STUDY ON PHOTO-INDUCED DYNAMICS IN TOPOLOGICALINSULATOR Bi2Te3 THIN FILMS

Kaishu Kawaguchi1, Ryo Mori1, Shinichiro Hatta2, Yuto Fukushima1, Hiroaki Tanaka1,
Ayumi Harasawa1, Tetsuya Aruga2, Takeshi Kondo1
1

ISSP, The University of Tokyo
Graduate School of Science, Kyoto University

2

INTRODUCTION
Spin-orbit interaction, where the orbital and spin components of electrons stronglyintertwine, gives rise to distinctive electronic properties. Notably, due to their intimate
relationship with topology, a variety of materials and functionalities in topological insulators
have been actively researched in recent years. These materials, represented by Bi2Se3, Bi2Te3,
and others, have the interesting feature of having unique spin-polarized conduction electrons
on their surfaces, even though their bulk is completely insulating [1]. Such novel materials
have expected to be a highly promising foundation for the design of new electronic devices,such as quantum computing, spintronics, and so on. Angle-Resolved Photoemission
Spectroscopy (ARPES) serves as a powerful experimental method to directly observe theelectronic structure of these topological insulators, allowing us to analyze the behavior ofelectrons. Furthermore, with the incorporation of time-resolved techniques, ARPES opens thepossibility to observe the dynamics of topological insulators in ultrafast time scales [2].
Time-resolved ARPES can observe how the electronic structure of materials changes by lightpulses, thereby presenting a new way to control quantum properties.
In this study, we apply these advanced methods to thin films of topological insulators. By
thinning these materials, interference effects can occur between surface and interface states,and potential modulation originating from the substrate becomes feasible. This will allow us
to introduce novel perturbation effects to the properties of topological insulators fromsubstrates. We anticipate that deepening our understanding of the dynamics and light-induced
transitions in topological insulator thin films from a band picture will pave new horizons in
opt-spintronics.
METHODSWe carried out experiments using a thin film of the topological insulator Bi2Te3, fabricatedon a p-type Si(111) substrate [3]. The research was measured by 10.7 eV pulse laser as aprobe light source to the ARPES apparatus [4]. The optical energy of the pump pulse is set at2.57 eV.
RESULTS AND DISCUSSION
Figure 1 presents the results from time-resolved ARPES measurements of Bi2Te3 thin films
at 2 QL (quintuple layer). A photo-induced potential shift, namely surface photovoltage [5],were clearly observed. The maximum energy shift is approximately 0.3 eV, indicating aconsiderably large surface photovoltage in comparison with prior studies about single crystalof Bi2Te3 [6]. The energy shift occurs on sub-nano seconds scale (Fig. 2), suggesting that thisphenomenon originates from charge transfer near the surface or interface; Bi2Te3 thin film isunlikely to induce sufficient charge transfer within thickness of 2 nm in this case, therefore
the 0.3 eV energy shift is likely to be due to photoexcitation of band bends originating fromthe p-type Si(111) substrate as the mechanism. Based on these results, utilizing a highly
doped substrate can be considered an effective method to induce a photoelectric field in thinfilms. In the future, the doping dependence of the substrate and the thickness dependence ofthe thin film will be studied to investigate the possibility of controlling the electronic state bysurface photovoltages.

Fig. 1 : Time-resolved ARPES results of Bi2Te3 thin film at 2 QL thickness. (a) is theground state without the pump pulse. (b) and (c) are the results at delay times of -1 ps(before arriving the pump pulse) and 0 ps (after arriving the pump pulse), respectively.

Fig. 2 : Time evolution of surface photovoltage amplitude estimated from the peak positionof the band dispersion, where delay time is the time difference between the pump andprobe pulse.

REFERENCES[1] H. Zhang et al., Nat. Phys. 5, 438 (2009).
[2] J. A. Sobota et al., Phys. Rev. Lett. 108, 117403 (2012).
[3] S. Hatta et al., Sci. Rep. 11, 5742 (2021).
[4] K. Kawaguchi et al., arXiv:2303.16466 (2023).
[5] D. K. Schroder, Meas. Sci. Technol. 12, R16 (2001).
[6] S. Ciocys et al., npj Quantum Materials. 5, 1–7 (2020).

FERMI SURFACE MAPPING OF TOPOLOGICAL INSULATORPB(BI0.3SB0.7)2TE4
Koichiro YAJI1, Yuya HATTORI2, Shunsuke YOSHIZAWA1, Shunsuke TSUDA1,
Youhei YAMAJI3, Yuto FUKUSHIMA4, Kaishu KAWAGUCHI4, Takeshi KONDO4,
Yuki TOKUMOTO5, Keiichi EDAGAWA5, and Taichi TERASHIMA2
1

