arXiv subjects

Y. Kinoshita

Publications and source records attributed to Y. Kinoshita.

9 recordsLinked to original sources

arXiv2026

Role of d-electron density of states in the quantum size effect of Pt-Ni and Pt-Pd nanoparticles

We investigated the quantum size effect (QSE) in bimetallic Pt$_{1-x}$Pd$_x$ and Pt$_{1-x}$Ni$_x$ nanoparticles, using $^{195}$Pt nuclear magnetic resonance measurements. The temperature and size dependencies of the anomaly in the nuclear spin-lattice relaxation rate divided by temperature $1/T_1T$ in the Pt$_{1-x}$Pd$_x$ nanoparticles suggest similar electron states between Pt and Pd atoms and are well understood by the QSE. The temperature and composition variations of $1/T_1T$ and Knight shift reveal a systematic increase in the density of states and reduction of the characteristic energy scale $T^*$ with increasing Ni content, consistent with the Kubo gap $\delta_{\mathrm{Kubo}}$. In contrast to Pt$_{1-x}$Cu$_x$ nanoparticles where the QSE is suppressed, the Pt$_{1-x}$Ni$_x$ nanoparticles exhibit clear signatures of quantum energy discretization. This discrepancy highlights the essential role of $d$-electrons in the manifestation of the QSE. Furthermore, analysis of the modified Korringa parameter $K(\alpha)$ suggests enhanced ferromagnetic correlations with increasing Ni concentration, approaching a ferromagnetic quantum critical regime. These results provide experimental evidence that $d$-electron density of states plays a crucial role in the manifestation of the QSE in the nanoparticles formed by the metallic $d$-electron atoms.

cond-mat.str-el↗

arXiv2024

Microwave Hall measurements using a circularly polarized dielectric cavity

We have developed a circularly polarized dielectric rutile (TiO$_2$) cavity with a high quality-factor that can generate circularly polarized microwaves from two orthogonal linearly polarized microwaves with a phase difference of $\pm π/2$ using a hybrid coupler. Using this cavity, we have established a new methodology to measure the microwave Hall conductivity of a small single crystal of metal in the skin-depth region. Based on the cavity perturbation technique, we have shown that all components of the surface impedance tensor can be extracted under the application of a magnetic field by comparing the right- and left-handed circularly polarized modes. To verify the validity of the developed method, we performed test measurements on tiny Bi single crystals at low temperatures. As a result, we have successfully obtained the surface impedance tensor components and confirmed that the characteristic field dependence of the ac Hall angle in the microwave region is consistent with the expectation from the dc transport measurements. These results demonstrate a significant improvement in sensitivity compared to previous methods. Thus, our developed technique allows more accurate microwave Hall measurements, opening the way for new approaches to explore novel topological quantum phenomena, such as time-reversal symmetry breaking in superconductors.

cond-mat.str-el↗

arXiv2023

A field-induced reentrant insulator state of a gap-closed topological insulator (Bi_{1-x}Sb_x) in quantum-limit states

In the extreme quantum limit states under high magnetic fields, enhanced electronic correlation effects can stabilize anomalous quantum states. Using band-tuning with a magnetic field, we realized a spin-polarized quantum limit state in the field-induced semimetallic phase of a topological insulator Bi_{1-x}Sb_x. Further increase in the field injects more electrons and holes to this state and results in an unexpected reentrant insulator state in this topological semimetallic state. A single-particle picture cannot explain this reentrant insulator state, reminiscent of phase transitions due to many-body effects. Estimates of the binding energy and spacing of electron-hole pairs and the thermal de Broglie wavelength indicate that Bi_{1-x}Sb_x may host the excitonic insulator phase in this extreme environment.

cond-mat.mes-hall↗

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↗

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↗

arXiv2020

Dynamic Evolution of Flux Distributions in a Pulse-driven Superconductor by High-speed Magneto-optical Imaging

The accurate understanding of flux dynamics is essential for the design and operation of superconducting circuits. The time evolution of flux-density distribution in an NbN strip by the transport current was observed using high-speed magneto-optical microscopy. It was determined that even for the dynamic penetration and exclusion of vortices under the transport current, the surface barrier is essential. This feature is important for the correct understanding of the complex behavior of state-of-the-art superconducting devices.

cond-mat.supr-con↗

arXiv2020

Spintronic superconductor in a bulk layered material with natural spin-valve structure

Multi-layered materials provide fascinating platforms to realize various functional properties, possibly leading to future electronic devices controlled by external fields. In particular, layered magnets coupled with conducting layers have been extensively studied recently for possible control of their transport properties via the spin structure. Successful control of quantum-transport properties in the materials with antiferromagnetic (AFM) layers, so-called natural spin-valve structure, has been reported for the Dirac Fermion and topological/axion materials. However, a bulk crystal in which magnetic and superconducting layers are alternately stacked has not been realized until now, and the search for functional properties in it is an interesting yet unexplored field in material science. Here, we discover superconductivity providing such an ideal platform in EuSn2As2 with the van der Waals stacking of magnetic Eu layers and superconducting Sn-As layers, and present the first demonstration of a natural spin-valve effect on the superconducting current. Below the superconducting transition temperature (Tc), the electrical resistivity becomes zero in the in-plane direction. In contrast, it, surprisingly, remains finite down to the lowest temperature in the out-of-plane direction, mostly due to the structure of intrinsic magnetic Josephson junctions in EuSn2As2. The magnetic order of the Eu layers (or natural spin-valve) is observed to be extremely soft, allowing one to easy control of the out-of-plane to in-plane resistivities ratio from 1 to infinity by weak external magnetic fields. The concept of multi-functional materials with stacked magnetic-superconducting layers will open a new pathway to develop novel spintronic devices with magnetically controllable superconductivity.

cond-mat.supr-con↗

arXiv2014

Terahertz radiation by optical rectification in a hydrogen-bonded organic molecular ferroelectric crystal, 2-phenylmalondialdehyde

Terahertz radiation by optical rectification has been observed at room temperature in a hydrogen-bonded organic molecular ferroelectric crystal, 2-phenyl malondialdehyde (PhMDA). The radiated electromagnetic wave consisted of a single-cycle terahertz pulse with a temporal width of $\sim$ 0.5 ps. The terahertz radiation amplitude divided by the sample thickness in PhMDA was nearly equivalent to that in a typical terahertz wave emitter ZnTe. This is attributable to a long coherence length in the range of 130 $\sim$ 800 $μ$m for the terahertz radiation from PhMDA. We also discussed the possibility of PhMDA as a terahertz wave emitter in terms of the phase-matching condition.

cond-mat.mtrl-sci↗

arXiv2014

Visualization of ferroelectric domains in boracite using emission of terahertz radiation

We report on the emission of terahertz radiation by irradiation of femtosecond laser pulses in non-centrosymmetric paraelectric and ferroelectric phases of Co$_3$B$_7$O$_{13}$I boracite. The Generation of the terahertz waves in both phases is caused by optical rectification via a second-order nonlinear optical effect. In the ferroelectric phase, we successfully visualized ferroelectric domains by analyzing the polarization state of the terahertz wave radiated from the crystal. In a large area of the crystal ($\sim$ 500 $\times$ 500 $μ$m$^2$), the observed polarization vector of the radiated terahertz wave was tilted from directions of spontaneous polarization, i.e., [100]$_{\rm cub}$, [010]$_{\rm cub}$, and [001]$_{\rm cub}$ in cubic setting, which can be explained by the presence of a ferroelectric 90$^\circ$ domain wall of the (101)$_{\rm cub}$ plane.

cond-mat.mtrl-sci↗