k//[001]) direction. (c) The 2D color code for (b). blue-red color
corresponds to spin polarization and the contrast of the color to theintensity of photoelectrons.
(d) Spin- integrated and (e)spin-resolved EDCs. All data were measured at 45K.

TOPOLOGICAL SURFACE STATE MODIFIED BY ADSORPTION OF
ORGANIC DONOR MOLECULE

Tatsuya Kitazawa1, Koichiro Yaji2, Kosuke Shimozawa1, Hiroshi Kondo1, Takayoshi
Yamanaka1, Hiroshi Yaguchi1, Yukiaki Ishida2, Kenta Kuroda2, Ayumi Harasawa2, Takashi
Iwahashi3, Yukio Ouchi3, Fumio Komori2, Shik Shin2, and Kaname Kanai1
1

Department of Physics, Faculty of Science and Technology, Tokyo University of Science
2
Institute for Solid State Physics, The University of Tokyo
3
Department of Materials Science and Engineering, Tokyo Institute of Technology

Topological insulators (TIs) have attracted much attention in quantum matter physics.
Topological surface states (TSS) have a helical-spin texture, in which the spin-locked
electrons are topologically protected against the backscattering from nonmagnetic impurities
due to time reversal symmetry1. This property of TSS will be important in next-generation
spintronic devices because it will allow lower-energy electronic transport. Thus, controlling
the TSS by creating topological interfaces with other materials is vital for using TIs in
spintronic devices. In 2016, Tu et al. reported a topological p-n junction on the surface of
Bi2-xSbxTe3-ySey, a three-dimensional TI fabricated by using electron-accepting organic
molecule F4-tetracyanoquinodimethane (TCNQ), and they achieved on-off switching of
charge current flow originating from TSS2, suggesting that electronic functional organic
molecules are suitable for TI spintronic devices. Recently, there have been several studies of
the TI interface with organic molecules toward using TIs in spintronic devices. Caputo et al.
examined the cobalt phthalocyanine (CoPc)/Bi2Se3 interface3 and observed a slight shift in the
Dirac point (DP) to higher binding energy by angle-resolved photoemission spectroscopy(ARPES) experiments, indicating charge transfer from CoPc to Bi2Se3. This means that the
CoPc/Bi2Se3 interface is a chemisorption system. However, a negligible shift in the Bi 4f corelevels was observed by X-ray photoemission spectroscopy (XPS), indicating little chargetransfer at the interface. These experimental results contradict each other. The interface
between TCNQ, a typical electron acceptor, and Bi2Se3 was studied by Pia et al.4 Theyobserved a DP shift to higher binding energy by ARPES and a shift of the Bi 4f core leveltoward lower binding energy accompanied by TCNQ deposition by XPS measurements.
These results indicate that there is a chemical interaction between adsorbed TCNQ and Bi2Se3.
Therefore, studies of the interfacial electronic structure of the TCNQ/Bi2Se3 interface should
consider chemical interactions, such as charge transfer.
These earlier experimental studies of the interface between the organic molecules and the TI
surface have led to contradictory interpretations of the interactions at the organic molecule/TI
interfaces. Little attention has been paid to the electronic structure of molecular layers,
although this is essential for properly evaluating the interfacial electronic structure. Also, it
can be pointed out that there are few reports on the electron donating molecules (n-type
dopant) although some researches on the electron-accepting molecules (p-type dopant) like
TCNQ and its derivatives have been reported.
In the present work, we directly observed the electronic structure both of the molecularlayers and the TI surface to fully understand the influences on the TSS upon the adsorption of
the molecules. We focused on the interface between tetrathianaphthacene (TTN) and Bi2Se3.
Bi2Se3 has been extensively investigated as a prototypical TI. A Bi2Se3 single crystal
comprises five-atom layers (Se-Bi-Se-Bi-Se) stacked along the [111] direction, which are
known as quintuple layers (QLs). The QLs are attached to each other by weak van der Waals(vdW) forces and the interlayer gap between QLs is called the vdW gap. We chose Bi2Se3
because it has a single Dirac cone at the Γ point in the (111) surface Brillouin zone and a wideband gap of ~ 0.3 eV. This simple electronic structure allows us to trace the DP shiftaccompanied by the molecular deposition easily. To create the organic molecular layer/TI
interface, we vacuum deposited TTN onto Bi2Se3. TTN is a π-conjugated molecule containingfour electron rich sulfur atoms and has a strong donor nature by a small ionization energy of ~4.4 eV. TTN functions as a strong n-type dopant for Bi2Se3 because the Fermi level of Bi2Se3
is lower than the highest occupied molecular orbital of TTN. The role of donor organic
molecules on the TI surface has not been investigated thoroughly, in contrast to acceptor
organic molecules or n-type dopants, like alkali metals and other inorganic materials. Organic
molecules have the advantages of requiring lower-cost methods for creating the interface with
TI and having a lower risk of intercalation into the bulk. Thus, organic molecules are goodcandidates for manipulating the TSS.
In this work, we observed the entire electronic structure of the TTN/Bi2Se3 interface by
examining the TTN thickness dependence of the interfacial electronic state using UPS, XPS,
and ARPES.
Figure 1 shows the Fermi surfaces and the energy band dispersions measured at the
TTN/Bi2Se3 interface for various TTN thicknesses by ARPES. For the 5Å-thick TTN, theDirac point (DP) shifted toward higher binding energy (E−EF = −0.42 ± 0.02 eV) and the
Fermi surface area of TSS increased. The hexagonal warping of the Fermi surface was also
observed. We also observed the emergence of a new state just below the Fermi level with aparabolic band dispersion and hexagonal Fermi surface upon TTN deposition. This new state
was identified as the two-dimensional electron gas state (2DEG) originating from the bulk
conduction band, which is induced by the band-bending derived from the increase in thenumber of TSS electrons.

Fig. 1: Development of the electronic structure at the TTN/Bi2Se3 interface. a) ARPES
spectra of Bi2Se3 and a TTN molecular layer of various thicknesses, showing the Fermi
surface and the energy band dispersion of photoemission intensity. The dashed lines labeled
DP represent the energy of DPs. These ARPES spectra are acquired along the momentum line
slightly off the ΓK line in the surface Brillouin zone. b) Positions of the DP (EDP) and thebottom of 2DEG band dispersion (E2DEG) and c) the Fermi surface area of TSS (STSS) and
2DEG (S2DEG).
REFERENCES[1] P. Roushan et al., Nature 460, 1106 (2009).
[2] N. H. Tu et al., Nat. Comm. 7, 13763 (2016).
[4] M. Caputo et al., Nano Lett. 16, 3409 (2016).
[5] A. D. Pia et al., Chem. Phys. Chem. 19, 2405 (2018).

