arXiv subjects

Yoshihiko Okamoto

Publications and source records attributed to Yoshihiko Okamoto.

At least 19 recordsLinked to original sources

arXiv2026

Intrinsic Magnetic Excitations and Heavy-Fermion Formation in the Frustrated Mn Pyrochlore System YMn$_{2+δ}$Zn$_{20-x}X_x$ ($X$ = In and Al) Revealed by Nuclear Magnetic Resonance and Nuclear Quadrupole Resonance Measurements

We performed nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) measurements to investigate the microscopic electronic states of the $d$-electron heavy-fermion candidates $\mathrm{YMn_{2+δ}Zn_{20-x}In_x}$ and $\mathrm{YMn_{2+δ}Zn_{20-x}Al_x}$. In these compounds, magnetic fluctuations of the Mn pyrochlore lattice are expected to play an important role in heavy-fermion formation; however, excess Mn atoms complicate the interpretation of the physical properties. Our spectral analysis reveals that In substitution exhibits much higher site selectivity and introduces significantly less disorder in local structure than Al substitution. The temperature dependence of the nuclear spin-lattice relaxation rate divided by temperature $1/T_1T$ measured by $^{55}$Mn-NQR shows a clear enhancement at low temperatures, indicating the development of low-energy excitations associated with heavy-fermion formation. However, its absolute magnitude is approximately 20 times smaller than that in the related compound YMn$_2$, which hosts stronger antiferromagnetic correlations, indicating that the magnetic interactions are substantially weakened by the enlarged Mn-Mn distance. These results demonstrate that the heavy-fermion state in this system arises from the Mn pyrochlore network and is more closely associated with frustration-induced magnetic excitations with low energy than with conventional antiferromagnetic quantum-critical fluctuations.

cond-mat.str-el↗

arXiv2026

Stabilizing Itinerant Electrons in a Corner-Sharing Kagomé Oxide Nd4Os3ZnO14

Kagome oxides provide a fertile platform for exploring exotic electronic states arising from geometrical frustration and characteristic band topology. Here, we report the synthesis of a 5d transition-metal kagome oxide, Nd4Os3ZnO14, obtained via high-temperature, high-pressure hydrothermal synthesis. Single-crystal X-ray diffraction reveals a two-dimensional kagome network formed by corner-sharing OsO6 octahedra, with a nominal osmium valence of +4.67. In-plane resistivity and hard X-ray photoelectron spectroscopy measurements indicate that the semimetallic electronic structure at room temperature evolves into a semiconducting ground state upon cooling, accompanied by a pronounced enhancement of hole mobility. Magnetic susceptibility measurements demonstrate localized Nd3+ moments without long-range magnetic order down to 2 K. The coexistence of a metallic kagome plane, strong spin-orbit coupling inherent to 5d electrons, and rare-earth magnetism establishes Nd4Os3ZnO14 as a promising platform for investigating correlated electron phenomena in kagome oxides within the itinerant regime.

cond-mat.str-el↗

arXiv2026

Quasi-One-Dimensional Electronic Nature of Ta4SiTe4 Underlying the Giant Thermoelectric Performance

Ta4SiTe4 is a one-dimensional van der Waals material that exhibits an exceptionally large thermoelectric power factor below room temperature. However, since this material has been available only in the form of acicular microcrystals, experimental exploration of the electronic properties responsible for its giant thermoelectric performance has long been challenging. In this study, we quantitatively evaluated the one-dimensional electronic nature of Ta4SiTe4 by combining micro-spot angle-resolved photoemission spectroscopy and transport measurements on focused-ion-beam-processed samples. The angle-resolved photoemission spectroscopy measurements reveal anisotropic band dispersions along and perpendicular to the crystallographic c axis. Consistently, transport measurements demonstrate that the resistivity perpendicular to the c axis is approximately five times larger than that along the c axis at 200 K. These results provide direct experimental evidence for the quasi-one-dimensional electronic character of Ta4SiTe4, which underlies its giant thermoelectric response reported previously, and offer fundamental insights into the role of electronic dimensionality in enhancing thermoelectric performance.