Research Center for Advanced Measurement and Characterization, NIMS
2

3International Center for Materials Nanoarchitectonics, NIMS

Center for Green Research on Energy and Environmental Materials, NIMS
4

Institute for Solid State Physics, The University of Tokyo

5

Institute of Industrial Science, The University of Tokyo

Topological materials are placed at a central research field in solid-state physics and materials
science. Topological surface states (TSSs) are formed through a phase transition from a trivial
insulator to a topological insulator, the so-called topological phase transition. The topologicalphase transition has been experimentally demonstrated in several systems, where the bulk bandinversion is artificially controlled by changing the composition and modulating the lattice
constant, etc. In contrast, experimental studies on the topological phase transition in pristine
topological materials have not been established so far, even though examining it helps us
understand the fundamental electronic properties of the topological materials. In the present
study, we have performed the Fermi surface mapping of Pb(Bi0.3Sb0.7)2Te4 to elucidate theelectronic band structure. Here, the Pb-based ternary compounds, Pb(Bi1-xSbx)2Te4, is known
as a strong topological insulator, which have been confirmed by angle-resolved photoemission
spectroscopy [1–3]. This work is part of a project to elucidate topological phase transitions inpristine materials.
Laser-ARPES measurements have been performed at the Institute for Solid State Physics,
The University of Tokyo [4]. The samples were cleaved with scotch tape in an ultra-highvacuum chamber. The photoelectrons were excited by lasers with photon energies of 6.994 eV[5] and 10.7 eV [6]. We used a p-polarized light. The sample temperature was kept at 30 K
during the measurements.
Figures 1(a) and (b) show the ARPES intensity map along GM and constant energy contours
of. Figure 1(b) shows constant energy contours Pb(Bi0.3Sb0.7)2Te4 with an excitation energy(hn) of 6.994 eV. Above the Dirac point (DP), the constant energy contours gradually change
from circular to hexagonal shapes: the constant energy contour at EB = 110 meV is circular, at
EB = 70 meV hexagonal, and at EB = 30 meV the conduction band appears at G. In contrast, astrong warping effect appear in the constant energy contours below the DP. The constant energy
contours at EB = 330 meV is elongated in GM.
Photoexcitation with a 10.7 eV laser allows us to investigate a wider wavenumber range[Fig. 1(c)]. The photoelectron intensity distribution of the constant energy contours with hn =10.7 eV is different from hn = 6.994 eV, resulting from the final state effect in photoemission.
In the contours at EB = 190 meV, corresponding to the DP, and EB = 260 meV, a sole surface
state (or surface resonance) is observed around G in each image, meaning that the valence band
maximum (VBM) for x = 0.70 is located at G. At EB = 330 meV, a steeply elongated surface
resonance is observed in G. Therefore, the energy dispersions of the surface resonances aremoderate along G while they are steep away from GM.
The previous theoretical calculations suggest that the VBM for x = 0.0 is located at k = ±0.3
Å-1 in G with the energy position comparable to the DP, while the VBM for PbSb2Te4 (i.e. x =Fig. 1 (a) ARPES intensity images recorded along GM for Pb(Bi0.3Sb0.7)2Te4. [7] (B,C) Constant energy
contours of the ARPES intensity taken with hn = 6.994 eV [7] and 10.7 eV photons, respectively.

1.0) is located at G with the energy 200 meV lower than the DP [1]. In the previous ARPES
study for x = 0.0, the band at k = ±0.3 Å-1 in G is located at 20 meV lower energy than the DP,
which is assigned as the VBM [3]. The present study reveals that in the case of x = 0.70, theband at k = ±0.3 Å-1 in G is found in a deeper binding energy side by 100 meV than the energyof the DP. Our ARPES suggests that the band energy at k = 0.3 Å-1 in G can be tunable by the
replacement of Bi with Sb.
REFERENCES[1] T. V. Menshchikova, S. V. Eremeev, E. V. Chulkov, Applied Surface Science 267, 1 (2013).
[2] S. Souma, K. Eto, M. Nomura, K. Nakayama, T. Sato, T. Takahashi, K. Segawa, and Y. Ando, Phys.
Rev. Lett. 108, 116801 (2012).
[3] K. Kuroda, H.Miyahara, M. Ye, S. V. Eremeev, Y.M. Koroteev, E. E. Krasovskii, E. V. Chulkov, S.
Hiramoto, C. Moriyoshi, Y. Kuroiwa, K. Miyamoto, T. Okuda, M. Arita, K. Shimada, H. Namatame,
M. Taniguchi, Y. Ueda, and A. Kimura, Phys. Rev. Lett. 108, 206803 (2012).
[4] K. Yaji, A. Harasawa, K. Kuroda, S. Toyohisa, M. Nakayama, Y. Ishida, A. Fukushima, S. Watababe,
C.-T. Chen, F. Komori, and S. Shin, Rev. Sci. Instrum. 87, 053111 (2016).
[5] T. Shimojima, K. Okazaki, and S. Shin S. J. Phys. Soc. Jpn. 84, 072001 (2015).
[6] K. Kawaguchi, K. Kuroda, Z. Zhao, S. Tani, A. Harasawa, Y. Fukushima, H. Tanaka, R. Noguchi,
T. Iimori, K. Yaji, M. Fujisawa, S. Shin, F. Komori, Y. Kobayashi, and Takeshi Kondo, submitted.
[7] K. Yaji, Y. Hattori, S. Yoshizawa, S. Tsuda, F. Komori, Y. Yamaji, Y. Fukushima, K. Kawaguchi, T. Kondo,
Y. Tokumoto, K. Edagawa, T. Terashima, submitted.

DIRECT OBSERVATION OF QUANTUM WELL STATES INDUCEDBY THE HYDROGEN-BONDED ORGANIC FRAMEWORKS FORMEDON METAL SURFACE
Kaname Kanai1, Rena Moue1, Hiroto Yamazaki1, Kaishu Kawaguchi2, Ryo Mori2, Yuto
Fukushima2, Ayumi Harasawa2, Shik Shin2, Takeshi Kondo2
Faculty of Science and Technology Tokyo University of Science1, The Institute for Solid State Physics,
The University of Tokyo2

