arXiv subjects

Kenta Kuroda

Publications and source records attributed to Kenta Kuroda.

At least 37 records · Page 2Linked to original sources

arXiv2022

Multipole polaron in the devil's staircase of CeSb

Rare-earth intermetallic compounds exhibit rich phenomena induced by the interplay between localized $f$ orbitals and conduction electrons. However, since the energy scale of the crystal-electric-field splitting is only a few millielectronvolts, the nature of the mobile electrons accompanied by collective crystal-electric-field excitations has not been unveiled. Here, we examine the low-energy electronic structures of CeSb through the anomalous magnetostructural transitions below the N$é$el temperature, $\sim$17 K, termed the 'devil's staircase', using laser angle-resolved photoemission, Raman and neutron scattering spectroscopies. We report another type of electron-boson coupling between mobile electrons and quadrupole crystal-electric-field excitations of the 4$f$ orbitals, which renormalizes the Sb 5$p$ band prominently, yielding a kink at a very low energy ($\sim$7 meV). This coupling strength is strong and exhibits anomalous step-like enhancement during the devil's staircase transition, unveiling a new type of quasiparticle, named the 'multipole polaron', comprising a mobile electron dressed with a cloud of the quadrupole crystal-electric-field polarization.

cond-mat.mtrl-sci↗

arXiv2022

Large anomalous Hall effect induced by weak ferromagnetism in the noncentrosymmetric antiferromagnet $\mathrm{Co}\mathrm{Nb}_3\mathrm{S}_6$

We study the mechanism of the exceptionally large anomalous Hall effect (AHE) in the noncentrosymmetric antiferromagnet $\mathrm{Co}\mathrm{Nb}_3\mathrm{S}_6$ by angle-resolved photoemission spectroscopy (ARPES) and magnetotransport measurements. From ARPES measurements of $\mathrm{Co}\mathrm{Nb}_3\mathrm{S}_6$ and its family compounds ($\mathrm{Fe}\mathrm{Nb}_3\mathrm{S}_6$ and $\mathrm{Ni}\mathrm{Nb}_3\mathrm{S}_6$), we find a band dispersion unique to the Co intercalation existing near the Fermi level. We further demonstrate that a slight deficiency of sulfur in $\mathrm{Co}\mathrm{Nb}_3\mathrm{S}_6$ eliminates the ferromagnetism and the AHE simultaneously while hardly changing the band structure, indicating that the weak ferromagnetism is responsible for the emergence of the large AHE. Based on our results, we propose Weyl points near the Fermi level to cause the large AHE.

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

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↗

arXiv2020

Three-dimensional electronic structure in ferromagnetic $\textrm{Fe}_3\textrm{Sn}_2$ with breathing kagome bilayers

A large anomalous Hall effect (AHE) has been observed in ferromagnetic $\textrm{Fe}_3\textrm{Sn}_2$ with breathing kagome bilayers. To understand the underlying mechanism for this, we investigate the electronic structure of $\textrm{Fe}_3\textrm{Sn}_2$ by angle-resolved photoemission spectroscopy (ARPES). In particular, we use both vacuum ultraviolet light (VUV) and soft x ray (SX), which allow surface-sensitive and relatively bulk-sensitive measurements, respectively, and distinguish bulk states from surface states, which should be unlikely related to the AHE. While VUV-ARPES observes two-dimensional bands mostly due to surface states, SX-ARPES reveals three-dimensional band dispersions with a periodicity of the rhombohedral unit cell in the bulk. Our data show a good consistency with a theoretical calculation based on density functional theory, suggesting a possibility that $\textrm{Fe}_3\textrm{Sn}_2$ is a magnetic Weyl semimetal.

cond-mat.mtrl-sci↗

arXiv2020

Observation of small Fermi pockets protected by clean CuO2 sheets of a high-Tc superconductor

The superconductivity of high transition temperature (Tc) occurs in copper oxides with carrier-doping to an antiferromagnetic (AF) Mott insulator. This discovery more than thirty years ago immediately led to a prediction about the formation of a small Fermi pocket. This structure, however, has not yet been detected, while it could be a key element in relating high-Tc superconductivity to Mott physics. To address this long-standing issue, we investigate the electronic structure of a five-layered Ba2Ca4Cu5O10(F,O)2 with inner CuO2 planes demonstrated to be cleanest ever in cuprates. Most surprisingly, we find small Fermi surface (FS) pockets closed around (pi/2,pi/2) consistently by angle-resolved photoemission spectroscopy (ARPES) and quantum oscillation measurements. The d-wave superconducting gap opens along the pocket, revealing the coexistence between the superconductivity and AF order in the same CuO2 sheet. Our data further indicate that the superconductivity can occur without contribution from the states near the antinodal region, which are shared by other competing excitations such as the charge density wave (CDW) and pseudogap states. This will have significant implications for understanding the superconductivity and puzzling Fermi arc phenomena in cuprates.

