STUDY ON THE HELICAL SPIN TEXTURE OF TOPOLOGICALSURFACE STATE IN SUPERCONDUCTOR 2M-WS2
Soonsang Huh1, Soohyun Cho2, Hua Bai3, Yuqiang Fang2, Fuqiang Huang2, Yuto
Fukushima1, Ayumi Harasawa1, Yunhao Lu3,, Gang Mu2 and Dawei Shen2,4 and Takeshi
Kondo1,5
1. Institute for Solid State Physics, University of Tokyo, Kashiwa 277-8581, Japan
2. Center for Excellence in Superconducting Electronics, State Key Laboratory of Functional
Materials for Informatics, Shanghai Institute of Microsystem and Information Technology, Chinese
Academy of Sciences, Shanghai 200050, China
3. Zhejiang Province Key Laboratory of Quantum Technology and Device, Department of Physics,
Zhejiang University, Hangzhou 310027, China
4. Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of
Sciences, Beijing 100049, China
5. Trans-scale Quantum Science Institute, University of Tokyo, Tokyo 113-0033, Japan.

Layered transition metal dichalcogenides (TMDs) of MX2 (M = Mo, W; X = S, Se, Te)
have been studied intensively because of a great interest in both fundamental and appliedresearch. Among the stable phases of TMDs, Monolayer 1T′-MX2 (M = Mo, W; X = S, Se,Te) were suggested as a promising candidate to realize quantum spin Hall (QSH) effect instrictly two-dimensional systems [1-4]. QSH insulators are expected to have two conductingedge channels with quantized electric conductance 2e2/h and opposite spin polarizations,which are robustly protected by the time-reversal symmetry. It could provide a possible
alternative to quantum electronic devices with low dissipation. However, because of thelimitation of monolayer, the TSS of QSH insulators in TMDs have been not addressed by theexperimental tools.
Recently, new stable structure phase 2M-MX2 have been found with the topological surface
state (TSS) and the superconductivity with the highest transition temperature 8.8 K among
TMDs [5-10]. Because the crystal structure of bulk 2M phase is identical with monolayer
1T′-MX2, 2M-MX2 have a similar band structure and TSS that is similar dispersion with TSS
of monolayer 1T′-MX2. The observation of TSS in 2M-MX2 can provide the information of
topological feature in monolayer 1T′-MX2 as well as own great interesting feature such as apossible candidate for the QSH insulator and the Majorana bound state together with thesuperconductivity [11].
We investigated the electronic and spin structure of the TSS in 2M-WS2 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 topological surface state with the higher momentum
resolution in Fig. 1(a). The hole-like band dispersion near the ky = 0 is corresponding to thelower part of the Dirac cone of topological surface state, and the crossing point is expected to
be located above the Fermi level. To show the helical spin structure of the topological surface
state, we exploit the spin-resolved ARPES. Figure 1(b) show the spin-resolved EDCs of sy
with red and blue colour code with spin up and spin down, respectively. The result indicatesthat the sign of sy is reversed between kL and kR. Moreover, we obtain that the in-planespin-polarization Py is reversed between kL and kR near the Fermi level, but anotherspin-polarization Px and Pz are negligible. It was expected that the topological surface state of
2M-WS2 show the helical spin
texture, thus allowing the opposite
spin direction between left andright momentum at the Fermi level.
In the view of the Fermi surface as
the schematic picture based on our
experimental results, the lower
Dirac cone branch of the
topological surface state in
2M-WS2 has the helical spin Figure 1(a) ARPES spectra of 2M-WS2 with the TSS obtained
texture with a clockwise rotation as from the band calculation and the possible direction of the spin
momentum based on the experimental results. The red (blue)
shown in Fig. 1(d). Given the arrow indicates the spin up (down). (b) Spin-resolved EDCs of
crystal and electronic structural the TSS at the left and right momentum cuts as marked in (a).
similarity between 2M-WS2 and The spin-resolved EDCs at kL and kR shows spin texture along
the Sy direction. (c) Representative spin-polarization magnitude
monolayer 1T′-MX2, our results for the TSS at kL and kR, whose obtained from the spin-resolved
can provide the general knowledge EDCs in (b). (d) Schematic picture of the helical spin texture
for the TSS at Fermi surface based on the spin-resolved EDC
about the edge conductance and and polarization. in (b) and (c)spin-based device as manufactured
with quantum spin Hall insulator,
monolayer 1T′-MX2.

