arXiv subjects

Koichiro Yaji

Publications and source records attributed to Koichiro Yaji.

At least 19 recordsLinked to original sources

arXiv2026

Momentum-dependent charge-density-wave gap formation in ZrTe_{2.98}Se_{0.02}

We investigated the energy gap formation across the charge density wave (CDW) transition inof ZrTe_{2.98}Se_{0.02}. By employing a laser photoemission microscopy, we clearly resolved one elliptical Fermi surface (FS) around the Brillouin zone (BZ) center, and two quasi-one-dimensional FSs along the BZ boundary. We further mapped the intensity difference between the FSs below and above the CDW transition temperature. We found that the energy CDW gap formation is limited to the momentum region 0.25 Å^{-1} < ky < 0.8 Å^{-1} along \bar{B}-\bar{D} line, which coincides with the location of one of the quasi-one-dimensional FSs. Characteristic momentum dependence in the energy CDW gap suggests the importance of both FS nesting and band-dependent electron-phonon coupling for understanding the CDW state in ZrTe_{3} system.

cond-mat.other↗

arXiv2026

Fermi surface of Kagome metal CsCr$_3$Sb$_5$ observed by laser photoemission microscopy

We investigated the Fermi surface (FS) in the paramagnetic state of Kagome metal CsCr$_3$Sb$_5$ by employing a laser photoemission microscopy. We found a circular FS and two hexagonal FSs around the Brillouin zone (BZ) center. Polarization-dependent measurements further enable us to detect small FS pockets at the BZ boundary. According to the density functional theory calculations, the orbital characters of the FSs were determined from their shape and orientations. We found that the size of the FS is strongly modified for the d$_{xz}$ orbital, suggesting the orbital-dependent correlation effect. These results provide an electronic basis for exploring the interplay of antiferromagnetic/charge density wave order and possible unconventional superconductivity in this compound.

cond-mat.str-el↗

arXiv2025

Induced magnetic moment at two-dimensional MXene/ferromagnetic interface evaluated by angle-dependent hard X-ray photoemission spectroscopy

Emergent ferromagnetism on the surface of recent two-dimensional (2D) MXene is investigated by X-ray magnetic circular dichroism (XMCD) and angle-dependent hard X-ray photoemission spectroscopy (HAXPES). Focusing on the Cr2N as one of the 2D-MXenes, the bilayers of Cr2N/Co and Cr2N/Pt are prepared by magnetron sputtering technique. XMCD reveals the induced magnetic moment of Cr in the Cr2N/Co interface, while it is not observed in the Cr2N/Pt interface at room temperature. To distinguish the possible origins of either the interlayer magnetic exchange coupling or the charge transfer at the interfaces, the additional controlled Cr2N/Cu bilayer, whose work function of Cu is consistent with Co, is prepared. HAXPES spectra for the Cr 2p core level near the interface of Cr2N/Cu is consistent with that of Cr2N/Co, indicating that the induced magnetic moment of Cr observed by XMCD for the Cr2N/Co can be attributed to the interlayer magnetic exchange coupling, rather than the charge transfer, which is a specific characteristics emerged at the interface with 2D-MXene.

cond-mat.mtrl-sci↗

arXiv2025

Topological delocalization against Anderson localization observed in Bi-doped PbSb2Te4 disordered topological insulator

We experimentally investigate the tendency of localization in the bulk and the topological surface states in topological insulators Pb(Bi1-xSbx)2Te4 (x=0.793-0.818) through detailed transport measurements. The bulk electronic states in the range 0.793<=x<0.818 are situated on the insulator side of the Anderson transition, as indicated by k_Fl values (where k_F is the Fermi wavenumber and l is the mean free path) falling below the Ioffe-Regel criterion (i.e., k_F l<1). In contrast, the topological surface states retain high mobility and even exhibit quantum oscillations, demonstrating their resilient nature against strong disorder. These findings highlight the delocalized nature of the topological surface states despite Anderson localization of the bulk electronic states.

cond-mat.mtrl-sci↗

arXiv2024

Moiré superlattices of antimonene on a Bi(111) substrate with van Hove singularity and Rashba-type spin polarization

Moiré superlattices consisting of two-dimensional materials have attracted immense attention because of emergent phenomena such as flat band-induced Mott insulating states and unconventional superconductivity. However, the effects of spin-orbit coupling on these materials have not yet been fully explored. Here, we show that single- and double-bilayer antimony honeycomb lattices, referred to as antimonene, form moiré superlattices on a Bi(111) substrate due to lattice mismatch. Scanning tunnelling microscopy (STM) measurements reveal the presence of spectral peaks near the Fermi level, which are spatially modulated with the moiré period. Angle-resolved photoemission spectroscopy (ARPES) combined with density functional theory calculations clarify the surface band structure with saddle points near the Fermi level, which allows us to attribute the observed STM spectral peaks to the van Hove singularity. Moreover, spin-resolved ARPES measurements reveal that the observed surface states are Rashba-type spin-polarized. The present work has significant implications in that Fermi surface instability and symmetry breaking may emerge at low temperatures, where the spin degree of freedom and electron correlation also play important roles.