cond-mat.mtrl-sci↗

arXiv2026

Molecular Orbital Degeneracy Lifting in a Tetrahedral Cluster System NbSeI

The lifting of degenerate electronic states, in which multiple electronic states share the same energy, is a fundamental issue in the physics of crystalline solids. In real materials, this problem has been extensively studied in transition metal compounds, where various quantum phenomena arise from the spin and orbital degeneracy of the d electrons on individual transition-metal atoms. In contrast, materials containing high-symmetry clusters composed of multiple transition-metal atoms are expected to exhibit more emergent phenomena due to the entanglement of the electronic degrees of freedom across multiple atoms. Here, we report the discovery of two distinct mechanisms of orbital-degeneracy lifting in NbSeI, which comprises Nb4 tetrahedral clusters with molecular orbital degrees of freedom and whose average crystal structure is predicted to host a flat-band metal. Below 106 K, NbSeI is found to be a nonmagnetic molecular orbital-ordered insulator. Above this temperature, the average structure becomes face-centered cubic without any superlattice, while the orbital degeneracy remains lifted by significant local distortions of Nb4 tetrahedra, which may be associated with a molecular orbital-liquid or orbital-frozen state. This noncooperative Jahn-Teller distortion stabilizes a nonmagnetic insulating state above 106 K, in stark contrast to the flat-band metal predicted from the average structure.

cond-mat.str-el↗

arXiv2025

Superconductivity in Ternary Zirconium Telluride Zr6RuTe2

Zr6CoAl2-type Zr6RuTe2 is found to show bulk superconductivity below the superconducting transition temperature Tc = 1.1 K, according to the electrical resistivity, magnetization, and heat capacity measurements using synthesized polycrystalline samples. This Tc exceeds that of Zr6MTe2 compounds in which M is other transition metals, indicating that M = Ru is favorable for superconductivity in Zr6CoAl2-type Zr6MX2.

cond-mat.supr-con↗

arXiv2025

Anisotropic superconducting gap probed by $^{125}$Te NMR in noncentrosymmetric Sc$_6M$Te$_2$ ($M$ = Fe, Co)

The superconducting gap symmetry is investigated by $^{125}$Te NMR measurements on Sc$_6M$Te$_2$ ($M$ = Fe, Co) without spatial inversion symmetry. The spin susceptibility obtained from the Knight shift $K$ is suppressed below the superconducting transition temperature, while leaving a finite value down to the lowest temperature ($\simeq 0.4$ K). The nuclear spin-lattice relaxation rate $1/T_1$ follows a power law against temperature $T$ without showing a coherence peak characteristic of the isotropic gap. The result implies a pairing admixture or a residual density of states under magnetic field. The normal metallic state has a Korringa scaling relation between $1/T_1T$ and the Knight shift, reflecting a weak electron correlation.

cond-mat.supr-con↗

arXiv2025

Cubic ReSTe as a High-Performance Thermoelectric Material

We report thermoelectric properties of sintered samples of undoped, W-doped, and Sb-doped ReSTe crystallized in a cubic MoSBr-type structure. All samples exhibited p-type thermoelectric properties. ReSTe and Re0.993W0.007STe exhibited the largest dimensionless figure of merit ZT, reaching 0.4 at 660 K. This high performance is attributed to large power factor owing to the degenerate semiconducting state realized by the strong spin-orbit coupling and low lattice thermal conductivity of the sintered samples. Furthermore, electronic band dispersion of ReSTe is almost flat at the bottom of the conduction band, suggesting that n-type ReSTe is expected to exhibit much higher performance than p-type ReSTe.

cond-mat.mtrl-sci↗

arXiv2024

Topological Semimetal KAlGe with Novel Electronic Instability

Compounds with the anti-PbFCl structure exhibit a variety of electronic instabilities and intriguing physical properties. NaAlSi and NaAlGe are similar topological nodal-line semimetals, but they have distinct properties. NaAlSi is a superconductor at 6.8 K, whereas NaAlGe is an insulator with a pseudogap of approximately 100 K. Using the potassium-indium flux method, we succeeded in synthesizing a single crystal of KAlGe, a new anti-PbFCl compound. First principles electronic structure calculations reveal that KAlGe is isoelectronic with NaAlSi and NaAlGe. KAlGe undergoes a metal-to-metal transition at 89 K and exhibits no superconductivity above 1.8 K. The low temperature phase has significantly lower carrier density and extremely high mobility, similar to Dirac electron systems. Furthermore, X-ray diffraction experiments show a structural change that breaks the fourfold symmetry during the phase transition. Electron-phonon interactions may be responsible for superconductivity in NaAlSi, whereas excitonic electron-hole interactions are thought to play an important role in KAlGe and possibly NaAlGe. Our findings demonstrate that fascinating physics lies within the compound family.

cond-mat.str-el↗

arXiv2024

Broken time-reversal symmetry detected by tunneling spectroscopy of superconducting Pd-doped CaAgP