REFERENCES[1] M. S. Lodge et al., Adv. Mater. 2021, 33, 2008029.
[2] Z. Fei et al., Nat. Phys. 2017, 13 677.
[3] X. Qian et al., Science 2014, 346, 6215.
[4] S. Wu et al., Science 2018, 359, 6371 76.
[5] Y. W. Li et al., Nat. Commu. 2021, 12, 2874.
[6] Y. Fang et al., Adv. Mater. 2019, 31, 1901942.
[7] X. Che et al., npj Quantum Mater. 2019, 4, 50.
[8] L. S. Wang et al., Rev. B 2020, 102, 024523.
[9] Y. Fang et al., J. Mater. Chem. C 2019, 7, 8551.
[10] P. Samarawickrama et al., ACS Omega 2021, 6, 4 2966.
[11] Y. Yuan et al., Nat. Phys. 2019, 15 1046.

ENHANCED RASHBA-SPLITTING OF AU(111) SURFACE STATES BY
THE MELAMINE HONEYCOMB LAYERS
Rena MOUE1, Yuta KOKUBO1, Kozo MUKAI2, Hirotaka MIZUSHIMA1, Yuto
FUKUSHIMA2, Kaishu KAWAGUCHI2, Takeshi KONDO2,3, Ayumi HARASAWA2, Jun
YOSHINOBU2, Shik SHIN2, Kaname KANAI1
1

2
3

Department of Physics, Faculty of Science and Technology,

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

Trans-scale Quantum Science Institute, The University of Tokyo.

In quasi-two-dimensional electron systems with broken spatial inversion symmetry, such as
metal surfaces and semiconductor interfaces, Rashba-type spin-orbit interactions (Rashba
effect) are known to occur through atomic spin-orbit interactions. The Rashba effect has
been actually demonstrated in quantum wells and thin films based on III-V semiconductors,
as well as in two-dimensional electron gases in the metal surface, and is expected to play animportant role as an elemental technology in spintronics.
Many studies have shown that the Rashba coupling can be modified by the adsorption of
atoms and molecules on metal surfaces and by distortions at the semiconductor interface. In
recent years, it has become possible to intentionally control the strength of the Rashba
coupling and design systems for various applications. For example, adsorption of noble gases
or organic molecules on noble metal (111) changes the strength of the Rashba coupling1.
Organic materials such as metal-organic frameworks (MOFs) and covalent organic
frameworks have been extensively studied over the past few decades and are potentiallyattractive for a variety of applications, including gas storage and separation, catalysis,
chemical sensors, and drug delivery2. On the other hand, new porous materials calledhydrogen-bonded organic frameworks (HOFs), which are constructed from pure organic or
metal-containing organic building blocks by hydrogen bonds, have recently attracted muchattention. Although many pioneering studies have been conducted on HOFs, their progresshas lagged behind that of MOFs and others. The main reason for the lag is that the structure ofmany HOFs collapses when solvent molecules are removed from the pores formed by weakhydrogen bonds. On the other hand, the instability of HOFs is caused by weak hydrogenbonds, but on the contrary, they are rather flexible compared to strong covalent orcoordination bonds. The use of this soft hydrogen bond is also beneficial for the flexibility of
the framework of porous materials. Carbon nitride molecules such as melamine(1,3,5-Triazine-2,4,6-triamine) and melem (2,5,8-triamino-heptazine) are typicalsupramolecular species that form HOFs through highly electronegative nitrogen-mediatedhydrogen bonds. The studies in these HOFs are rather well known for cyanuric acid andmelamine complexes, which have been studied for many years, where cyanuric acid andmelamine are combined by N-H···O and N-H···N hydrogen bonds to form rosette-shaped
superstructure3. On the other hand, melamine and melem are known to readily form highlyoriented monolayers consisting of two-dimensional ordered supramolecular structures byadsorption on metal surfaces (surface HOF: SHOF), without the help of solvent molecules.
This is probably due in large part to the flexibility of hydrogen bonding. Surfaces modified bysuch supramolecular structures are becoming powerful tools in applications such as chemicalsensors and nanoelectronics, thanks to their ability to exhibit molecular recognition features.
However, melamine is one of the most basic units in the systems that create such SHOFs, buteven for the structure of the SHOFs of melamine, the determination of their structures
remains arbitrary so far, although there are a few reported studies using STM4. Furthermore,
there are no reported studies that have conducted detailed investigations of the formation
process of melamine SHOFs, their electronic structure, or their interfacial electronic structurewith metal surfaces. Therefore, even now, many problems remain in understanding the basic
physical properties of melamine SHOFs as a prototype of SHOF.