The capability to control the electronic properties of the material surface is essential for thedevelopment and optimization of applications involving interfacial electron transfer, such as
optoelectronic devices and photovoltaic devices. Therefore, noble metal (111) surfaces are ofparticular interest due to their unique susceptibility to the presence of adsorbates due to theirelectronic structure. In fact, the two-dimensional free electron-like Shockley state (SS)localized at the Γ point of the noble metal (111) surfaces is easily modulated by periodic
potentials formed by regularly adsorbed atoms and molecules[1-3]. This change in SS at thenoble metal surface can be revealed by observing the change in its energy band dispersion.
For example, it is known to appear as a change in Rashba effect. The effect is an effect inwhich spin-orbit interactions due to spatial inversion symmetry breaking at the noble metal
surface causes spin splitting in the SS. The magnitude of the effect is highly dependent on its
surface states. It is also known that the formation of a molecular layer with corrals on thenoble metal can form quantum confinement by confining SS, that is, the surface free electrons,in the corrals. Recently, in order to investigate the latter, the focus has been on observing theeffects of self-organized ordered layers with large corrals on the surface electronic structure[3,4]. This is potentially problematic because the properties of the electronic states are inherently
sensitive to defects in the self-assembled molecular layer. Another approach to compensate
for the imperfections in self-assembly is to use a ring of molecules such that each individual
molecule acts as a corral to confine electrons. This allows each molecule to form nearly
identical confinement potential, forming a long-range defect-free quantum confined surface
states (QCSS)[5].
In this study, we investigatedthe effect of a self-assembled
film of [8]cycloparaphenylene([8]CPP) (figure 1a), which hasaring-shaped
molecular
structure, on the SS of Au(111).
The adsorption structure of[8]CPP was determined by
low-energy electron diffraction(LEED) and the energy band Figure 1. a) Molecular structures of [8]CPP. b) Experimental anddispersion
was
directly simulated LEED results for [8]CPP/Au(111) and schematic of theobserved by angle-resolved [8]CPP growth process on Au(111).The right half of the circle shows
the LEED pattern observed at the primary energy of Ep=25 eV
photoemission
spectroscopy (exp.). The left half of the circle shows the LEED pattern simulated(ARPES).
by calculation (cal.). The bottom row of b) shows a schematic of the
Figure1bshows
the growth process of the [8]CPP monolayer on Au(111) obtained fromexperimental(exp.)and the LEED results shown in the top row. When [8]CPP coverage is 3,
calculation (cal.) results of 5, and 8 Å, the adsorption structure is formed along the angular
direction of 28~32°, 25~35° and 15~45°) from the [-110] direction
LEED with stepwise increasing in Au(111).
coverage of [8]CPP monolayers
on Au(111). Furthermore, the adsorption structures of [8]CPP with different coverage were
determined from the LEED results. From left to right, the results correspond to about 3, 5, and8 Å coverage of [8]CPP, respectively, and it was confirmed that [8]CPP forms an adsorption
structure as if flatly adsorbed on the Au(111) surface, a result similar to previous studies[5]. In
addition, we confirmed that the growth direction of the adsorption structure diffused as thecoverage increased. In short, this study revealed for the first time that the adsorbed structures
gradually become multi-domain as the coverage increases.
The changes in SS observed by ARPES in accordance with the changes in the adsorption
structure of the revealed [8]CPP are shown in figure 2. Figures 2a, c, e, and g are band
mappings by ARPES, and figures 2b, d, f, and h are second-order derivative (SOD) results toclarify the features. Figures 2a and b show the SS of Au(111), where the spin splitting(Rashba splitting) due to the Rashba effect is clearly visible. In [8]CPP(3 Å)/Au(111), where
few multi-domains are observed, the bottom of the energy band dispersion representing the
SS is flattened (figures 2c, d). In [8]CPP (5 Å)/Au(111), where the multi-domains wereclearly observed, the flattened energy band dispersion (flat band) was obscured, but the
Rashba splitting was again confirmed with a shift of about 0.1 eV to the low binding side
(figures 2e, f). In addition, the Brillouin zone was modulated by the super-periodic structure
formed by [8]CPP, and a new energy
band dispersion was also observed
due to the folding of the Au(111) sp
band. In [8]CPP(8 Å)/Au(111),
where multi-domains dominate, theflat band disappear and the energy
band dispersion indicative of Rashba
splitting and folding are obscured(figures 2g, h). The analysis revealed
that the magnitude of Rashba
splitting is about 18% when [8]CPP
was adsorbed at about 5 Å. The flatband observed at 3 and 5 Åadsorption
suggestquantum Figure 2. Band mapping around the Γ point of a) clean Au(111)confinement
causedbythe by AEPES. The lower left corner of the figure shows theconfinement of surface electrons in measured orientation in the k-space in the schematic of thesurface Brillouin zone (SBZ). b) SOD of the band mapping by
the corral. However, the flat band ARPES of clean Au(111) versus energy. The black areas on theare weakly dispersive because the SOD represent the electronic states. Band mapping around theconfinement by organic molecules is Γ point of c) [8]CPP(3 Å)/Au(111), b) [8]CPP(5 Å)/Au(111)incomplete and the electrons trapped and b) [8]CPP(8 Å)/Au(111) by ARPES. Band mapping basedin the corrals interact with each other on the SODs of the corresponding ARPES results, d), f), h).
to form a dispersion.
In this study, we observed the formation of [8]CPP on Au(111). As a result, we revealed forthe first time the formation of multi-domains with increasing coverage. In addition, thesurface electronic states in each were observed by ARPES, which affected the Rashba effecton the Au(111) surface and suggested the formation of quantum confinement.
REFERENCES[1] C. R. Ast et al., Phys. Rev. Lett., 98, 186807 (2007).
[2] J. Zroff et al., Surface Scienece, 603, 354-358 (2009).
[3] I. Piquero-Zulaica et al., Nat. Commun., 8, 787 (2017).
[4] J. Zhang et al., Chem. Commun., 50, 12289-12292 (2014).
[5] B. N. Taber et al., J. Phys. Chem. Lett. 7, 3073-3077 (2016).

SPIN- AND ANGLE-RESOLVED PHOTOEMISSION STUDY OF MAX
PHASE COMPOUND ZR3SNC2
Takahiro Ito1,2, Manaya Mita2, Kiyohisa Tanaka3, Masashi Nakatake4, Yuto Fukushima5,
Kaishu Kawaguchi5, Takeshi Kondo5, Laurent Jouffret6, Hanna Pazniak7, Serge Quessada7
and Thierry Ouisse7
1

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

2

Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa, Nagoya 464-8603, Japan

3

Institute for Molecular Science, Okazaki 444-8585, Japan

4

Aichi Synchrotron Radiation Center, Seto, 489-0965, Japan

5

Institute for Solid State Physics, Th University of Tokyo, Kashiwa 277-8581, Japan

6

Université Clermont Auvergne, CNRS, ICCF, F-63000 Clermont–Ferrand, France

7

Univ. Grenoble Alpes, CNRS, Grenoble INP, LMGP, F-38000 Grenoble, France

MAX phases (Mn+1AXn, where M isan early transition metal, A belongs togroups 13-15 and X is either C or N, n
= 1 - 3) have recently attracted muchattention due to their possible
application to the production of a newclass of two-dimensional (2D) systems
called MXenes [1]. However, the bulk
electronic structure of MAX phases
has been studied mostly through ab
initio, DFT calculations, mainly due toa lack of single crystalline samples.
We have performed angle-resolved
photoemission spectroscopy (ARPES)
on several MAX phase single crystals
to directly investigate the electronic
structure of these systems [2-5].
Among the MAX phases, Zr3SnC2 ischaracterized by relatively higher twodimensionality than 211 phase due tothe thicker MX layer. In addition, thestrong spin-orbit coupling effect due tothe 4d transition metal Zr is expected.
We have performed SARPES study on
Zr3SnC2 to eucidate the electronic andspin structures with using He I lightsource, which is available for the spin
mapping at the wide momentum space.
Figures 1(a(b)) and (c) show theband structure (which compared with
DFT calculation along GM line of the
Sn-terminated surface) and theconstant energy ARPES image at 3.1