INVESTIGATION OF TOPOLOGICAL PROPERTIES IN
SUPERCONDUCTING TASE3
Chun Lin1, Ryo Noguchi1, Kenta Kuroda1, 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
3

Department of Physics, Tokyo University of Science

Department of Applied Physics, Hokkaido University
4

Department. of Physics, Gakushuin University

In recent years, a great deal of research activities are associated with the quantum materials
characterized by either the low dimensionality [1] or the electronic topology [2]. TaSe3 isknown as a quasi-1D semimetal with layered chain-like structure [Fig. 1 (a)] and has been
extensively discussed in the early ages for its superconducting properties and the absence of
charge density wave (CDW) [3]. These characteristics make it distinct from othertransition-metal trichalcogenides, which typically undergo multiple CDW transitions [3]. As
shown in Fig. 1 (b), it is considered that the absence of CDW stems from the relatively poor
nesting conditions in TaSe3 due to its more 3D-like electronic structure and CDW may be
induced by Cu intercalation [4,5] or in mesowires [6], both of which are expected to reducethe dimensionality. A recent first-principle calculation indicate that TaSe3 actually belongs toa strong topological insulator (TI) phase and moreover predict intriguing topological phasetransitions among strong TI, weak TI, Dirac semimetal, and normal insulator phases through aslight change of lattice parameters, as demonstrated in the topological phase diagram of Fig. 1(c) [7]. Therefore, TaSe3 offers a simple system to study the interplay between
superconductivity and topological phases, and the possibility of selecting dirrennt topologicalphase by changing physical parameters (temperature, element substitution etc.).
A direct investigation of the electronic structure of TaSe3 via angle-resolved photoemission
spectroscopy (ARPES) is consequently indispensable to elucidate its novel topologicalproperties. We have conducted laser-based spin-resolved ARPES (SARPES) measurements at
the Institute for Solid State Physics, the University of Tokyo last year and reported the firstunambiguously observation of the electronic structure of TaSe3 and the possible topologicalsurface states (TSSs) [8]. In present study, we show the high-resolution band structure of thepossible TSSs and confirm that these surface states are indeed spin-momentum locked.
Therefore, our results reveal a well-established TI phase in quasi-1D TaSe3.

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 with

hundreds of nano-meters in width (right) [6]. (c)Topological phase diagram of TaSe3 including
strong topological insulator (TI), weak TI, Diracsemimetal, and normal insulator phases [7].

Figures 2 (a,b) show the 3D Brillouinzone [7] and our recently calculated TSSs
at the X point on (-101) surface. Figure 2(c) is the schematic of the experimental
ARPES setup and the typical sample size.
In Figs. 2 (d,e), we present thehigh-resolution Fermi surface and theband dispersion at the X point. The
observed sharp and high-intense featuresnear EF are nicely consistent with our
calculations in Fig. 2 (b). To further
confirm whether the sharp electronic
states are spin-polarized, Fig. 2 (f) showsone
typicalspin-revolved
energydistribution curves and the corresponding
spin polarization at a momentum positionindicated by the dash line in Fig. 2 (e).
Taking advantage of our high-resolution
SARPES, the spin splitting is clearlyobserved for the spin up and spin downstates, uncovering a spin polarization of ~20%. We confirmed a reversal of the spinpolarization for an opposite momentum
position. Therefore, our results elucidate
the spin-momentum locked TSSs in TaSe3.
To
sum
up,
we
performed
high-resolution SARPES measurements on
TaSe3 focusing on the surface states, theresults confirm the spin-momentum locked
TSSs and hence a TI state.

Fig. 2 (a) 3D Brillouin zone (BZ) of TaSe3 [7]. (b)Calculated topological surface states (TSSs) at the X
points on (-101) surface [7]. (c) ARPES setup. Typicalsamples size is ~ 1×0.05×0.01 mm3. (d) Observed
surface-state Fermi surface on (-101) surface near the
X point at 10 K. (e) The corresponding high-resolution
dispersion at the X point with momentum positionindicated by the red arrow in (d). (f) Spin polarization
at the momentum position indicated by a dash line in(e). Red and blue curves are energy distribution curvesof spin up and spin down states, respectively.

REFERENCES[1] Y. Liu, et al., Nat. Rev. Mater. 1, 1 (2016).
[2] X.-L. Qi, S.-C. Zhang, Rev. Mod. Phys. 83, 1057–1110 (2011).
[3] P. Monceau, Adv. Phys. 61, 325 (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 (2019).
[7] S. Nie et al., Phys. Rev. B 98, 125143 (2018).
[8] C. Lin et al., ISSP Activity Report of Synchrotron Radiation Laboratory (2018).

ANGLE- AND SPIN-RESOLVED PHOTOEMISSION SPECTROSCOPYRESEARCH ON TYPE-II WEYL SEMIMETAL WTe2
Yuxuan Wan1,2, Lihai Wang3, Keisuke Koshiishi1, Masahiro Suzuki1, Peng Zhang ，Jaewook Kim4, Kenta Kuroda2, Koichiro Yaji2, Ryo Noguchi2, Ayumi Harasawa2, Shik Shin2,
Atsushi Fujimori1, Sang-Wook Cheong3, Takeshi Kondo2
2

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 Weyl fermions has been observed in particle physics research. On theother hand, some topological materials are expected to host electrons behaving as Weyl
fermions. These materials, called Weyl semimetals, have spin-polarized bands with cone-likeliner dispersions with zero-mass near Weyl points, which are generated when either timereversal or space-inversion symmetry is broken. The Weyl points are topologically protected,
and intriguingly, topological surface states connect a pair of Weyl points with opposite chirality,
forming arc-like Fermi surface (Fermi arc).
In some Weyl semimetals, the hole- and electron-like pockets touch each other at several
Weyl points. These kinds of Weyl semimetals are called type-II Weyl semimetals, and WTe2 westudy here is one of such candidates. In WTe2, thespace inversion symmetry along c-axis is broken,
thus the top and bottom surfaces should be different[1-3]. Theoretical calculations predict distinct Fermi
arc states for these two surfaces [4], which requiredetailed experimental investigation.
Experiment
The single crystals of WTe2 with large domains
were grown via the iodine vapor transport method[4]. Comparing with previous studies, the single
crystals have a much larger domain size. We
developed a new method to obtain a pair of samples
with opposite polar surfaces as shown in Fig. 1,
namely “sandwich method”. A single crystal ofsample was fixed on a metal substrate with silver
paste as usual ARPES experiment. Then we applied
silver paste on the top surface of the sample andcovered it with another metal substrate. After heating
the sample until the silver paste was hardened, we
separated the two substrates to cleave the sample. The
two pieces of sample would expose different surfaces.
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 of

Fig.1 (a) The method to get a pair ofsamples with polar surfaces. (b) and (c) The
Fermi surfaces on both the polar surfaces
by laser ARPES. (d) and (e) Banddispersions along high symmetry cuts.

surfaces of WTe2: surface A and B. We used the 7 eV laser ARPES to map the energy contours
and measured the band structures. We found two kinds of ARPES results on the polar surfaces.
SARPES measurements on candidate topological surface states were taken after the ARPES
measurements. We distinguished surface states from bulk states.
Results
We measured two kinds ofsurfaces of WTe2: surfaces A and B
with 7 eV laser-ARPES. In Fig. 1(b)-(e), we can see the arc-likesurface states on both surface A and
B. The surface states of the opposite
surfaces are different because of thelack of inversion symmetry as
expected.
Figure 2 shows the SARPES
results of the candidate topologicalsurface states. The spin polarizationof bulk bands inversed on the polar
surfaces, while surface states keep
the same direction. We also found asurface resonance state generated by
the mixture of the surface state andthe hole pocket, suggesting that thehole and electron pocket also
connect below the Fermi level,
which is not predicted by theoreticalstudies. The band structures arelinear near the contact point with a
Dirac-like cone.
We observed two kinds ofsurfaces 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 arecompatible with topological Fermi
arcs.