In this study, we determine the formation process and the structure of melamine SHOFs on
Au(111) by X-ray photoemission spectroscopy (XPS), low-energy electron diffraction(LEED) and their effect on the Au(111) surface electronic structure by angle-resolved
photoemission spectroscopy (ARPES) and spin-resolved and angle-resolved photoemission
spectroscopy (SARPES). The SHOF formed when melamine adsorbs on Au(111) is a simple
but most typical system, so clarifying its formation is inevitable for understanding theformation of other SHOFs. From the LEED results shown in Figure 1, melamine, uponadsorption on Au(111), immediately forms a unique hexagonal SHOF through cooperativeintermolecular hydrogen bonds. Many other organic molecules, whose main cohesive force isdispersion force, do not form such cooperative superstructures and usually involve molecular
deformation upon adsorption to a surface, but in the case of melamine SHOFs, molecular
deformation is minimized by multiple hydrogen bonds between the molecules. The SHOF
retains its two-dimensionality, like graphene adsorbed on Au(111). Theoretical calculations
also show that the SHOF induces a slight, but obvious, change in the position of nuclei in theoutermost layer of Au(111) in the surface vertical direction. This indicates that there is strong
orbital mixing between SHOF and Au(111), meaning that SHOF forms a new periodic
potential on the surface. Such a strong coupling between the SHOF and the metal surface
electron system has a significant effect on the electronic structure of the surface electron
system, leading to a reconfiguration of the surface charge density and a modulation of theelectrostatic potential in the outermost layers of the surface. As a result, it is shown tosignificantly enhance Rashba coupling in Shockley state (SS) of Au(111). The conclusions
drawn here indicate that SHOF may provide new guidelines in the manifestation of thespin-galvanic effect, the Edelstein effect, and the inverse Edelstein effect, which are attracting
attention as elemental technologies in spintronics.

Figure 1. a) Experimental and simulated LEED results for melamine/Au(111). The left half ofthe circle shows the LEED pattern observed at primary energy of Ep = 41.9 eV; the right halfof the circle shows the simulated LEED pattern. Red and blue arrows indicate diffraction
spots from two domains a and b with different pseudo-chirality, respectively. b) Schematic
drawing of the adsorption structure of domains a and b of melamine/Au(111).
REFERENCES[1] For example, F. Forster et al., Surf. Sci., 2006, 600, 3870–3874[2] For example, H. Li, M. Eddaoudi et al., Nature, 1999, 402, 276–279[3] For example, S. J. Makowski et al., Chem. Eur. J. 2012, 18, 3248–3257[4] For example, P. A. Staniec et al., J. Phys. Chem. C, 2007, 111, 886-893