Fig. 1: (a) Band structure along the ΓM line of Zr3SnC2.
(b) Same as (a) but compared with DFT calculation(light blue lines) of the Sn-terminated surface. (c)Constant energy ARPES image at 3.1 eV around the M
point. The definition of the spin polarization directions(Sx, Sy and Sz) projected on the sample surface areindicated by white arrows, respectively. (d-f) SARPES
spectra (S+:▲; S−:▽) at momentum cut#1 and #2 inFigs. 1(a) and (c) of Sx (a), Sy (b) and Sz (c), respectively.
(g-l) Enlarged SARPES spectra around 3.1 eV. Spinpolarization is shown at the bottom of each panel.

eV, respectively. From the comparison with DFT calculation, it has been found that theelectronic structure of Zr3SnC2 around the M point has been well reproduced by the Snterminated surface states, typically saddle-like dispersive feature just below the Fermi level.
While there is no clear sign of spin-polarization at the low binding energy side, we found that
non-negligible spin-polarization seems to be observed around 3.1 eV. In Fig. 1(i) and (l), outof-plane spin polarization (Sz) shows sizable Sz- sign at both cut#1 and #2. On the other hand,in-plane spin polarization Sx shows Sx+ sign at cut#2 (Fig. 1(j)). Furthermore, there is no clear
sign of in-plane spin polarization along y (Fig. 1(h) and (k)). From the observed spin textures,
we expect the possible existence of out-of-plane spin polarization at “bridge-like” band
dispersion around 3.1 eV as shown in the inset of Fig. 1(f), though the symmetrically required
antiparallel Sx- component has not been clearly observed. These results might suggest theeffect of Dresselhaus spin-orbit coupling causing the out-of-plane spin texture [6] at the Snterminated surface of Zr3SnC2. To clarify the mechanisms of the formation of the observed
spin-states and prove their existence, further ARPES study choosing surface termination as well
as DFT calculation of spin-texture will be performed in our future work.
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] D. Pinek et al., Phys. Rev. B 102, 075111 (2020).
[5] D. Pinek et al., Phys. Rev. B 104, 195118 (2021).
[6] L. L. Tao et al., Nature Commun. 9, 2767 (2018).

HIGH-RESOLUTION TEMPERATURE DEPENDENT SPINRESOLVED PHOTOEMISSION SPECTROSCOPY OF HALFMETALLIC FERROMAGNET La1-xSrxMnO3
Takayoshi YOKOYA1, Noriyuki KATAOKA1, Takanori WAKITA1, Hirokazu FUJIWARA2,
Yuto FUKUSHIMA2, Kaishu KAWAGUCHI2, Hiroaki TANAKA, Ryo MORI2, Ayumi
HARASAWA2, Takeshi KONDO2, Hiroshi KUMIGASHIRA2,
Yuji MURAOKA3
1
Research Institute for Interdisciplinary Science, Okayama University
2
Institute for Solid State Physics, The University of Tokyo
3
Institute of Multidisciplinary Research for Advanced Materials, Tohoku University
Half metallic ferromagnets have a unique spin dependent electronic structure near the Fermi
level (EF), where only one of the spin states crosses EF and the other has an energy gap acrossEF [1]. The unique electronic structure gives rise to many-body effects different from othersystems. Theoretically, the electronic states induced by many-body effects in half metals has
been proposed as non quasiparticle (NQP) states [2]. The experimental verification for NQP
states was made recently for a half metallic ferromagnet CrO2 by using high-resolution (HR)spin-resolved photoemission spectroscopy (SRPES) [3]. In the present research, we have
performed HRSRPES of another half metallic ferromagnet La1-xSrxMnO3 to experimentally
explore the characteristics and universality of NQP states. La1-xSrxMnO3 is one of the famous
perovskite materials, exhibiting colossal magnet resistance around Sr concentration x of 0.3.
Homoepitaxial films of La1-xSrxMnO3 were grown on the atomically-flat (001) surface ofNb-doped SrTiO3 substrates using a laser MBE method. Laser-based HRSRPES experiments
were performed at the Institute for Solid State Physics, The University of Tokyo [4]. The ppolarized light with hν = 6.994 eV was used to excite the photoelectrons. Photoelectrons wereanalyzed with a combination of a ScientaOmicron DA30L analyzer and a very-low-energyelectron-diffraction (VLEED) type spin detector. During the measurement, the instrumental
energy resolution was set to 30 meV to compensate small electron counts and the base pressure
was kept below 1 × 10#$ Pa. Calibration of the Fermi edge (EF) for the samples was achieved
using a gold reference. The data were taken at T = 20, 50, 90, 110, 135, 195 and 300 K. The
sample was annealed at 400 K under oxygen atmosphere prior to the experiment. We
magnetized the La1-xSrxMnO3(001) sample along the magnetic easy axis ([110] direction) by
bringing the sample close to a magnet. The approximate magnitude of the magnetic field at the
sample position was 700 Oe.
Figure 1 shows the temperature dependent HRSRPES data. At 20 K, the spectrum of the
majority spin states shows a Fermi edge, while that of the minority spin states shows negligible
intensity near EF. Accordingly, the polarization is nearly 100% near EF, providing spectroscopic
evidence for half metallicity of the sample at 20 K. In increasing temperature, the spectra of the
majority and minority spin states, as well as the polarization, do not show drastic changes till

195 K. At 300 K, the spectral shapes of the majority and minority spin states become similar
and the polarization reduces substantially. These results are consistent with the previous study[5]. However, we found that the temperature dependent polarization is different from theprevious study. In the present study, temperature dependent polarization keeps similar values
till 195K (T/TC ~ 0.56, where TC is a Curie temperature.), while that reduces at a much low
temperature in the previous study. We attribute the difference to the difference in the bulk
sensitivity of the present study. The steep reduction of polarization at EF, which is expectedfrom the growth of NQP, was not observed clearly within the experimental accuracy.