Fig.2 (a) and (e) The ARPES images of cut 3 on surface A andB, respectively. The red (pink)/blue (light-blue) dashed lines
indicate the band dispersions with sy-up/down polarization.
(b),(c),(f) and (g) Laser-SARPES images for sy -up anddown.(d) and (h) Energy distribution curves (EDCs) for up (red)and down(blue) along dashed line in (b), (c), (f), and (g). The
marks indicated the positions of the peaks: light-blue and pinkrounds for bulk states and blue and red for surfaces, which arealso plotted on (b), (c), (f), and (g) . (i)-(p) Similar set for spinx component along cut 2 on surface A and B.

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)

DEVELOPMENT OF TIME-, SPIN- AND ANGLE-RESOLVEDPHOTOEMISSION SPECTROSCOPY MACHINE AT ISSP
K. Kawaguchi, K. Kuroda, Z. Zhao, A. Harasawa, K. Yaji, R. Noguchi, S. Tani,
M. Fujisawa, S. Shin, F. Komori, Y. Kobayashi, T. Kondo
The Institute for Solid State Physics (ISSP), The University of Tokyo
Spin- and angle-resolved photoemission spectroscopy (SARPES) is a powerful technique to
directly access spin-polarized electronic structures [1]. In 2014, We have newly built thelaser-SARPES machine at ISSP, which is based on the 6.994-eV laser generated by KBBF
non-linear crystal and the double VLEED-type spin detector [2]. Thanks to the high-flux lightsource as well as the high-efficiency of the detectors, our SARPES machine enables us toreveal the various spin-polarized states and the related phenomena.
Recently, we have started to a project to upgrade our laser-SAPRES machine by combining
femtosecond laser pulses to perform pump-probe time-resolved SARPES (tr-SARPES). In
contrast to the standard SARPES, this technique allows us to investigate the non-equilibrium
spin-polarized electronic states and its spin dynamics disentangled from the charge dynamics.
We have installed a new 10.7 eV-laser pulse developed by Kobayashi group of ISSP [3],which is based on the Yb-doped fiver with the high repetition rate (1 MHz). The 10.7 eV laser
is generated by the third harmonic generation of the 347-nm driver in Xe gas. The
high-energy probe pulse will cover the entire first Brillouin Zone (BZ), which is a great
advantage compared to the conventional 6 eV-lasers based on Ti:sapphire laser.
In this activity report, we report the status of the machine development. At this stage, wehave detected the SARPES signals probed by 10.7 eV laser. We here will show its
demonstrations on polycrystalline-Au, and Bi/Si (111) films.
We characterize energy resolution bymeasuring polycrystalline gold plate(Fig. 1). By fitting analysis of theenergy distribution curves (EDCs) by a
Fermi-Dirac function convoluted by
Gaussian, we estimate the energy
broadening of the laser itself to be 20
meV. This value is reasonable if we
consider the band width of thefundamental laser generated by the
Yb-doped fiber. We also find that there
is no significant space charging effect,
which is the best advantage of thehigh-repetition laser.
Next, we mapped the band dispersionsof Bi/Si (111) film. Fig. 2(a) shows theobserved Fermi-surfaces covering the

Fig. 1: EDCs recorded by 10.7 eV laser on thepolycrystalline gold plate at 17 K. Dashed curve
is a fitting function to deduce the energy
resolution of the laser pulse itself.

BZ. We also measured the clear band dispersions of Bi/Si (111) in wide energy-momentum
windows [Fig. 2(b)]. This guarantees that our laser system is stable for SARPES and ability toinvestigate the spin-polarized band structure for entire first BZ. We are now installing thepump pulses to perform the pump-probe tr-SARPES measurement.

Fig. 2: (a) The Fermi surface of Bi/Si (111) films. 10.7 eV laser is p-polarization set up,
and analyzer slit is along ΓM line. (b) Band dispersion image along Γ-M direction.

References:[1] J. H. Dil, Electron. Struct. 1, 023001 (2019).
[2] K. Yaji et al., Rev. Sci. Instrum., 87, 053111 (2016).
[3] Z. Zhao et al., Opt. Exp. 25, 13517 (2017).

THE WEAK TOPOLOGICAL INSULATOR STATE IN ZRTE5Peng Zhang1, Ryo Noguchi1, Kenta Kuroda1, Chun Lin1, Kaishu Kawaguchi1, Koichiro Yaji2,
Ayumi Harasawa1, Simin Nie3, Hongming Weng4, V. Kandyba5, A. Giampietri5, A. Barinov5,
Qiang Li6, G.D. Gu6, Shik Shin7,1, and Takeshi Kondo1
1

Institute for Solid State Physics, University of Tokyo, Kashiwa, Chiba 277-8581, Japan. 2ResearchCenter for Advanced Measurement and Characterization, National Institute for Materials Science,
Tsukuba, Ibaraki 305-0003, Japan. 3Department of Materials Science and Engineering, Stanford
University, Stanford, California 94305, USA. 4Beijing National Laboratory for Condensed Matter
Physics and Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China. 5Elettra Sincrotrone Trieste, Basovizza, Italy. 6Condensed Matter Physics and Materials Science
Department, Brookhaven National Laboratory, Upton, NY, USA. 7Office of University Professor,
University of Tokyo, Kashiwa, Chiba 277-8581, Japan