cond-mat.supr-con↗

arXiv2020

Devil's staircase transition of the electronic structures in CeSb

Solids with competing interactions often undergo complex phase transitions with a variety of long-periodic modulations. Among such transition, devil's staircase is the most complex phenomenon, and for it, CeSb is the most famous material, where a number of the distinct phases with long-periodic magnetostructures sequentially appear below the Neel temperature. An evolution of the low-energy electronic structure going through the devil's staircase is of special interest, which has, however, been elusive so far despite the 40-years of intense researches. Here we use bulk-sensitive angle-resolved photoemission spectroscopy and reveal the devil's staircase transition of the electronic structures. The magnetic reconstruction dramatically alters the band dispersions at each transition. We moreover find that the well-defined band picture largely collapses around the Fermi energy under the long-periodic modulation of the transitional phase, while it recovers at the transition into the lowest-temperature ground state. Our data provide the first direct evidence for a significant reorganization of the electronic structures and spectral functions occurring during the devil's staircase.

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↗

arXiv2018

Rashba spin splitting of L-gap surface states on Ag(111) and Cu(111)

Spin-resolved band structures of L-gap surface states on Ag(111) and Cu(111) are investigated by spin- and angle-resolved photoelectron spectroscopy (SARPES) with a vacuum-ultra-violet laser. The observed spin textures of the Ag(111) and Cu(111) surface states agree with that expected by the conventional Rashba effect. The Rashba parameter of the Ag(111) surface state is estimated quantitatively and is 80% of that of Cu(111). The surface-state wave function is found to be predominantly of even mirror-symmetry with negligible odd contribution by SARPES using a linearly polarized light. The results are consistent with our theoretical calculations for the orbital-resolved surface state.

cond-mat.mtrl-sci↗

arXiv2018

Topological Dirac semimetal phase in the iron-based superconductor Fe(Te,Se)

Topological Dirac semimetals (TDSs) exhibit bulk Dirac cones protected by time reversal and crystal symmetry, as well as surface states originating from non-trivial topology. While there is a manifold possible onset of superconducting order in such systems, few observations of intrinsic superconductivity have so far been reported for TDSs. We observe evidence for a TDS phase in FeTe$_{1-x}$Se$_x$ ($x$ = 0.45), one of the high transition temperature ($T_c$) iron-based superconductors. In angle-resolved photoelectron spectroscopy (ARPES) and transport experiments, we find spin-polarized states overlapping with the bulk states on the (001) surface, and linear magnetoresistance (MR) starting from 6 T. Combined, this strongly suggests the existence of a TDS phase, which is confirmed by theoretical calculations. In total, the topological electronic states in Fe(Te,Se) provide a promising high $T_c$ platform to realize multiple topological superconducting phases.

cond-mat.supr-con↗

arXiv2017

Experimental determination of the topological phase diagram in Cerium monopnictides

We use bulk-sensitive soft X-ray angle-resolved photoemission spectroscopy and investigate bulk electronic structures of Ce monopnictides (CeX; X=P, As, Sb and Bi). By exploiting a paradigmatic study of the band structures as a function of their spin-orbit coupling (SOC), we draw the topological phase diagram of CeX and unambiguously reveal the topological phase transition from a trivial to a nontrivial regime in going from CeP to CeBi induced by the band inversion. The underlying mechanism of the topological phase transition is elucidated in terms of SOC in concert with their semimetallic band structures. Our comprehensive observations provide a new insight into the band topology hidden in the bulk of solid states.

cond-mat.mtrl-sci↗

arXiv2017

Direct Mapping of Spin and Orbital Entangled Wavefunction under Interband Spin-Orbit coupling of Rashba Spin-Split Surface States

We use spin- and angle-resolved photoemission spectroscopy (SARPES) combined with polarization-variable laser and investigate the spin-orbit coupling effect under interband hybridization of Rashba spin-split states for the surface alloys Bi/Ag(111) and Bi/Cu(111). In addition to the conventional band mapping of photoemission for Rashba spin-splitting, the different orbital and spin parts of the surface wavefucntion are directly imaged into energy-momentum space. It is unambiguously revealed that the interband spin-orbit coupling modifies the spin and orbital character of the Rashba surface states leading to the enriched spin-orbital entanglement and the pronounced momentum dependence of the spin-polarization. The hybridization thus strongly deviates the spin and orbital characters from the standard Rashba model. The complex spin texture under interband spin-orbit hybridyzation proposed by first-principles calculation is experimentally unraveled by SARPES with a combination of p- and s-polarized light.

cond-mat.mtrl-sci↗

arXiv2016

Spin Texture in Type-II Weyl Semimetal WTe2

We determine the band structure and spin texture of WTe2 by spin- and angle-resolved photoemission spectroscopy (SARPES). With the support of first-principles calculations, we reveal the existence of spin polarization of both the Fermi arc surface states and bulk Fermi pockets. Our results support WTe2 to be a type-II Weyl semimetal candidate and provide important information to understand its extremely large and nonsaturating magnetoresistance.

cond-mat.mtrl-sci↗