ELECTRONIC STATES OF TOPOLOGICAL SEMIMETALSSTUDIED BY SPIN- AND ANGLE-RESOLVED PHOTOEMISSION
SPECTROSCOPY
Y.X. Wan1, K. Kuroda1, C. Bareille1, Y. Ishida1, A. H. Mayo2,3,
H. Takahashi 2, 3, M. S. Bahramy1,4, C. Lin1, Y. Arai1，H. Tanaka1,
Y.Y. Dong1, M. Hashimoto5, D.H. Lu5, T. Muro6, S. Ishiwata2, 3, T. Kondo1
1The Institute for Solid State Physics, The University of Tokyo
2Department of Applied Physics, The University of Tokyo
3Graduate School of Engineering Science, Osaka University
4Center of Spintronics Research Network (CSRN), Osaka University
5Department of Physics and Astronomy, The University of Manchester
6Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory
7Japan Synchrotron Radiation Research Institute (JASRI)

INTRODUCTION
By breaking the time reversal symmetry in Dirac semimetals, one would obtain
topological Weyl semimetals or nodal line semimetals [1]. However, usually only extremely
large external magnetic field can realize such topological phase transition, which is
challenging in the laboratory. A layered phosphide, EuP3, has been reported to archive
topological phase transition when external magnetic field is applied: it would be a magnetic
Weyl semimetal in perpendicular magnetic field, but nodal-line semimetal in in-planemagnetic field [2]. This material has drawn increasing attention for it can be an ideal platform
to study the topological phase transition, as well as its promising
application in engineering.
We have observed of the band structure in α-EuP3 byangle-resolved photoemission spectroscopy (ARPES) with synchrotron
lightsource. We used resonant photoemission to directly confirm the f-p
coupling. Further, with the high-resolution ARPES, we discovered an
unexpected band splitting in the valence band. This band splitting
occurs only along ky direction without any global symmetry breaking.
To compare with the results, we also measured an isostructural material
SrP3, and found similar splitting. This result suggests that the crystal
structure contributes to the band splitting. We need to confirm Fig.1 Band dispersion of EuP3,
whether this kind of band splitting is intrinsic by measure their spin showing an unconventional
splitting.
texture with SARPES.

EXPERIMENTS
During the experiment, all EuP3 and SrP3 samples were cleaved in situ with top posts, and theab plane [(001) surface] was exposed and measured. The ab plane is flat and shiny, suggestingthat the compounds is metallic. It should be noted that there is another cleavage except (001)surface but no band dispersion can be observed from that.
The experiment was carried out with the spin-resolved laser ARPES (SARPES) at ISSP, theUniversity of Tokyo. The light source is 7 eV laser from KBBF crystal.

RESULTSWe measured the in-plane spin texture of EuP3 and SrP3. Although due to the matrix
element effect, we cannot directly observe the two splitting band dispersions, we found thatthe valence bands are spin-polarized. The spin textures of both materials are Dresselhaus-like,suggesting the space reversion symmetry is broken by unknown mechanism. However, weneed further study to confirm whether the splitting is a combination of Dresselhaus andZeeman effects because we have not excluded the out-of-plane spin polarization. μSR candirectly demonstrate direct evidence of the spin fluctuation [3]. We also need theoretical study
to explain why Dresselhaus effect occurs in the centrosymmetric EuP 3 and SrP3 crystals.

Fig. 2 Spin texture of EuP3. The valence band is spin-polarized along both x and y directions, suggesting a
Dresselhaus-like band splitting.

REFERENCES[1] M. Koshino et al., Phys. Rev. B 93, 045201 (2016)[2] A. H. Mayo et al., Phys. Rev. X 12, 011033 (2022)[3] J. Z. Ma et al., Sci. Adv. 5, eaaw4718 (2019).

LASER-SARPES STUDY OF SPIN-POLARIZED ELECTRONICSTATES FOR PB(BI1-XSBX)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