Two distinct topological insulators exist in three dimensions: strong topological insulator(STI) and weak topological insulator (WTI). Among them, STI have been widely studied in
the past decades both in theories and experiments. There are 2D spin-momentum locked Diraccones on all the surfaces, in which the perfect backscattering is prohibited, while generalscattering away from 180 degree still exists. On other hand, there are many theory studies on
WTI but experiments are still rare. WTI host surface states only on the side surfaces. They arethought to be weak since two adjacent layers in even-layer WTI may couple with each other,leading to a topologically trivial phase. However, it is found that WTI surface states are
actually robust as a results of delocalization. Furthermore, surface bands of WTI generally
have a quasi-1D dispersion. Similar to the quantum spin Hall edge states, there are lessscattering channels than STI, and smaller scattering at the side surface is expected. However,experiments on surface states of WTI are rare, due to the difficulty to prepare a large anduniform side surface of layered materials. Cleavage is generally useful for top surface, andsuch techniques proved its power on STI. In nature, most materials can only be cleaved fromtop surface, and it is difficult to prepare a side surface. The only example is Bi4I4, which has
two cleaved surface at the same time (specially) [1]. ZrTe5 is another candidate of WTI [2].
Although there are some spectroscopic studies on the top surface, there is no study on the sidesurface. The key evidence for WTI of ZrTe5, the results from side surface, is missing, and theproperties of the surface band is absent.
ZrTe5 has a quasi-1D crystal structure. The layer distance for the top surface is ∼ 7.3 Å, and itis ∼ 6.9 Å for the side surface. As a result of the large layer distances, in principle, it should bepossible to cleave both surfaces. However, Te-Te bonding exists between adjacent layers forthe side surface, making it much more difficulty to cleave than the top surface stacked by van
der Waals forces. We successfully cleaved the side surface of WTe5, and measured its band
structure with laser ARPES. We resolved a Fermi surface only dispersing along the chain
direction (a), as displayed in Fig. 1a. The ARPES intensity plot along ka shows a hole-likeband. By comparing with the calculations [2], we found this band should be the surface band.
The spin polarization is expected for the surface band.
We measured two pairs of EDCs (Fig.1a) by spin-resolved ARPES from three different
samples. The ka locations are also shown in Fig.1b. The spin polarization curves along the b, aand c directions are displayed in Fig.1c - e. In each panel of Fig.1c - e, the five curves withthe same color (blue or red) are taken at two different kb (kb ∼ 0 and 10, indicated in Fig.1a)

-10
-15
-10 -5

L
0

5

-0.1
-0.2
-0.3

10

-10 -5ka (degree)Surface band

H

0
-5

H

0

L

-10 -5 0

5 10

ka (degree)

Polarization

f

0.4
0.2

ka (degree)dSpin Pb

L
5 10

eSpin Pa

Spin Pc

0.0-0.2

Background

Surface band

-0.4
0.4
0.2

g
Spin Pb

EDC 2hSpin Pa

Spin Pc

0.0-0.2

average and remove background

Polarizationc

H

Background

0.0

5

E - EF (eV)kb (degree)

10

EDC 2

0.1

EDC 1

b

15

BackgroundaSurface bandfrom three samples (one sample at only kb ∼ 0, in total 6 - 1 = 5 curves). These five curves
show no obvious difference, confirming the repeatability of the measurements. We notice thatthe spin polarization curves in the background range (< -0.15 eV) are not zero but have offsets.
Such constant polarization in the background may come from the asymmetry of the spin +and spin - channels, or may come from the matrix-element effect in the photoemission process,which is often observed in spin-resolved ARPES. We averaged the five polarization curves
with the same color in Fig.1c - e, and subtracted the resulting curves by a constant to remove
the background polarization. The results are displayed in Fig.1f - h. It is clear that the spin ispolarized along b direction, tangent with the Fermi surface, forming the quasi-1D
spin-momentum locking pattern. Our results prove the existence of topological surface stateson the side surface, and the weak topological insulator nature of ZrTe5.

EDC 1

-0.4
-0.25 -0.20 -0.15 -0.10 -0.05 0.00

-0.20 -0.15 -0.10 -0.05 0.00

-0.20 -0.15 -0.10 -0.05 0.00

E - EF (eV)Figure 1. Spin polarization calibration. (a) Fermi surface of the side surface. The redand blue dots indicate the locations in k-space of the spin-polarized EDCs in (c - h). (b)Intensity plot of the band structure along ka and its MDC curvature plot. The red and bluelines indicate the ka locations of the spin-polarized EDCs. Due to the quasi-1D feature of
the Fermi surface, the band structure at kb ∼ 0 and kb ∼ 10 in (a) are basically the same.
(c - e) Raw spin polarization curves along b, a and c directions measured at the indicatedlocations in (a) and from different samples. The polarization curves at the same ka arealmost the same (independent of kb locations or samples). So the curves are justdistinguished by their ka values with red and blue colors. (f - h) Spin polarization curves
along b, a and c directions, obtained by averaging the same-color curves in (c - e),respectively. Since the background part (< -0.15 eV) should show no spin polarization,
we subtracted a constant for all the curves to remove the background spin polarization.

References:[1] R. Noguchi. et al. Nature 566, 518 (2019).
[2] H. Weng, Dai, X. & Fang, Z. Phys. Rev. X 4, 011002 (2014).

