arXiv subjects

Shin-ichi Kimura

Publications and source records attributed to Shin-ichi Kimura.

At least 19 recordsLinked to original sources

arXiv2026

Optical investigation of the electronic structure of a ferromagnetic Weyl semimetal CeAlSi

To investigate electronic states during the ferromagnetic transition in a magnetic Weyl semimetal CeAlSi, we measured temperature-dependent optical conductivity [$σ_1(ω)$] spectra and compared them with DFT+DMFT band calculations. The $σ_1(ω)$ spectrum did not change significantly across the ferromagnetic ordering temperature ($T_C$), suggesting that the Ce 4f states are almost localized. DFT+DMFT calculations with almost localized Ce 4f states successfully reproduced the spectral shape and the unchanged $σ_1(ω)$ spectra across $T_C$. The dynamic effective mass evaluated from the extended Drude model is very small, which DFT+DMFT calculations also reproduce, but the scattering probability at even lower temperatures suggests ferromagnetic fluctuations. These results suggest that the interaction intensity between the Weyl fermions and Ce 4f states is very weak, as reproduced by DFT+DMFT calculations.

cond-mat.str-el↗

arXiv2026

Control of band structure in the altermagnetic candidate MnTe by temperature and strain

The temperature and strain dependences of the optical conductivity spectrum of hexagonal manganese telluride (MnTe) were measured, revealing absorption in the terahertz (THz) region from spin-split bands to acceptor levels. The temperature dependence of the THz absorption peak is consistent with that of a ferromagnetic phase transition, even though MnTe exhibits no net magnetism. The temperature dependence was attributed to a change in the altermagnetic electronic structure. Under negative uniaxial strain, the THz peak shifts to the high-energy side, suggesting spin-splitting bands at energies away from EF, consistent with the theoretical prediction that the spin-splitting angle decreases. The observed behavior of the THz peak clearly shows that MnTe has the altermagnetic electronic structure. Additionally, a Fano-like asymmetric line shape in the optical phonon absorption was observed, possibly originating from interactions with the alternative spin-split bands.

cond-mat.str-el↗

arXiv2025

Anisotropic Non-Fermi Liquid and Dynamical Planckian Scaling of a Quasi-Kagome Kondo Lattice System

At the quantum critical point of correlated materials, a non-Fermi liquid state appears where electron correlations continuously develop to very low temperatures. The relaxation time of the interacted electrons, namely quasiparticles, is scaled with the Planckian time, $\hbar/k_{\rm B}T$. However, there is a debate over whether heavy-fermion systems can obey the Planckian time. In the optical conductivity spectra, the Drude response will appear as the scaling of $\hbarω/k_{\rm B}T$ as the dynamical Planckian scaling (DPS). Here, we report the non-Fermi liquid behavior in the Drude response of a candidate for such materials, the quasi-kagome Kondo lattice CeRhSn. Even though the material shows a strong valence fluctuation, renormalized Drude responses observed at the photon energy below 100 meV are characterized by non-Fermi-liquid-like scattering rate $1/τ$. The heavy carriers' Drude response only for the Ce quasi-kagome plane obeyed DPS below 80 K, suggesting the anisotropic quantum criticality with the strong $c \textrm{-} f$ hybridization.

cond-mat.str-el↗

arXiv2024

Photo-induced phase transition on black samarium monosulfide

To investigate the role of the excitons for the origin of the pressure-induced phase transition (BGT) from the black-colored insulator (BI) to the golden-yellow-colored metal (GM) of samarium monosulfide (SmS), optical reflectivity, Sm $3d$ X-ray absorption spectroscopy (XAS), and X-ray diffraction (XRD) with the creation of excitons by photoexcitation (PE) are reported. In the pump-probe reflectivity measurement, following a huge reflectivity change of about 22 %, three different relaxation times with a vibration component were observed. The fast component with the relaxation time ($τ$) of less than 1 ps is due to the excitation and relaxation of electrons into the conduction band, and the slowest one with $τ> {\rm several} 100$ ps originates from the appearance of the photo-induced (PI) state. The components with $τ\sim 10$ ps and vibration originate from the appearance of the PI state and the interference between the reflection lights at the sample surface and the boundary between the BI and PI states, suggesting that the electronic structure of the PI phase is different from that of the BI state. XAS spectra indicate that the Sm mean valence is shifted from the Sm$^{2+}$ dominant to the intermediate between Sm$^{2+}$ and Sm$^{3+}$ by PE but did not change to that of the GM phase across BGT, consistent with the reflectivity data. The XRD result after PE shows that the PI state has much less lattice contraction than the GM phase. These results suggest that the BGT cannot be achieved solely by creating excitons after PE but requires other effects, such as a lattice contraction.