The appearance of broken time-reversal symmetry (TRS) in superconducting states is an intriguing issue in solid-state physics because of the incompatibility of the spontaneous magnetic field and the Meissner effect. We identify broken TRS in Pd-doped CaAgP (CaAg$_{0.9}$Pd$_{0.1}$P) by tunneling spectroscopy through the magnetic field response of conductance spectra. CaAg$_{0.9}$Pd$_{0.1}$P is a nodal-line semimetal with exotic electronic states such as drumhead surface states and surface superconductivity. Tunneling conductance spectra acquired at the side surfaces of CaAg$_{0.9}$Pd$_{0.1}$P under an applied magnetic field exhibit broad zero-bias peaks with small asymmetric structures. Surprisingly, the asymmetric structures are reversed exactly by flipping the field direction. On the basis of an analysis which stands on the formula of tunneling junctions for unconventional superconductors, these results are consistent with the pair potential of the superconductivity breaks the TRS and is strongly coupled to an external magnetic field. We reveal the novel character of superconducting nodal-line semimetals by developing the TRS sensitivity of tunneling spectroscopy. Our results serve as an exploration of broken TRS in superconducting states realized in topological materials.

cond-mat.supr-con↗

arXiv2024

Superconductivity in Hexagonal Zr6CoAl2-Type Zr6RuBi2 and Zr6FeBi2

We report the synthesis and electronic properties of polycrystalline samples of Zr6MBi2 (M = Ru and Fe) crystallizing in the hexagonal Zr6CoAl2-type structure. Based on their electrical resistivity, magnetization, and heat capacity data, Zr6RuBi2 and Zr6FeBi2 are found to exhibit bulk superconductivity below Tc = 4.9 and 1.4 K, respectively. Although Zr6RuBi2 is most likely a conventional superconductor, the considerably higher Tc for M = Ru than that for M = Fe differs from the trend in Tc for Zr6CoAl2-type superconductors reported thus far. The superconductivity of an amorphous solid-solution phase, which may hinder elucidation of the superconducting properties of Zr6MBi2, is also discussed.

cond-mat.supr-con↗

arXiv2024

Electronic Self-Organization in the β-Pyrochlore Oxide CsW2O6

In this review, I present the electronic properties of the beta-pyrochlore oxide CsW2O6 and other related materials. At 215 K, CsW2O6 exhibits an electronic phase transition to a nonmagnetic insulating state, which exhibits a complex self-organization of 5d electrons. In this phase transition, various factors, such as geometrical frustration of the pyrochlore structure, moderately strong electron correlation, Jahn-Teller-like distortion, phase transition to the insulating state, and formation of chemical bonds in solids, which are powerful driving forces for achieving a wide variety of electronic properties in transition metal oxides, play important roles. In CsW2O6, the interplay of these factors has led to the emergence of an electronic phase transition that preserves the cubic symmetry, three-dimensional nesting of Fermi surfaces, a possible charge order with a fractional valence satisfying the Anderson condition, and an equilateral-triangular trimer formation by a three-centered-two-electron bond. In addition to the introduction of these unique features of CsW2O6, each of them has been compared to those of other materials to provide an overview of the electronic properties of a wide variety of related materials, which can contribute to a complete understanding of the vast and infinite electronic phenomena in transition metal oxides.

cond-mat.mtrl-sci↗

arXiv2024

Superconductivity in Ternary Zirconium Telluride Zr6MTe2 with 3d Transition Metals

We report the synthesis, electronic properties, and electronic states of Zr6MTe2 (M = Cr, Mn, Fe, and Co), which is isostructural to a recently discovered superconductor family Sc6MTe2. Based on the electrical resistivity and heat capacity data measured at low temperatures, Zr6FeTe2 is found to show bulk superconductivity below Tc = 0.76 K. Zr6CoTe2 also exhibited zero resistivity due to superconductivity below 0.13 K. In contrast, Zr6+dMn1-dTe2 does not show superconductivity but instead exhibits strong magnetism, which most likely prevents the formation of superconductivity in this material. The electronic properties and electronic states of Zr6MTe2 are discussed in comparison with those of Sc6MTe2.

cond-mat.supr-con↗

arXiv2023

Superconductivity in Ternary Germanide ScPdGe and Silicide ScPdSi

The electronic properties of ScPdGe and ScPdSi, crystallizing in the hexagonal ZrNiAl and orthorhombic TiNiSi structures, respectively, are investigated. ScPdGe and ScPdSi are found to show bulk superconductivity below 0.9 and 1.7 K, respectively, based on electrical resistivity and heat capacity data measured using synthesized polycrystalline samples. First principles calculations indicate the presence of large contributions of Sc 3d and Pd 4d electrons at the Fermi energy in both materials. The electronic properties and electronic states of these materials are discussed in comparison with those of several superconductors containing scandium and a 4d transition metal element.

cond-mat.supr-con↗

arXiv2023

Anisotropic Optical Conductivity Accompanied by a Small Energy Gap in One-Dimensional Thermoelectric Telluride Ta4SiTe4