Observation of spin polarized surface states in polar Weyl semimetal
Td-MoTe2
M. Sakano1, Y. Tanaka1, M. S. Bahramy1, 2, H. Takahashi1,3, K. Kuroda4, A. Harasawa4, K.
Yaji4, S. Shin4, S. Ishiwata1,3, K. Ishizaka1, 2
1Quantum-Phase Electronics Center (QPEC) and Department of Applied Physics, The
University of Tokyo
2
RIKEN Center for Emergent Matter Science (CEMS)
3
Graduate School of Engineering Science, Osaka University
4
Institute for Solid State Physics, The University of Tokyo
The electronic structure in the low temperature phase of Td-MoTe2, which is theoretically predicted
as a type-II Weyl semimetal, has been intensively investigated by angle-resolved photoelectron
spectroscopy [1-6]. Until now, some segment-like band features resembling Fermi arcs areexperimentally observed, which are indicative of the topological surface states connecting the bulk
Weyl nodes [5,6]. However, since the complicated electronic structure near the Fermi level makes it
difficult to be observed, the interpretations of ARPES results on the observed Fermi-arc-like bands arestill controversial. Here we investigate the spin polarizations of the Fermi arc features for both polar
terminations by utilizing the high-resolution spin and angle-resolved photoelectron spectroscopy(SARPES) [7].
Td-MoTe2 has the noncentrosymmetric crystal structurewith polarization along the stacking direction (c-axis), so
it has the “top” and “bottom” polar domains. The top
surface is denoted by (001) and the bottom surface isdenoted by (00-1). According to the band calculations,
there are eight Weyl points in the first Brillouin zone of
Td-MoTe2. Their energy levels and coordinates are
predicted to be (E, kx, ky, kz) = (EF + 6.7 meV, ±0.185,±0.013,0) (W1s : near the Γ-X line) and (EF + 59 meV, ±0.181, ±0.053, 0) (W2s : the far from Γ-X line) by thetheoretical research[21]. Fig. 1(a) and (b) show the energy
contours at EF + 6.7 meV for the (001) and (00-1) surfaces
respectively. On the (001) surface, an arc-like surface state
dispersing from the W1 can be seen between the hole
pocket and the electron pocket. In addition, there isanother surface state that crosses the Γ-X line near the
Fig. 1 The calculated energy contour
electron pocket, indicated by “SS”. On the (00-1) surface,at EF + 6.7 meV for (001) (a) andthere is a sharp surface state that connects the pair of W1s(00-1) (b) surfaces.
with different chirality, indicated by the white arrow in
Fig. 1(b).
Fig. 2(a) shows the energy contour at EF on the (001) surface obtained by ARPES at 20 K. The two
types of sharp photoelectron intensities are observed indicated by the pink solid brackets and the green
curve. The pair of segments is the possible Fermi arcs connecting W1 and W2 from the comparison
with the calculation shown in Fig. 1(a). The state indicated by the green curve is also a surface state
but can be distinguished from the Fermi arc.
Next, looking at the energy contours at EF + 20 meV for the (00-1) surface shown in Fig. 2(b), a shortsegment of the photoelectron intensity can be observed at kx ~ 0.20 Å-1 along the Γ-X line, as indicatedby the pink marker. This segment is the possible Fermi arc connecting the pair of W1 across the Γ-Xline, as shown in the calculation for the (00-1) surface (Fig. 1(b)).
To investigate the spin structures of the topological surface states, we perform SARPES along Fermi
arcs and the SS. On the (001) surface, the SARPES measurements were performed along the orange
lines (#0~#7) shown in Fig 2(a). Figs. 2(c)~(e) show the spin-resolved spectrums along #0 ~ #7 for theSx, Sy and Sz respectively. The measurement position (#0 ~ #7) in momentum space is calculated fromthe detector angle shown on the horizontal axis of Figs. 2(c)-(e). The red and blue curves represent thephotoelectron intensities of the up spins and down spins, respectively. From the ARPES results shownin Fig. 2(a), there are the Fermi arc and the SS, on #0, #2~#7. The peak positions shown in the spin
resolved momentum distribution curves (MDCs) are the Fermi arc and SS peak positions obtained byfitting the MDCs by the normal ARPES measurement shown in Fig 2(a). In the spin-resolved
spectrums for Sx shown in Fig. 2(c), the only one peak is observed on the #1, whereas two peakstructures are observed on the #2 reflecting the two different surface states, so we can say that the Arc
and the SS can be measured by SARPES. The spin resolved MDCs for Sy (Fig. 2(d))shows that the SS
has Sy up polarization on the Γ-X line, the up-spin spectrums have the two-peak structure in the regionslightly off the Γ-X line (#2-#5), so we can see that the Sy of the Fermi arc near W1 is up on the (001)surface. For Sz, the Arc and SS seem to have the opposite polarization near the Γ-X line, but they havelarge Sz-up polarization far from the Γ-X line. Next, we move on to the results of the (00-1) surface.
Fig. 2(b) shows the energy contour at EF + 20 meV on the (00-1) surface. SARPES is performed along
the orange line in Fig. 2(b), so the peak in the 3D spin-resolved MDCs shown in Fig. 2(f)-(h)corresponds to the peak of the Arc. There seem to be no difference in the photoelectron intensities of
the up and down of Sx, but the MDC of Sy-down ARPES intensities shown in Fig. 2(g) has a sharp
peak which is consistent with the peak position of the Arc observed by the normal ARPES. Finally,
the MDC of Sz-up ARPES
intensities shown in Fig. 2(h) has apeak structure at the same position
with the result of the normal
ARPES, so we can see the Arc
have
the
small
Sz-downpolarization.
We focus on the spin components
along y of the Arc in the vicinity ofthe Γ-X line. The Arc of the (001)surface has Sy-up polarization (Py ~0.4) just outside of the pair of W1
at ky ~ ±0.01 Å-1. On the other
hand, the Arc on the (00-1) surface
at ky = 0 has Sy-down polarization(Py ~ -0.6). In other words, the signof the spin polarizations of the
Fermi arcs on the top and bottom
surface reverse at the W1s, thejunction point between the Arcs.
This feature seems to reflect the
Berry flux emerging from the
Weyl points, as the source and sinkof Berry flux in momentum space.
REFERENCES[1] A. A. Soluyanov, et al.,Fig.2 ARPES energy contour at EF for (001) (a) and at EF + 20
Nature 527, 495(2015).
meV for (00-1) (b). (c)-(e) 3D SARPES momentum distribution[2] Y. Sun, et al.,Phys. Rev. B
curves along the orange lines in (a). (f)-(h) 3D SARPES
92 161107 (2015).
momentum distribution curves corresponding to the orange[3] Z. Wang, et al.,Phys. Rev.
line in (b).
Lett. 117, 056805(2015).
[4] L. Huang, et al., Nat. Mater.
15, 1155 (2016).
[5] A. Tamai, et al., Phys. Rev. X 6, 031032 (2016).
[6] M. Sakano, et al., Phys. Rev. B 95, 121101 (2017).
[7] K. Yaji et al., Rev. Sci. Instrum. 87, 053111 (2016).

SARPES STUDY OF A PB ATOMIC LAYER ON GE(111)1

Koichiro Yaji1 and Fumio Komori2
Research Center for Advanced Measurement and Characterization,
National Institute for Materials Science

2

Division of Nanoscale Science, Institute for Solid State Physics, The University of Tokyo