cond-mat.str-el↗

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

Two-dimensional heavy fermion in a monoatomic-layer Kondo lattice YbCu$_2$

The Kondo effect between localized $f$-electrons and conductive carriers leads to exotic physical phenomena. Among them, heavy-fermion (HF) systems, in which massive effective carriers appear due to the Kondo effect, have fascinated many researchers. Dimensionality is also an important characteristic of the HF system, especially because it is strongly related to quantum criticality [S. Sachdev, Science 288, 475 (2000)]. However, perfect two-dimensional (2D) HF materials have not been reported yet. Here, we report the surface electronic structure of the monoatomic-layer Kondo lattice YbCu$_2$ on a Cu(111) surface observed by synchrotron-based angle-resolved photoelectron spectroscopy. The 2D conducting band and the Yb 4$f$ state, located very close to the Fermi level, are observed. These bands are hybridized at low-temperature, forming the 2D HF state, with an evaluated coherent temperature of about 30 K. The effective mass of the 2D state is enhanced by a factor of 100 by the development of the HF state. Furthermore, clear evidence of the hybridization gap formation in the temperature dependence of the Kondo-resonance peak has been observed below the coherent temperature. Our study provides a new candidate as an ideal 2D HF material for understanding the Kondo effect at low dimensions.

cond-mat.str-el↗

arXiv2023

Observation of Electronic Structure Modification in the Hidden Order Phase of CeCoSi

CeCoSi with no local inversion symmetric crystal structure ($P4/nmm$) exhibits a phase transition of unknown origin (Hidden Order: HO) at about 12 K ($T_0$) above the antiferromagnetic transition temperature ($T_{\rm N}=9.4$ K). The electronic structure change across $T_0$ was investigated with high-precision optical reflection spectroscopy. The optical spectrum changed from a typical metallic behavior above $T_0$ to a gap-like structure at around 15 meV below $T_0$. The gap-like structure was unchanged across $T_{\rm N}$ except for the narrowing of the Drude component of carriers due to the suppression of magnetic fluctuations. This result suggests a slight change from the typical metallic electronic structure above $T_0$ to that with an energy gap near the Fermi level in the HO phase. The change in electronic structure in the HO phase was concluded to be due to electron/valence instability.

cond-mat.str-el↗

arXiv2023

Current-Induced Metallization and Valence Transition in Black SmS

A strongly-correlated insulator, samarium mono-sulfide (SmS), presents not only the pressure-induced insulator-to-metal transition (IMT) with the color change from black to golden-yellow but also current-induced IMT (CIMT) with negative resistance. To clarify the origin of the CIMT of SmS, the electronic structure change has been investigated by optical reflectivity and angle-integrated photoelectron spectra by applying an electric current. At lower temperatures than about 100 K, where the nonlinear $V$-$I$ curve has been observed, the carrier density rapidly increases, accompanied by decreasing relaxation time of carriers with increasing current. Then, the direct gap size increases, and the mean valence changes from Sm$^{2+}$-dominant SmS to the mixed-valent one with increasing current. These results suggest that the CIMT originates from increasing the Sm $4f$-$5d$ hybridization intensity induced by the applied current.

cond-mat.str-el↗

arXiv2022

Growth of 2D topological material Bi on InSb(111)B with fractal surface structures

Bismuth (Bi) atomic layers are known as 2D topological materials with variety of the electronic structures and topological orders depending on the number of stacking layers. Recently, it is reported that few layers of Bi grown on semiconductor substrate InSb(111)B exhibit the Sierpiński-triangle (ST) fractal patterns on the surface. In this work, we have grown Bi layers on InSb(111)B and traced the evolution of the atomic and electronic structures of Bi. The surface atomic structures and growth modes were monitored by using reflective high-energy electron diffraction and core-level photoelectron spectroscopy. It is suggested that the single layer of the ST-phase Bi grows on InSb(111)B and the following Bi deposition causes layer-by-layer growth up to nominally 4 atomic layers. Diffuse band dispersion and quantum well states observed by angle-resolved photoelectron spectroscopy are consistent with the small surface domains and variation of the thickness even during the layer-by-layer growth region. The further Bi evaporation changes the growth mode to the 3D island formation. The unveiled growth behavior of Bi on InSb(111)B would provide a new interesting playground to study 2D topological electronic structure of quasi-periodic 2D atomic layers.