cond-mat.mes-hall↗

arXiv2024

Optical study on electronic structure of the locally non-centrosymmetric CeRh$_2$As$_2$

The electronic structures of the heavy-fermion superconductor CeRh$_2$As$_2$ with the local inversion symmetry breaking and the reference material LaRh$_2$As$_2$ have been investigated by using experimental optical conductivity ($σ_1(ω)$) spectra and first-principal DFT calculations. In the low-temperature $σ_1(ω)$ spectra of CeRh$_2$As$_2$, a $4f$-conduction electron hybridization and heavy quasiparticles are clearly indicated by a mid-infrared peak and a narrow Drude peak. In LaRh$_2$As$_2$, these features are absent in the $σ_1(ω)$ spectrum, however, it can nicely be reproduced by DFT calculations. For both compounds, the combination between a local inversion symmetry breaking and a large spin-orbit (SO) interaction plays an important role for the electronic structure, however, the SO splitting bands could not be resolved in the $σ_1(ω)$ spectra due to the small SO splitting size.

cond-mat.str-el↗

arXiv2023

Visualization of spin-polarized electronic states by imaging-type spin-resolved photoemission microscopy

Harnessing electron spin is crucial in developing energy-saving and high-speed devices for the next generation. In this scheme, visualizing spin-polarized electronic states aids in designing and developing new materials and devices. Spin-resolved photoemission spectroscopy provides information on the spin-polarized electronic states. To investigate the spin-polarized electronic states in microscopic materials and devices, spin-resolved photoemission spectroscopy requires spatial resolution in a sub-micrometer scale. Here we show the imaging-type spin-resolved photoemission microscopy (iSPEM) with an ultraviolet laser developed at the National Institutes for Materials Science (NIMS). Our iSPEM achieves a spatial resolution of 420 nm, drastically improving by more than an order of magnitude compared to conventional spin-resolved photoemission spectroscopy instruments. Besides, the multi-channel spin detector significantly reduces the data acquisition time by four orders of magnitude compared to the conventional instruments. The iSPEM machine elucidates the spin-polarized electronic states of sub-micrometer scale materials, polycrystals, device structure samples, and so on, which have yet to be the target of conventional spin-resolved photoemission spectroscopy.

cond-mat.mtrl-sci↗

arXiv2023

Anomalously large spin-dependent electron correlation in nearly half-metallic ferromagnet CoS$_2$

The spin-dependent band structure of CoS$_2$ which is a candidate for a half-metallic ferromagnet was investigated by both spin- and angle-resolved photoemission spectroscopy and theoretical calculations, in order to reappraise the half-metallicity and electronic correlations. We determined the three-dimensional Fermi surface and the spin-dependent band structure. As a result, we found that a part of the minority spin bands is on the occupied side in the vicinity of the Fermi level, providing spectroscopic evidence that CoS$_2$ is not but very close to a half-metal. Band calculations using density functional theory with generalized gradient approximation showed a good agreement with the observed majority spin $e_g$ bands, while it could not explain the observed band width of the minority-spin eg bands. On the other hand, theoretical calculations using dynamical mean field theory could better reproduce the strong mass renormalization in the minority-spin $e_g$ bands. All those results strongly suggest the presence of anomalously enhanced spin-dependent electron correlation effects on the electronic structure in the vicinity of the half-metallic state. We also report the temperature dependence of the electronic structure across the Curie temperature and discuss the mechanism of the thermal demagnetization. Our discovery of the anomalously large spin-dependent electronic correlations not only demonstrates a key factor in understanding the electronic structure of half-metals but also provides a motivation to improve theoretical calculations on spin-polarized strongly correlated systems.

cond-mat.str-el↗

arXiv2023

Spin-polarized saddle points in the topological surface states of the elemental Bismuth revealed by a pump-probe spin-resolved ARPES

We use a pump-probe, spin-, and angle-resolved photoemission spectroscopy (ARPES) with a 10.7 eV laser accessible up to the Brillouin zone edge, and reveal for the first time the entire band structure, including the unoccupied side, for the elemental bismuth (Bi) with the spin-polarized surface states. Our data identify Bi as in a strong topological insulator phase ($Z_2$=1) against the prediction of most band calculations. We unveil that the unoccupied topological surface states possess spin-polarized saddle points yielding the van Hove singularity, providing an excellent platform for the future development of opto-spintronics.