The formation mechanism of a topological surface state (TSS) is well established, where thebulk band inversion between the top of the valence band and the bottom of the conduction bandis required. The phase transition from a trivial insulator to a topological insulator is known as a
topological phase transition. The topological phase transition can be controlled by tuning thecomposition, the lattice constant, and the film thickness of the system [1-4]. Understanding themodification in an electronic structure induced by topological phase transitions is essential not
only for fundamental interests but also for applications.
A family of Pb-based ternary compounds, Pb(Bi1-xSbx)2Te4, has been revealed to be
topological insulators [5-8]. In the previous angle-resolved photoemission spectroscopy(ARPES) studies [7,8], the TSSs above a Dirac point (DP) have been clearly observed, where
the TSSs exist in a projected bulk band gap. On the other hand, the dispersive feature is unclear
below DP, which might be due to the existence of the bulk electronic states in the close vicinityof the TSSs. In addition, no spin-resolved ARPES (SARPES) study has been reported so far.
In the present study, we have investigated spin-polarized electronic states of Pb(Bi1xSbx)2Te4 (x = 0.70, 0.79) by SARPES using a laser (laser-SARPES). Our laser-SARPES
reveals Rashba-type occupied surface resonances near the TSSs. Besides, the TSSs
continuously connects to a branch of the Rashba-type surface resonances. Our results provide
direct evidence that the present system undergoes the
topological phase transition.
Laser-ARPES and -SARPES measurements havebeen performed at the Institute for Solid State Physics,
The University of Tokyo [9]. The samples were cleavedwith scotch tape in an ultra-high vacuum chamber
below the base pressure of 2.0 x 10-8 Pa. The
photoelectrons were excited by a quasi-continuouswave laser with a photon energy of 6.994 eV. Theenergy resolutions for laser-ARPES and -SARPES
were set to 5 and 10 meV, respectively. The sample
temperature was kept at 30 K during the measurements.
Figure 1 displays an ARPES intensity map for x =_ _0.70 taken along the GM direction. Overall dispersive
features of the TSS and the bulk conduction bands
agree with the previous studies [7,8]. The precise band
structure near the Dirac point has been demonstrated
thanks to the high-resolution laser-ARPES. The Dirac

Fig. 1 ARPES intensity map for x = 0.70
_ _along GM. A dashed curve represents the
projected bulk band edge determined by aleading edge of the corresponding
photoelectron intensity.

point (DP) is observed at the binding energy of
190 meV. The photoelectron intensity from the
TSS in the bulk band gap is prominent. The TSS
penetrates into the projected bulk bands at k =±0.05 Å-1, indicating that the surface stateschange to surface resonances. The surface
resonances disperse in the binding energy rangeof 0.2-0.5 eV. One can notice here that the
spectrum widths of the surface resonances arebroadened compared with those of the TSSs. We
also find several bands below EB ~ 0.7 eV.
Laser-SARPES measurements have been
performed for x = 0.70 and x = 0.79 samples_ _[Fig. 2(a,b)] in the GM direction with a ppolarized light. The dispersive feature for x=0.79
is quite similar to that for x = 0.70. Here, the DP
for x=0.79 is located above EF. The observed_

spin direction is inverted with respect to the G Fig. 2 SARPES images for (a) x = 0.70 and (b) x_ _point, meaning the TSSs possess a helical spin = 0.79 along GM. (c) Spin- and angle-resolved
texture. More interestingly, spin components spectra for x = 0.79.
opposite to the TSS are observed below DP,
corresponding to the spin textures of the surface resonances.
The band structures of the TSS and the surface resonances are identified from spin-resolved
energy distribution curves [Fig. 2(c)]. The surface resonances are spilt into two in the energy
direction in |k| = 0.1–0.2 Å-1. The surface resonance that appears in the lower binding energy
side possesses the same spin orientation as the TSS. Another surface resonance dispersing in
the energy range of 0.2–0.3 eV possesses the spin direction opposite to that of the counterpart.
_In addition, the spin polarizations of the surface resonances are inverted with respect to the G
point. Thus, we conclude that the surface resonances exhibit the Rashba-type spin splitting.
More importantly, the upper branch of the surface resonances is continuously connected to the
TSS. Our result provides direct evidence that the system undergoes the topological phasetransition.
REFERENCES[1] S.-Y. Xu et al., Science 332, 560 (2011).
[2] M. Brahlek et al., Phys. Rev. Lett. 109, 186403 (2012).
[3] Y. Zhang et al., Nat. Phys. 6, 584 (2010).
[4] C. Lin et al., Nat. Mater. 20, 1093 (2021).
[5] T. V. Menshchikova et al., JETP Lett. 93, 15 (2010).
[6] H. Jin et al., Phys. Rev. B 83, 041202(R) (2011).
[7] S. Souma et al., Phys. Rev. Lett. 108, 116801 (2012).
[8] K. Kuroda et al., Phys. Rev. Lett. 108, 206803 (2012).
[9] K. Yaji et al., Rev. Sci. Instrum. 87, 053111 (2016).