cond-mat.mtrl-sci↗

arXiv2022

Photo-induced nonlinear band shift and valence transition in SmS

The photo-induced band structure variation of a rare-earth-based semiconductor, samarium monosulfide (SmS), was investigated using high-harmonic-generation laser-based time-resolved photoelectron spectroscopy. A nonlinear photo-induced band shift of the Sm 4f multiplets was observed. The first one is a shift to the high-binding-energy side due to a large surface photovoltage (SPV) effect of approximately 93 meV, comparable to the size of the bulk band gap, with a much longer relaxation time than 0.1 ms. The second one is an ultrafast band shift to the low binding energy side, which is in the opposite direction to the SPV shift, suggesting an ultrafast valence transition from divalent to trivalent Sm ions due to photo-excitation. The latter energy shift was approximately 58 meV, which is consistent with the energy gap shift from ambient pressure to the boundary between the black insulator and golden metallic phase with the application of pressure. This suggests that the photo-induced valence transition can reach the phase boundary, but other effects are necessary to realize the golden metallic phase.

cond-mat.mtrl-sci↗

arXiv2022

Surface valence transition in SmS by alkali metal adsorption

The electronic structure changes of SmS surfaces under potassium (K) doping are elucidated using synchrotron-based core-level photoelectron spectroscopy and angle-resolved photoelectron spectroscopy (ARPES). The Sm core-level and ARPES spectra indicate that the Sm mean valence of the surface increased from the nearly divalent to trivalent states, with increasing K deposition. Carrier-induced valence transition (CIVT) from Sm$^{2+}$ to Sm$^{3+}$ exhibits a behavior opposite to that under conventional electron doping. Excess electrons are trapped by isolated excitons, which is inconsistent with the phase transition from the black insulator with Sm$^{2+}$ to the gold metal with Sm$^{3+}$ under pressure. This CIVT helps to clarify the pressure-induced black-to-golden phase transition in this material, which originates from the Mott transition of excitons.

cond-mat.str-el↗

arXiv2021

Bulk-Sensitive Spin-Resolved Resonant Electron Energy-Loss Spectroscopy (SR-rEELS): Observation of Element- and Spin-Selective Bulk Plasmons

We have developed a spin-resolved resonant electron energy-loss spectroscopy (SR-rEELS) in the primary energy of 0.3--1.5 keV, which corresponds to the core excitations of $2p$-$3d$ absorption of transition metals and $3d$-$4f$ absorption of rare-earths, with the energy resolution of about 100~meV using a spin-polarized electron source as a GaAs/GaAsP strained superlattice photocathode. Element- and spin-selective carrier and valence plasmons can be observed using the resonance enhancement of core absorptions and electron spin polarization. Furthermore, bulk-sensitive EELS spectra can be obtained because the primary energy corresponds to the mean free path of 1--10~nm. The methodology is expected to provide us novel information of elementary excitations by resonant inelastic x-ray scattering and resonant photoelectron spectroscopy.

physics.ins-det↗

arXiv2021

Optical evidence of local and itinerant states in Ce- and Yb-heavy-fermion compounds

The electronic properties of Cerium (Ce) and ytterbium (Yb) intermetallic compounds may display a more local or more itinerant character depending on the interplay of the exchange interactions among the $4f$ electrons and the Kondo coupling between $4f$ and conduction electrons. For the more itinerant case, the materials form heavy-fermions once the Kondo effect is developed at low temperatures. Hence, a temperature variation occurs in the electronic structure that can be traced by investigating the optical conductivity ($σ(ω)$) spectra. Remarkably, the temperature variation in the $σ(ω)$ spectrum is still present in the more localized case, even though the Kondo effect is strongly suppressed. Here, we clarify the local and itinerant character in the electronic structure by investigating the temperature dependence in the $σ(ω)$ spectra of various Ce and Yb compounds with a tetragonal ThCr$_2$Si$_2$-type crystal structure. We explain the temperature change in a unified manner. Above temperatures of about 100 K, the temperature dependence of the $σ(ω)$ spectra is mainly due to the electron-phonon interaction, while the temperature dependence below is due to the Kondo effect.

cond-mat.str-el↗

arXiv2020

One-dimensionality of the spin-polarized surface conduction and valence bands of quasi-one-dimensional Bi chains on GaSb(110)-(2$\times$1)