Strongly spin-orbit coupled materials, such as topological insulators and Rashba systems, areplaced at a central research field in condensed matter physics not only because of fundamental
interests but also spintronic applications. In the standard model of the strongly spin-orbit
coupled systems, the spin direction is perpendicular to both the direction of surface normal andthe momentum of an electron. Recently, in contrast to the scenario of the spin-momentum
locking, the spin-orbital locking model was reported in several papers [1]. Also, in our previous
studies [2,3], we demonstrated the spin-orbit coupling of surface states on Bi(111) and Bi2Se3
with spin- and angle-resolved photoemission spectroscopy using a vacuum ultraviolet laser(laser-SARPES). Here we show the spin-orbit coupling of surface states of a monolayer Pb film
on Ge(111) (Pb/Ge(111)-b) investigated by laser-SARPES. Moreover, we demonstrate theincident-light-angle dependence of SARPES.
SARPES measurements were performed at E-building in the Institute for Solid State Physics[4]. The Pb/Ge(111) sample was in-situ prepared in a molecular beam epitaxy chamber which
was directly connected to the analysis chamber. We used an n-type Ge(111) substrate. The cleansurface was obtained with the several cycles of Ar+ ion sputtering with the ion energy of 0.5
kV and subsequent annealing at 600 °C. Pb was evaporated from an aluminum crucible. The
thickness of Pb and the orderliness of the monolayer film was judged from low-energy-electrondiffraction pattern. The photoelectrons were excited by 6.994-eV photons and were analyzed
with a ScientaOmicron DA30-L analyzer equipped with very-low-energy-electron-diffraction
type spin detectors. The electron deflector function is useful for studying the light-incidentangle dependence of SARPES. The effective Sherman function was 0.28. The sample
temperature was kept at 40K during themeasurements.
Figure 1 shows an ARPES image recorded
along GK in the surface Brillion zone of
Pb/Ge(111)-b, being parallel to the mirror
plane of the surface. We find several bands
around G, which are ascribed to thesubsurface states of Ge(111) [5]. The bands
crossing the Fermi level (EF) around k ~ 0.400.45 Å-1 are Pb-derived states, where thesplitting is due to the Rashba effect [6]. The
bands lying in the energy range of 0.3-0.8 eV
below EF around k > 0.3 Å-1 are also Pbderived states and show semiconducting Fig. 1 The ARPES image of Pb/Ge(111)-b along
behavior. The overall shapes of the bands GK recorded with the photon energy of 6.994 eV.
observed with hn = 6.994 eV well agree with The color arrangement gives the information of thephotoelectron intensity.
those with hn = 21 eV reported in ourprevious study [7].
The SARPES measurements were performed with p- and s-polarized lights [Fig. 2], which
could be regarded as orbital-selective SARPES. Here, the light incident angles were set to 20°
and 40°. We used the spin detector for resolving the y-spin component, which was perpendicularto the mirror plane of the surface. As shownin Fig. 2, we clearly demonstrate that theobserved spin polarizations are inverted
upon switching the light polarizations. This
indicates that the symmetric and antisymmetric orbitals are independently
coupled to mutually opposite spins as inother spin-orbit coupled systems such as
Bi(111) and Bi2Se3. The total spinpolarization is obtained from the integrationsof the orbital-selective spin polarizations
with p- and s-polarized lights. For the lightincident angle of 20°, the spin-polarized
branches show opposite spin directions eachother, which is in agreement with thescenario of the Rashba effect. On the other
hand, for the 40° incidence, the total spinpolarizations are in the same directions,which is against the Rashba scenario and Fig. 2 Laser-SARPES images for the Pb-derived
violates the time reversal symmetry. surface states along GK recorded with the light
Besides, the theoretical calculation suggests incident angles of 20° (upper panels) and 40° (lower
that these branches exhibit the Rashba-type panels) with the p-polarized light (left panels) and thespin texture [7]. Therefore, we conclude that s-polarized light (center panels). The right panels
the spin polarization observed with SARPES represent the orbital-integrated SARPES image.
White and black colors give the information of thewith the incident angle of 40° is largely negative-positive spin polarizations. The left-upper
influenced by the final-state effect in panel is a schematic drawing of the experimental
photoemission process. It is likely that the geometry.
photoionization cross sections between thep- and s-polarizations are different. We have to mind that the total spin polarization is notalways derived from the integrations of the orbital-resolved ones.
REFERENCES[1] Z.-H. Zhu et al., Phys. Rev. Lett. 110, 216401 (2013), Y. Cao et al., Nat. Phys. 9, 499 (2013), H.
Zhang et al., Phys. Rev. Lett. 111, 066801 (2013), Z. Xie et al., Nat. Commun. 5, 3382 (2014).
[2] K. Kuroda et al., Phys. Rev. B 94, 165162 (2016).
[3] K. Yaji et al., Nat. Commun. 8, 14588 (2017).
[4] K. Yaji et al., Rev. Sci. Instrum. 87, 053111 (2016).
[5] K. Yaji et al., J. Electron Spect. Rel. Phenom. 201, 92 (2015).
[6] K. Yaji et al., Nat. Commun. 1, 17 (2010).
[7] K. Yaji et al., Phys. Rev. B 86, 235317 (2012).

HIGH-RESOLUTION SPIN-RESOLVED ELECTRONIC STRUCTURE
OF TOPOLOGICAL HEAVY-FERMION MATERIALS
Andrés F. Santander-Syro, Franck Fortuna, Maximilian Thees
Institut des Sciences Moléculaires d'Orsay (ISMO), Université Paris-Saclay and CNRS, France

Cris Adriano
Instituto de Física “Gleb Wataghin”, UNICAMP, Campinas-SP, 13083-859, Brazil

Ce-based intermetallic compounds have interesting physical properties that arise from theinterplay between Ruderman-Kittel-Kasuya-Yoshida (RKKY) magnetic interaction,
crystalline electric field and Fermi surface (FS) effects in connection to a strong
hybridization between the Ce3+ 4f electrons and the conduction electrons. Ce-based materials
often present non-trivial ground states, such as heavy fermion, unconventional
superconductivity and non-Fermi-liquid behavior, which frequently appear in the vicinity of amagnetically ordered state. Interestingly, some of these properties, seem to be favored tooccur in low symmetry systems such as tetragonal structure, well-known examples are thefamilies of CeMIn5, Ce2MIn8 (M = Co, Rh, Ir, Pd), CePt2In7 and CeCuSi2.
In this context, recent attention was given to the CeTX2 family (T = transition metal; X =pnictogen), which hosts both ferromagnetic (FM) and antiferromagnetic (AFM) memberswith complex magnetic behavior [1]. In particular, in this project we investigated, using
laser-excited spin- and angle-resolved photoemission spectroscopy (Laser-SARPES), thecompound CeCuBi2 that crystallizes in the tetragonal ZrCuSi2-type structure (P4/nmm [1]).
Previous studies show that CeCuBi2 has an AFM ordering at TN = 16 K with a large magnetic
anisotropy [1].
As shown in Figure 1, in the AFM phase we detected a dispersive hole-like state with afinite spin polarization. This state may arise from unpaired or incompletely paired moments
near the material surface, as also observed in other AFM heavy-fermion systems [2]. Further
data analyses and complementary spin-integrated ARPES measurements are currently being
performed to clarify its origin.

Figure 1. (a) laser-ARPES energy-momentum map showing a hole-like state right below the Fermi level in
CeCuBi2. (b) Bottom panel: spin-resolved energy distribution curves along the momentum marked by the redline in (a); top panel: net spin polarization.

REFERENCES[1] Cris Adriano et al., Phys. Rev. B 90, 235120 (2014).
[2] A. Chikina et al., Nat. Commun. 5, 1-7 (2014).

Study of spin structure in domain-regulated alloy of atomic layer
Ting-Yu Chen1, Li-Cheun Yang2, Santosh Chiniwar1, H. -T. Jeng1, Woei Wu Pai3,
S. -J. Tang1,2,
1. Department of Physics, National Tsing Hua University, Hsinchu 30013, Taiwan
2. National Synchrotron Radiation Research Center (NSRRC), Hsinchu 30076,
Taiwan3. Center for Condensed Matter Sciences, National Taiwan University, Taipei 106,
TaiwanThe enhanced Rashba effect of a 2-dimestional (2D) surface alloy has been a centerof interest in surface-science community. A typical Rashba effect in a 2D system ismainly contributed by the out-of-plane electric field caused by the inversion –symmetry
breaking perpendicular to the surface plane. Consequently, the k momentum ofdegenerate surface-state bands locked with opposite in-plane spin polarizations shift inpositive and negative directions, respectively, forming a concentric 2D constant-energy
contour (CEC). However, previous studies on 2D surface alloys revealed the enhanced
Rashba effect correlated with possible in-plane electric field and thus out-of-plane spinpolarizations. Such effect was attributed to the second-order warping term of spin-orbit
interaction in addition to the first-order term for the typical Rashba effect.
Our group found that the surface-state bands of GeAg2 surface alloy formed on
Ag(111) exhibits anomalous splitting centered at surface zone center Γ as shown in
Fig. 1. The splitting is obvious in one symmetry direction ΓM Ag2Ge but negligible inanother ΓK Ag2Ge . The corresponding 2D CECs centered at Γ is displayed in Fig. 2(a),exhibiting an entangled configuration of inner hexagon and outward snowflakes thatrotate 30° with each other.