cond-mat.mtrl-sci↗

arXiv2021

Selective observation of surface and bulk bands in polar WTe2 by laser-based spin- and angle-resolved photoemission spectroscopy

The electronic state of WTe2, a candidate of type-II Weyl semimetal, is investigated by using laser-based spin- and angle-resolved photoemission spectroscopy (SARPES). We prepare the pair of WTe2 samples, one with (001) surface and the other with (00-1) surface, by "sandwich method", and measure the band structures of each surface separately. The Fermi arcs are observed on both surfaces. We identify that the Fermi arcs on the two surfaces are both originating from surface states. We further find a surface resonance band, which connects with the Fermi-arc band, forming a Dirac-cone-like band dispersion. Our results indicate that the bulk electron and hole bands are much closer in momentum space than band calculations.

cond-mat.mtrl-sci↗

arXiv2021

Visualization of the strain-induced topological phase transition in a quasi-one-dimensional superconductor TaSe3

Control of the phase transition from topological to normal insulators can allow for an on/off switching of spin current. While topological phase transitions have been realized by elemental substitution in semiconducting alloys, such an approach requires the preparation of materials with various compositions, thus it is quite far from a feasible device application, which demands a reversible operation. Here we use angle-resolved photoemission spectroscopy (ARPES) and spin-resolved ARPES to visualize the strain-driven band structure evolution of the quasi-1D superconductor TaSe3. We demonstrate that it undergoes reversible strain-induced topological phase transitions from a strong topological insulator phase with spin-polarized, quasi-1D topological surface states, to topologically trivial semimetal and band insulating phases. The quasi-1D superconductor TaSe3 provides a suitable platform for engineering the topological spintronics, for example as an on/off switch for spin current robust against impurity scattering.

cond-mat.mtrl-sci↗

arXiv2021

Visualization of optical polarization transfer to photoelectron spin vector emitted from the spin-orbit coupled surface state

Similar to light polarization that is selected by a superposition of optical basis, electron spin direction can be controlled through a superposition of spin basis. We investigate such a spin interference occurring in photoemission of the spin-orbit coupled surface state in Bi2Se3 by using spin- and angle-resolved photoemission spectroscopy combined with laser light source (laser-SARPES). Our laser-SARPES with three-dimensional spin detection and tunable laser polarization including elliptical and circular polarization enables us to directly visualize how the direction of the fully-polarized photoelectron spin changes according to the optical phase and orientation of the incident laser polarization. By this advantage of our laser-SARPES, we demonstrate that such optical information can be projected to the three-dimensional spin vector of the photoelectrons. Our results, therefore, present a novel spin-polarized electron source permitting us to optically control the pure spin state pointing to the arbitrary direction.

cond-mat.mtrl-sci↗

arXiv2021

Atomic-layer Rashba-type superconductor protected by dynamic spin-momentum locking

Spin-momentum locking is essential to the spin-split Fermi surfaces of inversion-symmetry broken materials, which are caused by either Rashba-type or Zeeman-type spin-orbit coupling (SOC). While the effect of Zeeman-type SOC on superconductivity has experimentally been shown recently, that of Rashba-type SOC remains elusive. Here we report on convincing evidence for the critical role of the spin-momentum locking on crystalline atomic-layer superconductors on surfaces, for which the presence of the Rashba-type SOC is demonstrated. In-situ electron transport measurements reveal that in-plane upper critical magnetic field is anomalously enhanced, reaching approximately three times the Pauli limit at $T = 0$. Our quantitative analysis clarifies that dynamic spin-momentum locking, a mechanism where spin is forced to flip at every elastic electron scattering, suppresses the Cooper pair-breaking parameter by orders of magnitude and thereby protects superconductivity. The present result provides a new insight into how superconductivity can survive the detrimental effects of strong magnetic fields and exchange interactions.

cond-mat.supr-con↗

arXiv2021

Material design with the van der Waals stacking of bismuth-halide chains realizing a higher-order topological insulator

The van der Waals (vdW) materials with low dimensions have been extensively studied as a platform to generate exotic quantum properties. Advancing this view, a great deal of attention is currently paid to topological quantum materials with vdW structures. Here, we provide a new concept of designing topological materials by the vdW stacking of quantum spin Hall insulators (QSHIs). Most interestingly, a slight shift of inversion center in the unit cell caused by a modification of stacking is found to induce the topological variation from a trivial insulator to a higher-order topological insulator (HOTI). Based on that, we present the first experimental realization of a HOTI by investigating a bismuth bromide Bi4Br4 with angle-resolved photoemission spectroscopy (ARPES). The unique feature in bismuth halides capable of selecting various topology only by differently stacking chains, combined with the great advantage of the vdW structure, offers a fascinating playground for engineering topologically non-trivial edge-states toward future spintronics applications.