ELECTRONICSTRUCTURES
OF
HALF-METALLICFERROMAGNET La1-xSrxMnO3 BY HIGH-RESOLUTION SPINRESOLVED PHOTOEMISSION SPECTROSCOPY
T. Setoguchi1, N. Kataoka1, K. Kawaguchi2, Y. Fukushima2, A. Harasawa2, K. Kuroda2,
T. Kondo2, S. Shin2, T. Wakita1,3, Y. Muraoka1,3, and T. Yokoya1,31Graduate School of Natural Science and Technology, Okayama University, Okayama 700-8530, Japan

2

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

3

Research Institute for Interdisciplinary Science, Okayama University, Okayama 700-8530, Japan

Half metallic ferromagnets have a unique spin dependent electronic structure near the
Fermi level (EF), where only one of the spin states cross EF [1]. The unique electronic
structure gives rise to many body effects different from other systems. Theoretically, theelectronic states induced by many-body effects in half metals has been proposed as non
quasiparticle (NQP) states [2]. The experimental verification for NQP states has been
made recently for a half metallic ferromagnet CrO2 by using high-resolution (HR) spinresolved photoemission spectroscopy (SRPES) [3]. In the present research, we have
performed HRSRPES of another half metallic ferromagnet La1-xSrxMnO3 toexperimentally explore the characteristics and universality of NQP states. La1-xSrxMnO3
is one of the famous perovskite materials, exhibiting colossal magnet resistance aroundSr concentration x of 0.3. The previous SRPES study has provided evidence for half
metallic electronic structure of La1-xSrxMnO3 at 40 K[4]Homoepitaxial films of La1-xSrxMnO3 were grown on the atomically-flat (001) surfaceof Nb-doped SrTiO3 substrates using a laser MBE method. Laser-based SRPES
experiments were performed at the Institute for Solid State Physics, The University ofTokyo [7]. The p-polarized light with hν = 6.994 eV was used to excite the photoelectrons.
Photoelectrons were analyzed with a combination of a ScientaOmicron DA30L analyzer
and a very-low-energy-electron-diffraction (VLEED) type spin detector. The energy andangular resolutions were set to 6 meV and 0.3˚ (corresponding to ~0.005 Å) for spinintegrated ARPES and 10 meV and 1˚ (corresponding to ~0.02 Å) for SARPES,
respectively. During the measurement, the instrumental energy resolution was set to 20
meV and the base pressure was kept below 1 × 10#$ Pa. Calibration of EF for the
samples was achieved using a gold reference. The data were taken at T = 20, 50, 100 and
150 K. Clean surfaces of the samples for all measurements were obtained by in situ
annealing at 200℃ for 20 minutes. We magnetized the LSMO(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.
We obtained HRSRPES data at 20, 50, 100, and 150K. Compared to the minority spinstates showing negligible intensity around the near-EF region, the minority spin states
have enhanced intensity with a Fermi edge at all measured temperatures. These results
are in line with the previous SRPES studies reporting that LSMO is a half metallic
ferromagnet. However, we found that polarization is slightly smaller than 100%. We
tentatively attribute the reduced polarization to detecting the information of the substrate.
We plan to measure HRARPES with thicker films in the future experiment.
ACKNOWLEDGEMENT
This work was partially supported by JSPS KAKENHI Grand Numbers(JP20H0185313) from MEXT. We acknowledge M. Minohara and H. Kumigashira forproviding the samples.
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] J.-H. Park et al., Nature 392, 794-796 (1998).
[5] K. Yaji et al., Rev. Sci. Instrum. 87, 053111 (2016).