Fig. 1: The measured energy band dispersions for Ag2Ge on Ag(111) in two symmetry directions

(a)

(b)

Fig. 2: (a) The measured 2D constant energy contours at −1.0 eVfor surface-state bands of Ag2Ge
surface alloy on Ag(111) centered at Γ and (b) the corresponding fitting result.

We employed the spin-orbit interacting model of the first-order Rashba term and thesecond-order Warping term to simulate the measured CECs in Fig. 2(a); the match isvery agreeable as shown by the orange fitting curve in Fig. 2(b). Moreover, the fitting
result shows that the contribution to the splitting is dominantly from the Warping term,
being in complete contrast to the behaviors in most surface alloys where the first-order
Rashba term dominates. If it is the case, the spin polarizations of the surface-state bandsof Ag2Ge surface alloy centered at Γ would be dominantly out-of-plane. This
motivated us to come to the Synchrotron Radiation Laboratory in the Institute for Solid
State Physics, The University of Tokyo, Japan to use the spin- and angle-resolved
photoelectron spectroscopy (SARPES) to measure the spin texture of surface state
bands of Ag2Ge surface alloy. Figure 3 shows the scanning-tunneling-microscopy(STM) image of Ag2Ge surface alloy coexisting with striped-phase germanene which
form triangle arrays enclosing the Ag2Ge surface alloy. We believe the triangle arrays
break the in-plane symmetry to cause the in-plane electric field.
Figure 4 shows the preliminary results of our spin-resolved photoemission
measurement carried out in SARPES machine time in Nov 2019. The applied magnetic
field was along the out-of-plane direction (z direction). Indeed one can see the energy
distribution curves (EDCs), taken at off-normal 8°, corresponding to two opposite
directions (±z) of magnetic fields reveal difference in intensities for the two peaks ofthe split surface-state bands. However, it is a bit abnormal that the two peaks of red
EDC are both higher than those of blue EDC. I think the surface normal of our sample
was not completely aligned with the detector axis during measurment. In our next
SARPES machine, we will be much more careful in those measurement details.

Fig. 3: The STM image of Ag2Ge surface alloy coexisting with striped-phase germanene in the form of
triangle arrays.

Fig. 4: The preliminary result of spin-resolved photoemission measurement on the split surface-state
bands of Ag2Ge surface alloy. Red and blue EDCs correspond to opposite directions (±z ) of magnetic
fields.

Spin resolved ARPES on interfacial electronic states of Bi/NbSe2
R. Stania1, H. Jung1,2, H.W. Yeom1,2
1Center for Artificial Low Dimensional Electronic Systems, Institute for Basic Science (IBS),
Pohang 37673, Republic of Korea
2Department of Physics, Pohang University of Science and Technology (POSTECH), Pohang
37673, Republic of Korea

Introduction
Bulk, as well as low-dimensional, bismuth (Bi) have provided extremely rich platforms toadvance our knowledge in condensed matter physics, ranging from high thermoelectric
properties[1] to giant Rashba effect with highly spin-polarized electrons [2][3]. Moreover,there have recently been much interest in the topological classification of Bi, and it has beenreported that pure Bi is topologically non-trivial in bulk [4][5], as well as in thin film[6].
More recently, it has also been reported that due to the higher-order topology, Bi also
possesses topologically protected hinge (edge) states even in bulk form [7]. Such new
identifications of Bi are expected to open up further possibilities for Bi-based structures,
particularly when combined with other novel materials.
In order to utilize the rich properties of Bi-based structures, both to explore further
fundamental aspects and for future device applications, it is essential to understand theelectronic interactions of low-dimensional Bi on various substrates, particularly at theirinterfaces (e.g., topological quantum computations when combined with superconductors [7]).
Of special interest are two-dimensional (2D) layered van-der-Waals materials (vdW) which
are known to host exceptional spin-related properties and have strong potential for spintronic
applications [8].
Thus, in our experiments, we aimed to investigate the electronic structures of ultra-thin Bi andits interfacial electronic states on a transition metal dichalcogenide (TMDC). In particular,
2H-NbSe2 not only shows a phase transition from metallic state to charge density wave (CDW)
state [9], but also towards superconducting state at one of the highest transition temperatures
amongst the TMDCs [10]. In addition, NbSe2 was recently predicted to be topologically nontrivial [11]. Thus, Bi/NbSe2 is an ideal playground to explore such interactions and an
important step towards the investigation of topological superconductivity and Majorana
fermions.
Experimental details
The NbSe2 samples were cleaved under UHV conditions and the surface quality wasconfirmed using LEED and He IIα. The Bi layers were grown in a separate chamber andcoverage was determined using He IIα excitation. The high resolution ARPES and SARPES
measurements were performed using a laser light source with a photon energy of 6.994 eV atthe laser-SARPES machine developed at ISSP [12]. All photoemission experiments werecarried out at sample temperatures below 35 K under UHV condition (about 1x10-8 Pa).

Results and discussion
By using high resolution ARPES we were able to distinguish the bands around the Γ point
which not only shows the expected bands for freestanding few-layer Bi(111) but also
additional bands that could not be clearly allocated and may resemble interface states (Figure1a). Spin dependent measurements reveal a strong spin texture (Figure 1b) for the TSS around
EB=0.7 eV and especially for the surface state near the Fermi level, which was expected to beless strong polarized for few BL Bi films. More structural investigation and theoretical
calculation will be required in order to determine the origins of the sub-bands.