Fig. 1. Temperature dependent HRSRPES spectra and polarizations of La1-xSrxMnO3. The
red triangles, bule triangles, and green circles show the majority spin spectrum, the minority
spin spectrum, and polarization, respectively.
ACKNOWLEDGEMENT
This work was partially supported by JSPS KAKENHI Grand Numbers (JP20H0185313)
from MEXT.
REFERENCES[1] M.I. Katsnelson et al., Rev. Mod. Phys. 80, 315-378 (2008).
[2] V.Y. Irkhin and M.I. Katsnelson, Usp. Fiz. Nauk 164, 705 (1994).
[3] H. Fujiwara et al., Phys. Rev. Lett. 121, 257201 (2018).
[4] K. Yaji et al., Rev. Sci. Instrum. 87, 053111 (2016).
[5] J.-H. Park et al., Nature 392, 794-796 (1998).

LIGHT POLARIZATION DEPENDENCE OF LOW-ENERGYANGLE-RESOLVED PHOTOEMISSION SPECTROSCOPY OF A
TRANSITION METAL DICHALCOGENIDE
Hiroaki Tanaka
The Institute for Solid State Physics, The University of Tokyo

Angle-resolved photoemission spectroscopy (ARPES) has been utilized to observe the band
dispersion of crystals experimentally [1]. In ARPES measurements, X-ray or vacuum
ultraviolet (VUV) light is irradiated on the crystal surface, and emitted photoelectrons areanalyzed. The energy and momentum conservation laws determine the relationship between
the Bloch wavevector and the binding energy, in other words, the band dispersion of solids.
As well as the band dispersion determined by ARPES, the photoemission intensity gives
deeper insight into the electronic structure of solids because it is determined by the matrix
element between the initial states (wave functions of the ground state) and the final states(approximated to plane waves). For example, the light polarization dependence of ARPES
spectra has revealed orbital contributions of valence bands in the iron-based superconductor
FeSe [2, 3]. Our study aims to quantitatively discuss the photoemission intensity distribution
by combining the high-resolution ARPES and the photoemission intensity calculations basedon the first-principles calculations.
We used the laser-based ARPES setup constructed at the Institute for Solid State Physics,
the Univ. of Tokyo [4]. The photon energy was 7 eV, and the light polarization varied from θ= 0° (s-polarization) to θ = 180°, where θ = 90° is the p-polarization. The measurement
temperature was around 60 K, and the energy resolution was 13 meV.
We measured the photoemission spectra of the transition metal dichalcogenide 1T-TiS2 [Fig.
1(a)]. TiS2 is a simple one-layered material, so we don’t need to consider the termination
surface dependence. No structure transition, such as the charge density wave phase, has beenreported. Therefore, TiS2 is suitable for quantitatively analyzing the photoemission intensity
by experiments and numerical simulations. The band dispersion of 1T-TiS2 has two holebands near the Γ point [Fig. 1(b)]. Since these bands have only weak kz dispersion, as
described by slightly thick slab band dispersion, we can expect that they can be observed
whatever photon energy is used.
Figures 1(c) and 1(d) summarize our ARPES measurements. When we used s-polarized
light, the heavy hole band [blue curves in Fig. 1(c)] strongly contributed to the photoemission
intensity. On the other hand, when p-polarized light was used, the light hole band [red curves
in Fig. 1(d)] was dominant. This difference reflects the orbital selectivity of thephotoemission process and its dependence on incident light polarization.
We investigated the light polarization dependence by changing the angle θ with a 10° step.
Since the λ/2 waveplate installed in the laser-ARPES setup can be mechanically controlled,
we could measure the polarization dependence automatically using a Python program. Figure
1(d) shows the result. As discussed above, the peak position in the p-polarized lightmeasurements is about 50 meV lower than in the s-polarized case. More important is that thepolarization dependence of the peak position exhibits a smooth curve. When we consider thebulk band dispersion comprising heavy and light hole bands, one of two peaks or both peaks
should appear. However, we observed that the spectrum peak could appear between two holebands, even though the VUV-ARPES using 7 eV laser is considered relatively bulk sensitive
due to the long inelastic mean free path [5]. This result clearly shows that the surface
electronic structure should be considered to discuss the polarization dependence of ARPES
spectra.

Figure 1: ARPES measurements of 1T-TiS2. (a) Crystal structure. (b) Calculated slab band dispersion along theΓK (kx) direction. (c) Experimental band dispersion taken by s- and p-polarized light. The blue and red curves
represent peak positions extracted from energy distribution curves in s- and p-polarization measurements. (d)Light polarization dependence of the peak position at kx = -0.1 Å-1 and 0.1 Å-1.

In summary, we investigated the polarization dependence of ARPES spectra using thetransition metal dichalcogenide 1T-TiS2 and a high-resolution VUV-ARPES machine
equipped with a 7 eV laser. We observed that the peak position of the hole bands oscillated bychanging the light polarization. Such a continuous change cannot be explained by the bulk
band dispersion of 1T-TiS2, so we will need to employ a slab system and detailed
photoemission intensity calculations to reproduce the results theoretically.
REFERENCES[1] J. A. Sobota et al., Rev. Mod. Phys. 93, 025006 (2021).
[2] M. Ti et al., Phys. Rev. X 9, 041049 (2019).
[3] R. P. Day et al., npj Quantum Materials 4, 54 (2019).
[4] K. Yaji et al., Rev. Sci. Instrum. 87, 053111 (2016).
[5] M. P. Seah and W. A. Dench, Surf. Interf. Anal. 1, 2 (1979).

SURFACE STATE SPIN POLARIZATION MEASUREMENT OF Co
DOPED LaFeAsO
Soonsang Huh1, Yuto Fukushima1, Kaishu Kawaguchi1, Ayumi Harasawa1, Felix Anger2,
Wurmehl Sabine2, Bernd Büchner2, 3, and Takeshi Kondo1
1. The Institute for Solid State Physics (ISSP), The University of Tokyo, Kashiwa, Chiba 277-8581,
Japan
2. Leibniz Institute for Solid State and Materials Research, IFW-Dresden, 01069, Dresden, Germany
3. Institute of Solid State Physics, TU Dresden, 01069, Dresden, Germany