We investigated the optical properties of single crystals of one-dimensional telluride Ta4SiTe4, which shows high thermoelectric performance below room temperature. Optical conductivity estimated from reflectivity spectra indicates the presence of a small energy gap of 0.1-0.15 eV at the Fermi energy. At the lowest energy, optical conductivity along the Ta4SiTe4 chain is an order of magnitude higher than that perpendicular to this direction, reflecting the anisotropic electron conduction in Ta4SiTe4. These results indicate that coexistence of a very small band gap and anisotropic electron conduction is a promising strategy to develop a high-performance thermoelectric material for low temperature applications.

cond-mat.mtrl-sci↗

arXiv2023

Superconductivity in Ternary Scandium Telluride Sc6MTe2 with 3d, 4d, and 5d Transition Metals

We report the discovery of bulk superconductivity in Sc6MTe2 with seven kinds of transition-metal elements M. The critical temperatures for M = 3d elements are higher than those for 4d and 5d elements and increase in the order of M = Ni, Co, and Fe with the highest Tc of 4.7 K in Sc6FeTe2. First principles calculations indicate the presence of significant contribution of Fe 3d orbitals at the Fermi energy, which most likely enhance the Tc of Sc6FeTe2. The upper critical field for M = Os is considerably enhanced by the strong spin-orbit coupling. These results show Sc6MTe2 to constitute a unique family of d-electron superconductors, in which d electrons of 3d and 5d M atoms strongly influence the superconducting properties.

cond-mat.supr-con↗

arXiv2023

Structural and Electronic Properties of a Triangular Lattice Magnet NaPrTe$_2$ Compared with NaNdTe$_2$ and NaTbTe$_2$

NaPrTe2, NaNdTe2, and NaTbTe2 are found to be triangular lattice magnets with the alpha-NaFeO2 structure, where lanthanoid atoms with 4f electrons form a triangular lattice, based on the structural analysis and physical property measurements of synthesized polycrystalline samples. The alpha-NaFeO2 structure is a new polymorph of NaPrTe2, which has been reported to crystallize in the cubic LiTiO2 structure. Polytypism in NaPrTe2 was discussed based on the structural parameters determined by the Rietveld analysis. NaPrTe2 is suggested to be in the proximity of the phase boundary between the LiTiO2 and alpha-NaFeO2 types, as compared to NaNdTe2 and NaTbTe2, indicating that this compound might be interesting from the perspectives of the dimensional control of geometrically frustrated lattices. The magnetic susceptibility and heat capacity data indicated that NaPrTe2 do not show long-range magnetic order or a spin-glass transition above 2 K.

cond-mat.mtrl-sci↗

arXiv2023

Inelastic Neutron Scattering Study of the Spin Dynamics in the Breathing Pyrochlore System LiGa0.95In0.05Cr4O8

The A-site ordered chromate spinels LiGa1-xInxCr4O8 host a network of size-alternating spin-3/2 Cr3+ tetrahedra known as a 'breathing' pyrochlore lattice. For the x=0.05 composition, the complex magneto-structural ordering observed in the parent x=0 material is replaced by a single transition at Tf=11 K, ascribed to the collinear nematic order caused by strong spin-lattice coupling. We present here an inelastic neutron scattering study of the spin dynamics in this composition. Above Tf , the dynamical scattering function S(Q,E) is ungapped and quasi-elastic, similar to undoped LiGaCr4O8. Below Tf , the spectral weight splits between a broad inelastic feature at 5.8 meV and toward the elastic line. The former feature can be ascribed to spin precessions within antiferromagnetic loops, lifted to finite energy by the effective biquadratic spin-lattice term in the spin Hamiltonian.

cond-mat.str-el↗

arXiv2022

Large Magnetic-Field-Induced Strains in Sintered Chromium Tellurides

Sintered samples of Cr3Te4 and Cr2Te3 are found to show large strains accompanied by large volume changes under a magnetic field. In Cr3Te4, volume increases of deltaV/V = 500-1170 ppm by applying a magnetic field of 9 T are observed over the entire temperature range below 350 K. At room temperature, the deltaV/V value exceeds 1000 ppm, which is considerably larger than the maximum values reported for Cr-based magnets thus far and is comparable to the room-temperature value of forced-volume magnetostriction in invar alloys. Cr2Te3 show a large deltaV/V of 680 ppm when applying a magnetic field of 9 T at 200 K. Both samples display particularly large volume increases around the Curie temperature, where they also show negative thermal expansion due to microstructural effects, suggesting that the cooperation between anisotropic lattice deformation associated with the magnetic ordering and microstructural effects is essential for the manifestation of the large magnetic-field-induced volume changes.

cond-mat.mtrl-sci↗