Fig. 1 a) Spin integrated measurement of Bi/NbSe2 along the ΓK direction; b) spin dependent EDC along thedashed line in a) with red (blue) corresponding to out of plane spin up (down) measurements and green thepolarization

References[1] B. Poudel, et al., Science, 320, 634 (2008)[2] Y. Koroteev, et al., Phys. Rev. Lett., 93, 046403 (2004)[3] T. Hirahara, et al., Phys. Rev. B, 76, 153305 (2007)[4] S. Ito, et al., Phys. Rev. Lett., 117, 35 (2016)[5] Y. Ohtsubo, et al., New J. Phys., 18, 123015 (2016)[6] I.K. Drozdov, et al,. Nat. Phys., 10, 664 (2014)[7] F. Schindler, et al,. Nat. Phys., 14, 918 (2018)[8] D. Pensin, A.H. MacDonald Nature Mater., 11, 409 (2012)[9] D.E. Moncton, et al., Phys. Rev. Lett., 34, 734 (1975)[10] R. F. Frindt Phys. Rev. Lett., 28, 299 (1972)[11] M. S. Bahramy, et al., Nature Mater., 17, 21 (2017)[12] K. Yaji, et al., Rev. Sci. Instrum., 87, 053111 (2016)

HIGH-RESOLUTION SPIN- AND ANGLE-RESOLVEDPHOTOEMISSION SPECTROSCOPY ON DOPED MAGNETICTOPOLOGICAL INSULATORS
Keisuke Fukutani1,2, Chandan De1, Roland Stania1,2, Koichiro Yaji3, Kenta Kuroda3,
Jaeyoung Kim1,2, and Han-Woong Yeom1,41Center for Artificial Low dimensional Electronic Systems, Institute for Basic Science (IBS), Republicof Korea2
Pohang Accelerator Laboratory, Pohang University of Science and Technology (POSTECH),
Republic of Korea
3
The Institute for Solid State Physics, The University of Tokyo, Japan
4
Depertment of Physics, Pohang University of Science and Technology (POSTECH), Republic of
Korea

The topological classifications in condensed matter physics have led to a paradigm shift in
our understanding of the properties of solids, where the fundamental characteristics of matterscan be determined not only by their symmetries and many-body interactions but also by their
topological orders [1]. Such new classification schemes, have allowed for the predictions anddiscoveries of various new types of materials, leading to the realizations of intriguing
electronic properties [2-7]. Among them, the interplay between the topological order andmagnetism (broken time-reversal symmetry) has garnered much attentions as it is expected tolead to various exotic phenomena, such as quantum anomalous Hall effect [8] and topologicalmagnetoelectric effect [9], and have been extensively studied to date [10].
Recently, a discovery of an intrinsic antiferromagnetic topological insulator MnBi2Te4 has
been reported [11-13], followed by extensive investigations by various experimental andtheoretical means on this material as well as its hetero-structural cousins(MnBi2Te4)n(Bi2Te3)m [14-17]. On the other hand, despite the expected broken time-reversal
symmetry, the electronic band structures of these materials are variously reported with regard
to the presence of the band gap in the Dirac cone [18-23], which is at the heart of topologicalelectronic properties. While there are several proposed hypotheses to explain suchdiscrepancies [20,22,23], the general consensus has not been reached.
The keys to resolve such issue can be provided by high-resolution spin- and angle-resolved
photoemission spectroscopy (SARPES), which allows us to simultaneously probe thetopological surface Dirac band and their momentum-dependent spin textures. Since it is
known for these materials that the microscopic domain sizes and the presence of bulk bandprojections, pronounced above the photon energies of hv ~10 eV [11], can obscure theunambiguous observation of the Dirac bands, utilizations of the micro-spot-sized, laser-based(hv = 7 eV) SARPES available at the Institute for Solid State Physics (ISSP) at the Universityof Tokyo would be an ideal experimental probe to tackle the present issue.
In our experiments, we have performed laser-SARPES measurements on the pristine as well
as Sb-doped (MnBi2Te4)n(Bi2Te3)m at the temperatures above and below the magnetic phasetransition temperatures and have revealed their electronic structures as well as thecorresponding spin textures near the Γ point in the surface Brillouin zone. Fig. 1 shows the
ARPES intensity plots and the corresponding Fermi surface mapping for the pristine
MnBi2Te4 and MnBi4Te7. It can be clearly seen that the both materials possess gapless Diraccones centered at the Γ point with the Dirac point (ED) located at the binding energy of ~2.7
eV with no clearly identifiable band gap. These observations are consistent with some of theearlier ARPES studies [20-23], and likely point to the possibility that near the surface, themagnetic configurations are different from that of the bulk (i.e., possible surface magnetic

reconstructions), so as to provide the effective protection from gap-opening even in theabsence of the time-reversal symmetry [15]. In order to identify the magnetic configurations
near the surface and to identify the possible origin of the gapless Dirac cones, we have further
obtained the spin-resolved ARPES data for the pristine as well as the Sb-doped samples. The
spin-resolved data show unconventional spin textures for topological surface states and are
currently under further investigation in conjunction with theoretical calculations as well astransport measurements.

Figure 1: ARPES intensity maps for (a-c) MnBi2Te4 and (d-f) MnBi4Te7. (a) and (b) schematically show thelayer-stacking sequences for each of the materials, where the topmost layers are determined to be both MnBi2Te4.
(b) and (e) show the Fermi surface mappings and (c) and (f) show the band dispersions around the Γ point. The
Dirac point ED, where the upper and lower Dirac cones meet in the gapless dispersions are indicated by thearrows for both materials. The data were obtained with the s-polarized 7-eV laser at the temperature of T = 10 K.

REFERENCES[1] M. Z. Hasan and C. L. Kane, Rev. Mod. Phys. 82, 3045 (2010).
[2] C.-Z. Chang et al., Science 340, 167 (2013).,[3] C.-K. Chiu et al, Rev. Mod. Phys. 88, 035005 (2016).
[4] P. Zhang et al., Science 360, 182 (2018),[5] C.X. Trang et al., Nat. Commun. 11, 159 (2020).
[6] L. Fu et al., Phys. Rev. Lett. 100, 096407 (2008).
[7] A. Das et al., Nat. Phys. 8, 887 (2012)[8] F. D. M. Haldane, Phys. Rev. Lett. 61, 2015 (1988).
[9] X.-L. Qi et al., Phys. Rev. B 78, 195424 (2008).
[10] Y. Tokura et al., Nat. Rev. Phys. 1, 126 (2019).
[11] M. M. Otrokov et al., Nature 576, 416 (2019).
[12] E. D. L. Rienks et al., Nature 576, 423 (2019).
[13] Y. Gong et al., Chinese Phys. Lett. 36, 076801 (2019).
[14] R. C. Vidal et al., Phys. Rev. X 9, 041065 (2019).
[15] Y.-J. Hao et al., Phys. Rev. X 9, 041038 (2019).
[16] L. Ding, et al., Phys. Rev. B 101, 020412(R) (2020).
[17] R. C. Vidal et al., Phys. Rev. B 100, 121104 (2019).
[18] J. Wu et al. Science Advances 5, eaax9989 (2019).
[19] K. N. Gordon et al., arXiv:1910.13943 (2019).
[20] P. Swatek et al., Phys. Rev. B 101, 161109(R) (2020).
[21] B. Chen et al., Nat. Commun. 10, 4469 (2019).
[22] Y.J. Chen et al., Phys. Rev. X 9, 041040 (2019).
[23] Y. Fu et al., Phys. Rev. B 101, 161113(R) (2020).