cond-mat.mtrl-sci↗

arXiv2020

Scaling law for the Rashba-type spin splitting in quantum well films

We use laser-based spin- and angle-resolved photoemission spectroscopy (laser-SARPES) with high-resolution, and experimentally determine, for the first time, the Rashba-parameters of quantum well states (QWSs) systematically changing with the film thickness and the quantum numbers, through the observation of the Ag films grown on an Au(111) substrate. The data are very well reproduced by the theoretical calculations based on the density functional theory. Most importantly, we find a scaling law for the Rashba parameter ($α_{\rm R}$) that the magnitude of $α_{\rm R}$ is scaled by the charge density at the interface and the spin-orbit coupling ratio between the film and the substrate, and it is expressed by a single straight line regardless of the film thickness and the quantum numbers. The new finding not only is crucial to understand the Rashba effect in QWSs but also gives a foundation of film growth engineering to fine-tune the spin splitting in 2D heterostructure systems.

cond-mat.mtrl-sci↗

arXiv2020

Observation and control of the weak topological insulator state in ZrTe5

A quantum spin Hall insulator hosts topological states at the one-dimensional edge, along which backscattering by nonmagnetic impurities is strictly prohibited and dissipationless current flows. Its 3D analogue, a weak topological insulator (WTI), possesses similar quasi-1D topological states confined at side surfaces of crystals. The enhanced confinement could provide a route for dissipationless current and better advantages for applications relative to the widely studied strong topological insulators. However, the topological side surface is usually not cleavable and is thus hard to observe by angle-resolved photoemission spectroscopy (ARPES), which has hindered the revealing of the electronic properties of WTIs. Here, we visualize the topological surface states of the WTI candidate ZrTe5 for the first time by spin and angle-resolved photoemission spectroscopy: a quasi-1D band with spin-momentum locking was revealed on the side surface. We further demonstrate that the bulk band gap in ZrTe5 is controlled by strain to the crystal, realizing a more stabilized WTI state or an ideal Dirac semimetal state depending on the direction of the external strain. The highly directional spin-current and the tunable band gap we found in ZrTe5 will provide an excellent platform for applications.

cond-mat.mtrl-sci↗

arXiv2019

Fermi level tuning of one-dimensional giant Rashba system on a semiconductor substrate: Bi/GaSb(110)-(2x1)

We fabricated spin-polarized surface electronic states with tunable Fermi level from semiconductor to low-dimensional metal in the Bi/GaSb(110)-(2$\times$1) surface using angle-resolved photoelectron spectroscopy (ARPES) and spin-resolved ARPES. The spin-polarized surface band of Bi/GaSb(110) exhibits quasi-one-dimensional character with the Rashba parameter $α_{\rm R}$ of 4.1 and 2.6 eVÅ\ at the $\barΓ$ and $\bar{\rm Y}$ points of the surface Brillouin zone, respectively. The Fermi level of the surface electronic state is tuned in situ by element-selective Ar-ion sputtering on the GaSb substrate. The giant Rashba-type spin splitting with switchable metallic/semiconducting character on semiconductor substrate makes this system a promising candidate for future researches in low-dimensional spintronic phenomena.

cond-mat.mes-hall↗

arXiv2018

Multiple topological states in iron-based superconductors

Topological insulators and semimetals as well as unconventional iron-based superconductors have attracted major recent attention in condensed matter physics. Previously, however, little overlap has been identified between these two vibrant fields, even though the principal combination of topological bands and superconductivity promises exotic unprecedented avenues of superconducting states and Majorana bound states (MBSs), the central building block for topological quantum computation. Along with progressing laser-based spin-resolved and angle-resolved photoemission spectroscopy (ARPES) towards high energy and momentum resolution, we have resolved topological insulator (TI) and topological Dirac semimetal (TDS) bands near the Fermi level ($E_{\text{F}}$) in the iron-based superconductors Li(Fe,Co)As and Fe(Te,Se), respectively. The TI and TDS bands can be individually tuned to locate close to $E_{\text{F}}$ by carrier doping, allowing to potentially access a plethora of different superconducting topological states in the same material. Our results reveal the generic coexistence of superconductivity and multiple topological states in iron-based superconductors, rendering these materials a promising platform for high-$T_{\text{c}}$ topological superconductivity.

cond-mat.supr-con↗