Surface electronic structure and its one-dimensionality above and below the Fermi level ($E_{\rm F}$) were surveyed on the Bi/GaSb(110)-(2$\times$1) surface hosting quasi-one-dimensional (Q1D) Bi chains, using conventional (one-photon) and two-photon angle-resolved photoelectron spectroscopy (ARPES) and theoretical calculations. ARPES results reveal that the Q1D electronic states are within the projected bulk bandgap. Circular dichroism of two-photon ARPES and density-functional-theory calculation indicate clear spin and orbital polarization of the surface states consistent with the giant sizes of Rashba-type SOI, derived from the strong contribution of heavy Bi atoms. The surface conduction band above $E_{\rm F}$ forms a nearly straight constant-energy contour, suggesting its suitability for application in further studies of one-dimensional electronic systems with strong SOI. A tight-binding model calculation based on the obtained surface electronic structure successfully reproduces the surface band dispersions and predicts possible one- to two-dimensional crossover in the temperature range of 60--100~K.

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↗

arXiv2019

Magneto-Optics of the Weyl Semimetal TaAs in the THz and IR Regions

The magnetic-field dependence of optical reflectivity [$R(ω)$] and optical conductivity [$σ(ω)$] spectra of the ideal type-I Weyl semimetal TaAs has been investigated at the temperature of 10 K in the terahertz (THz) and infrared (IR) regions. The obtained $σ(ω)$ spectrum in the THz region of $\hbarω\leq15$ meV is strongly affected by the applied magnetic field ($B$): The Drude spectral weight is rapidly suppressed and an energy gap originating from the optical transition in the lowest Landau levels appears with a gap size that increases in proportion to $\sqrt{B}$, which suggests linear band dispersions. The obtained THz $σ(ω)$ spectra could be scaled not only in the energy scale by $\sqrt{B}$ but also in the intensity by $1/\sqrt{B}$ as predicted theoretically. In the IR region for $\hbarω\geq17$ meV, on the other hand, the observed $R(ω)$ peaks originating from the optical transitions in higher Landau levels are proportional to linear-$B$ suggesting parabolic bands. The different band dispersions originate from the crossover from the Dirac to the free-electron bands.

cond-mat.str-el↗

arXiv2019

Optical and photoelectrical studies on anisotropic metal-insulator transition of RuAs

The anisotropic changes in the electronic structure of a metal-to-insulator transition (MIT) material, RuAs, with two-step phase transition are reported by using polarized optical conductivity [$σ(ω)$] spectra, angle-integrated photoelectron (PE) spectra, and band calculations based on local density approximation (LDA). Both the PE and $σ(ω)$ spectra not only in the high-temperature (HT) phase but also in the low-temperature (LT) phase as well as the energy gap formation owing to the MIT were almost consistent with those derived from the LDA band calculations, so the fundamental electronic structure in the HT and LT phases can be explained without electron correlations. However, the electronic structure in the middle phase between the HT and LT phases has not been clarified. The polarized $σ(ω)$ spectra revealed not only the anisotropic energy gap formation but also the anisotropic gap-opening temperature, i.e., the energy gap along the $c$ axis in the HT phase starts to open near the higher transition temperature, but that along the $b$ axis opens below the lower transition temperature. The finding suggests that the two-step MIT originates from the anisotropic energy gap formation.

cond-mat.str-el↗

arXiv2018

Evidence for a preformed Cooper pair model in the pseudogap spectra of a Ca10(Pt4As8)(Fe2As2)5 single crystal with a nodal superconducting gap

For high-Tc superconductors, clarifying the role and origin of the pseudogap is essential for understanding the pairing mechanism. Among the various models describing the pseudogap, the preformed Cooper pair model is a potential candidate. Therefore, we present experimental evidence for the preformed Cooper pair model by studying the pseudogap spectrum observed in the optical conductivity of a Ca10(Pt4As8)(Fe2As2)5 (Tc = 34.6 K) single crystal. We observed a clear pseudogap structure in the optical conductivity and observed its temperature dependence. In the superconducting (SC) state, one SC gap with a gap size of Δ = 26 cm-1, a scattering rate of 1/τ = 360 cm-1 and a low-frequency extra Drude component were observed. Spectral weight analysis revealed that the SC gap and pseudogap are formed from the same Drude band. This means that the pseudogap is a gap structure observed as a result of a continuous temperature evolution of the SC gap observed below Tc. This provides clear experimental evidence for the preformed Cooper pair model.

cond-mat.supr-con↗