The discovery of the superconductivity in F-doped LaFeAsO has opened new era foriron-based superconductors and brought renew interest in high TC superconductivity research[1]. Extensive and intensive studies have shown that the Oxypnictide system, so called 1111
crystal structure system, exhibits relatively high Tc than other systems. Up to now,
SmFeAsO1-x (TC ~ 55 K) has been observed with the highest Tc in bulk single crystal atambient pressure among iron-based superconductors [2]. For this reason, understanding themechanism that why high Tc favor in this crystal structure become one of the main issues in
the research field.
In this regard, several angle-resolved photoemission spectroscopy (ARPES)
experiments were conducted to understand the electronic structure, which is believed toprovide important clues to the high TC mechanism. However, the 1111 system shows apolarized cleaved surface arising from both Lanthanide Oxides layer and Iron Pnictogen layer,
resulting in a complex electronic
structure. For example, [LaO]+ layerand [FeAs]- layer exists in LaFeAsO.
This leads to coexistence of surface
and bulk electronic state. Although
there are several studies reporting
surface and bulk state [3, 4], still it is
not clearly defined.
In this study, ARPES
measurement were performed on 6%
Co-doped LaFeAsO single crystal
which is synthesized by solid state
crystal growth technique [5]. Figures
1 (a) and (b) show Fermi surface taken
at 10 K with photon energy of 40 eV
and 7 eV, respectively. Fermi surface
area of 7 eV data corresponds to reddashed box in Fig. 1 (a). Two large[Figure 1] Fermi surface taken by (a) 40 eV and (b) 7 eV. (c)circular pockets are observed in both High symmetry cut data taken along Γ – M direction. (d)data. High symmetry cut along Γ – M Spin resolved energy distribution curve. Red and blue line+
direction is shown in Fig. 1 (c). Two indicates Y and Y spin direction measurement result,respectively.
electron band like dispersion isobserved around k = 0.4 Å -1. According to previous calculations, the observed bands are
expected to be the surface state. We tested the spin polarization of the this observed surface
states. Surprisingly, opposite spin polarization is observed between the two bands. Spinpolarization is likely due to the presence of inversion symmetry breaking in this surface state.
However, the cause of this spin polarization needs to be elucidated through further
experimental/theoretical studies.
REFERENCES[1] Y. Kamihara, T. Watanabe, M. Hirano, and H. Hosono, J. Am. Chem. Soc. 130, 3296 (2008).
[2] Z.-A. Ren et al., Chin. Phys. Lett. 25, 2215 (2008)[3] L. X. Yang et al., Phys. Rev. B 82, 104519 (2010).
[4] P. Zhang et al., Phys. Rev. B 94, 104517 (2016).
[5] R. Kappenberger et al., J. Cryst. Growth 483, 9 (2018).

THICKNESS DEPENDENCE OF SPIN-POLARIZED BANDSTRUCTURE IN TOPOLOGICAL INSULATOR Bi2Te3 THIN FILMS
Kaishu Kawaguchi1, Ryo Mori1, Yuto Fukushima1, Hiroaki Tanaka1, Shinichiro Hatta2,
Ayumi Harasawa1, Tetsuya Aruga2, Takeshi Kondo1
1

2

ISSP, The University of Tokyo
Graduate School of Science, Kyoto University

INTRODUCTION
Topological insulators, with their surface state exhibiting spin-polarized electronic states on
the surface, hold promising applications in spintronics and quantum information, and so on. In
this context, employing thin films to enhance surface effects is a key approach. Under thissituation, interference effects occur between the electronic states present at the surface andinterface [1]. And additionally, energy splits in band dispersion also occur originating fromthe substrate effects [1,2]. In particular, substrate effects, when viewed from a macroscopic
perspective, can be considered as a kind of the
Rashba effect resulting from broken inversion
asymmetry from the interface between thin films
and substrate, called structure inversion
asymmetry (SIA) [2]. This offers excitingprospects of merging different physical properties
through junctions with superconductors [3] andso on. However, these SIA largely depend on thesubstrate type and controlling them can bechallenging due to the varying origins andquantitative differences of potential modulation.
Therefore, it is crucial to reveal the more
microscopic origins of SIA, details about thepotential structure and its origins that werepreviously unclear. For this purpose, we have Fig. 1 : Band-dispersive depiction ofextensively investigated the thickness and energy-splitting structures originating
substrate dependency of the energy splitting from the hybridization of topologicalstructure in the thin films of the topological surface and interface states and SIA in
insulator Bi2Te3, to explore how potential topological insulator thin films.
modulation can be controlled.
METHODS
A challenge in this research lies in the fact that each energy-split band dispersion is
localized at the surface and interface. Therefore, it is difficult to identify the interface side in
Angle-Resolved Photoemission Spectroscopy (ARPES) due to the decay of photoelectron
intensity within the thin film. This complication, along with band broadening, can lead to
obscured energy splitting, making observation impossible—a phenomenon reported inprevious studies that calls for a solution. In this study, by using spin-resolved ARPES(SARPES) technics, we aimed to clearly separate the energy splitting structure based on the
difference in spin polarization direction derived from the reversed helicity of the surface andinterface.
For this study, considering the importance of insulating films in applications, experiments
were conducted using a CaF2/Si(111) substrate prepared by depositing a CaF2 thin film on ap-type Si(111) substrate. Bi2Te3 ultra-thin films were prepared in ultra-high vacuum using a
Bi2Te3 deposition source [4]. The SARPES apparatus combines a Very Low Energy Electron
Diffraction (VLEED) spin detector and a 7-eV laser to form a high-efficiency, high-resolution
system [5] capable of decomposing even fine spin-split structures for efficient observation.

RESULTS AND DISCUSSION
Figure 2 shows the thickness dependency of the Bi2Te3 thin film made on the CaF2/Si(111)substrate. A clear band structure can be observed, indicating high-quality thin film. Also, it isevident that a gap is formed at the Dirac point for less than 4 QL, aligning with previous
research. Although the ARPES image does not reveal any energy-split behaviour, successful
observation of the splitting structure was achieved by conducting spin-resolved measurements(Fig. 3). Furthermore, upon investigating the thickness dependency, an energy inversion of
the spin-splitting structure was observed from 1 to 3 QL. Such an inversion phenomenon has
never been observed in previous studies, making our research a first in this regard. Also, as
the film thickness increases, a shift of the band to higher energy was noticed.
As possible origins of the energy split at the surface and interface, crystal structure
distortion and peculiar charge transfer are considered. About structure distortion, it isespecially noticeable when the film is thin [6]. On the other hand, if the charge is polarized
towards the Bi2Te3 side, the effect becomes significant when the film is thick. Therefore, it isbelieved that this inversion phenomenon of spin-splitting depending on the thickness arose
due to the competition between these two effects. To understand these origins in more detail,it will be necessary to examine the doping dependence and to verify it through calculations.

Fig. 2 : ARPES results of the thickness dependence of Bi2Te3 thin films, for thicknesses of(a) 1 QL, (b) 2 QL, (c) 3 QL, and (d) 8 QL.

Fig. 3 : (a) 1 QL and (b) 3 QL, the peak positions obtained by fitting each spin-resolved
ADC (angular distribution curve) spectrum are plotted on the ARPES image. The filling
points indicate the spin up (red) and down (blue) peak positions for higher intensities andopaque points for lower intensities. (c) is ADC fitting result of (a) at E - EF = -0.15 eV.

REFERENCES[1] H. Zhang et al., Nat. Phys. 5, 438 (2009).
[2] Y. Zhang et al., Nat. Phys. 6, 584–588 (2010).
[3] H. Yi et al., Nat. Mater. 21, 1366–1372 (2022).
[4] S. Hatta et al., Sci. Rep. 11, 5742 (2021).
[5] K. Yaji et al., Rev. Sci. Instrum. 87, 053111 (2016).
[6] G. Landolt et al., Phys. Rev. Lett. 112, 057601 (2014).

QUANTUM CONFINED SURFACE STATES FORMED IN A
HEXAGONAL CORRAL IN SELF-ASSEMBLEDHYDROGEN-BONDED ORGANIC FRAMEWORKS
Kaname Kanai1, Rena Moue1, Hiroto Yamazaki1, Kozo Mukai2, Kaishu Kawaguchi2, Yuto
Fukushima2, Fumihiko Ozaki2, Ryo Mori2, Takeshi Kondo2, Ayumi Harasawa2, Shik Shin2,
Jun Yoshinobu2
Faculty of Science and Technology Tokyo University of Science1, The Institute for Solid State Physics,
The University of Tokyo2

The ability to control and modify the electronic properties of material surfaces is essential
for the realization of next-generation devices involving electron and spin transfer at theinterface between material surfaces and electrodes. In the field of surface science, theproperties of many surface states and techniques for the modification of these surface states
have been pursued. In particular, the Shockley state (SS), which is a pseudo-two-dimensional
free electronic surface state on the (111) surface of an fcc-metal, is sensitive to various
molecular adsorptions and has been the subject of many studies. However, SS is easily andquickly dissipated by irregularly adsorbed molecules. Meanwhile, for regularly arranged
molecules on the surface, SS survives, and in some cases, its properties are significantly
modified[1]. This indicates that the sensitivity of SS to adsorbates originates from a situationin which spatially spread pseudo-free electrons in two dimensions are easily scattered by theperiodic potentials newly created parallel to the surface by the adsorbed molecular array. Insuch a situation, electron waves scattered by adsorbates regularly arranged to form void
spaces, referred to as corrals, are expected to create standing waves inside the corrals.
In recent years, several studies have mentioned that molecules with a macrocyclic structure,
superstructures made by adsorbing molecules on their surfaces and metal clusters, can create
standing waves of electrons and have demonstrated quantum confinement in these structures.
However, the formation of such quantum confined surface states (QCSSs) associated withmolecular adsorption is mostly caused by accidentally self-assembled superstructures, andneither the corral size nor its shape is easily tunable. In this study, we propose anotherapproach that is relatively robust to self-assembly imperfections, namely quantum
confinement based on hydrogen-bonded organic frameworks (HOFs).
Carbon nitride molecule, melem (2,5,8-triamino-tri-s-triazine) has three-fold symmetry.
Melem forms HOFs with characteristic structures by forming multiple hydrogen bonds
between adjacent molecules in the crystal or on the surface. In general, many carbon nitride
compounds that use hydrogen bonding as one of their cohesive forces have large voids intheir crystal structures, which is also true for HOFs formed on the surface (SHOF). The
SHOF is expected to form a graphite-like structure with a long-range order rather than alocalized structure. Under these conditions, the SHOFs formed on the fcc-metal (111) surface
are expected to produce identical confinement potentials in the corral created in theirstructures, resulting in the formation of a two-dimensional network of nearly identical QCSSs.
In this study, SHOFs were formed on Au(111) by melem, and their electronic structures
were investigated by angle-resolved photoemission spectroscopy (ARPES). The presence ofslightly distorted hexagonal corrals in the SHOF of melem with a honeycomb (HC) structureis expected to lead to quantum confinement of SSs of Au(111). Meanwhile, the confinement
potential is created as a result of the slight displacement of the Au(111) surface atoms in thesurface vertical direction, and the magnitude of the potential formed is small.
The SS of clean Au(111) observed by ARPES (Figure 1a) shows parabolic band dispersion
as a quasi-two-dimensional free-electron system. The SS band exhibits a large Rashba
splitting characteristic of Au(111). This large splitting is because the SS of Au(111) has astrongly asymmetric wave function near the outermost nuclei of the Au atoms. The effective

mass of the SS electron was estimated to be m* = (0.274 ± 0.020) me. the coveragees of themelem layers at 1 and 2 Å correspond to the coverages of the submonolayers, and the melem
layer has a HC structure. The band mapping results of Melem(1 Å)/Au(111) (Figure 1b)
clearly show that the SS band shape of Au(111) is different from that of pristine Au(111).
ARPES band mapping showed no Rashba splitting and a single broad weak energy band. In
contrast, a localized state with a narrow dispersion width, which does not exist in pristineAu(111), is newly formed at approximately 0.40 and 0.47 eV. Since the highest occupied
molecular orbital of the melem appears below a binding energy of 2 eV, these localized states
are not derived from the melem itself.
To examine the detailed band dispersion of this localized state, curve fitting was performed
for each energy distribution curve, and the results are shown by black dashed lines in Figure
1b. This result also shows that the dispersion width is extremely narrow compared with that of
the SS. In contrast, this level was slightly curved, indicating slight dispersion. The band
mapping results of Melem(2 Å)/Au(111) by ARPES intensity (Figure 1c) were very similar to
those of Melem(1 Å)/Au(111), with a localized state observed near the bottom of the SS. In
contrast, the fitting analysis shows that the dispersion widths of the two localized states areslightly smaller than those of Melem(1 Å)/Au(111), indicating a more localized nature. The
discrete states present in Melem(1 Å)/Au(111) and Melem(2 Å)/Au(111) suddenly
disappeared in both the ARPES (Figure 1d) of Melem(3 Å)/Au(111). From the results of
LEED experiments, when the coverage of the melem layer reached 3 Å, a monolayer wasformed in which the closed-packed structure was dominant. This abrupt change in the surface
electronic structure strongly suggests that the localized states originates from the HC structure
and that this is the QCSSs in the corral of the HC structure. Melem(3 Å)/Au(111), where theclosed-packed structure is dominant, does not have a corral similar to the HC structure, which
can be interpreted as the disappearance of the localized states observed in the HC structure
because quantum confinement of the surface electrons does not occur.

Figure 1. Band mapping around the G point (kx = 0 Å-1) of a) clean Au(111), b) melem(1Å)/Au(111), c) melem(2 Å)/Au(111), and d) melem(3 Å)/Au(111) by angle-resolved
photoemission spectroscopy (ARPES).
REFERENCE[1] R. Moue et al., Adv. Mater. Inter., 2022, 2201102 (2022).

STUDY ON SPIN STRUCTURE OF THE SURFACE STATE IN TYPE-Ⅱ
WEYL SEMIMETAL NbIrTe4
Soonsang Huh1, Soohyun Cho2, Wei Xia3, 4, Soonsang Huh1, Yuto Fukushima1, Ayumi
Harasawa1, Takeshi Kondo1, 5, Yanfeng Guo3, 4 and Dawei Shen2*
1. Institute for Solid State Physics, University of Tokyo, Kashiwa 277-8581, Japan
2. National Synchrotron Radiation Laboratory, University of Science and Technology of China,
Hefei 230029, China
3. School of Physical Science and Technology, Shanghai Tech University, 201210, Shanghai,
China
4. Shanghai Tech Laboratory for Topological Physics, Shanghai Tech University, 201210,
Shanghai, China
5. Trans-scale Quantum Science Institute, University of Tokyo, Tokyo 113-0033, Japan.

The topological Weyl semimetals (TWS) originate from their unusual bulk electronic
structure and have the nondegenerate bands crossing in a single point of k space, so-called the
Weyl point. This can be realized in the quantum materials where either time-reversal or
inversion symmetry, or both, is broken [1-3]. The symmetry tends to produce copies of Weyl
points and separate the Weyl nodes with different non-zero Chern number in the Brillouinzone. On the boundary, topological edge states exist at edges of 2D planes and disappear atedges of other planes in which the separation points between two types of planes are Weyl
points [1]. Therefore, the Fermi surface of the TWS simply consist of only two such points
and show an opened line that connect each pair of Weyl nodes with opposite chirality, whichis called a Fermi arc [1-3]. Moreover, in MoxW1−xTe2 compound, another type of the TWS
has been discovered with called type-II TWS whose Weyl nodes are highly anisotropic andtilted, breaking the Lorentz symmetry [1-5]. Given the feature of the band touching at the
Weyl node, the Fermi surface at the Weyl nodes of the type-II WSM exhibit the electron andhole pockets [2-5]. This distinction feature of the band touching near the Weyl node may givea rise to marked differences in the thermodynamics and response to external magnetic field[2-5].
Recently, it has been reported that (Nb, Ta)IrTe4 is a ternary TWS of type-II [3-5]. This
compound crystallizes in a non-centrosymmetric orthorhombic lattice with space group
Pmn21 (No. 31) and have van der Waals interaction within layers [3-5]. Figure 1a show theside and top view of the crystal structure in NbIrTe4, which show that the Nb and Ir atoms areoctahedrally coordinated by Te atoms. The structural symmetry and a stacking configurationof NbIrTe4 is identical structure with the orthorhombic 1Td-MX2 (M = Mo, W; X = S, Se, Te)
as the first suggestion with the type-II WSM, but NbIrTe4 has a unit cell doubled along b
direction and the substitution from M atoms in 1Td-MX2 to Nb and Ir atoms in NbIrTe4 [3-5].
Because of the similarity in the crystal structure, NbIrTe4 has the Nb-Ir chain as shown in the
top view of Fig. 1a, which is similar with the Mo-Mo or W-W chain that can be related to theunusual electronic transport in 1Td-MX2. The Nb-Ir chain is along the a and Γ to X directionin the crystal and Brillouin zone (BZ) coordinates in Figs. 1a and 1b, respectively. Figure 1b

shows the orthorhombic bulk BZ and projected surface BZ onto the (001) surface with thehigh symmetry points.
In this study, we investigate the electronic and spin structure of the surface state in NbIrTe4
utilizing high-resolution angle- and spin- resolved photoemission spectroscopy. The
low-energy photoemission spectra taken from the laser-based ARPES with 6.994 eV photons
clearly describe the fine dispersion of the surface state near the Y point with the higher
momentum resolution in Figs. 1 (c) and (d). In previous ARPES study in Refs. [5], NbIrTe4
show the Weyl points between the Γ and Y points, and the Weyl nodes would be connectedby the surface state along ky direction in Fig. 1(c). In our band calculation, the surface state at
Y point show the spin-polarization along the +sy or -sy direction, but we cannot obtain the
opposite sign of the spin-resolved energy distribution curves (EDCs) between -kF and +kF as
shown in Figs. 1(e) and (f). For the experiment failure to show the spin texture in the surface
state, we speculate that there could be a possibility to be a photoelectron interference effects
in the surface state [6].

Figure 1. (a) The top and side view of the crystal structure of NbIrTe4. (b) The orthorhombic
bulk BZ and projected surface BZ onto the (001) surface with the high symmetry points. (c)Experimentally obtained Fermi surface at 6.994 eV and 15 K. (d) The ARPES spectra along the
S-Y-S direction. The spin-resolved EDCs of the surface state at -kF (e) and +kF (f) as marked ind. The spin-resolved EDCs represent spin texture along the sy direction.

REFERENCES[1] C. Zhang et al., Annu. Rev. Mater. Res. 2020, 50:131-153.
[2] A. A. Soluyanov et al., Nature 2015, 527 495.
[3] Ilya Belopolski et al., Nat. Commun. 2017, 8 942.
[4] E. Haubold et al., Phys. Rev. B 2017, 95 241108(R).
[5] S. A. Ekahana et al., Phys. Rev. B 2020, 102 085126.
[6] Z.-H. Zhu et al., Phys. Rev. Lett. 2014, 112 076802.