arXiv subjects

Koshi Takenaka

Publications and source records attributed to Koshi Takenaka.

At least 19 recordsLinked to original sources

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↗

arXiv2025

Uniform electronic states and $s$-wave superconductivity in a strongly disordered high-entropy compound (RuRhPdIr)$_{0.6}$Pt$_{0.4}$Sb

High-entropy compounds, where multiple elements occupy a single crystallographic site in a highly disordered manner, challenge conventional understandings of electronic structure based on periodicity and well-defined band dispersion. Here, we report a detailed nuclear magnetic resonance study of the high-entropy superconductor (RuRhPdIr)$_{0.6}$Pt$_{0.4}$Sb, revealing a spatially homogeneous electronic environment in the normal state, in stark contrast to its crystallographically disordered lattice. The superconducting state exhibits a small but solid Hebel-Slichter coherence peak followed by a significant decrease in the nuclear spin-lattice relaxation rate, providing compelling evidence for fully gapped $s$-wave pairing. Our findings not only deepen the understanding of superconductivity in highly disordered quantum materials but also open a new pathway for exploring novel superconducting states in entropy-stabilized systems.

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

Chemical Pressure Tuning of Multipolar and Magnetic Orders in Ba$_2$(Cd$_{1-x}$Ca$_x$)ReO$_6$ Double Perovskites

Double perovskite compounds containing 5d transition metal elements have been extensively studied as platforms for multipolar order phenomena stemming from spin-orbit-entangled 5d electrons. In this study, we examine the interplay between crystal structure, multipolar order, and magnetic order in solid solutions of double perovskites with the 5d1 electronic configuration: Ba2CdReO6 and Ba2CaReO6, which exhibit distinct electronic orders. The substitution of larger Ca2+ ions for Cd2+ in Ba2CdReO6, systematically increases the lattice constant with increasing the amount of substitution x. Although the spin-orbit-entangled J = 3/2 state remains intact upon substitution, both the quadrupolar order below Tq = 25 K and the canted antiferro-magnetic (AFM) order below Tm = 12 K in Ba2CdReO6 are progressively suppressed as x increases. Magnetization measurements reveal that the canted AFM order is suppressed at x = 0.6, transitioning to a colinear AFM order, while the quadrupolar order persists up to x = 0.9. The experimental electronic phase diagram, summarizing the dependence of electronic orders on lattice constants, aligns well with the theoretical phase diagram considering electric quadrupolar interactions [G. Chen et al., Phys. Rev. B 82, 174440 (2010)]. This correspondence confirms that chemical pressure induced by substitution effectively tunes the interaction between 5d electrons. The results highlight the potential of chemical pressure to modulate multipolar interactions, paving the way for novel multipolar properties in 5d electron systems.

cond-mat.str-el↗

arXiv2025

Superconductivity in High-Entropy Antimonide M$_{1-x}$Pt$_x$Sb (M = equimolar Ru, Rh, Pd, and Ir)

The high-entropy concept was applied to the synthesis of transition-metal antimonides, M1-xPtxSb (M = equimolar Ru, Rh, Pd, and Ir). High-entropy antimonide samples crystallized in a pseudo-hexagonal NiAs-type crystal structure with a P63/mmc space group were successfully synthesized through a conventional solid-state reaction and subsequent quenching. A detailed investigation of the composition and equilibration conditions confirmed the reversible phase transition between a multi-phase state at low temperature and an entropy-driven single-phase solid solution at high temperatures. Electrical resistivity, magnetization, and heat capacity measurements of single-phase M1-xPtxSb (x = 0.2) samples revealed a bulk superconducting transition at 2.15(2) K. This study demonstrates that the high-entropy concept provides numerous opportunities for the discovery of new functional materials such as superconductors.

cond-mat.supr-con↗

arXiv2025

Topological Electronic Structure and Transport Properties of the Distorted Rutile-type WO$_2$

We elucidate the transport properties and electronic structures of distorted rutile-type WO2. Electrical resistivity and Hall effect measurements of high-quality single crystals revealed the transport property characteristics of topological materials; these characteristics included an extremely large magnetoresistance of 13,200% (2 K and 9 T) and a very high carrier mobility of 25,700 cm2 V-1 s-1 (5 K). First-principles calculations revealed Dirac nodal lines (DNL) near the Fermi energy in the electronic structure when spin-orbit interactions (SOIs) were absent. Although these DNLs mostly disappeared in the presence of SOIs, band crossings at high-symmetry points in the reciprocal space existed as Dirac points. Furthermore, DNLs protected by nonsymmorphic symmetry persisted on the ky = π/b plane. The unique transport properties originating from the topological electronic structure of chemically and thermally stable WO2 could represent an opportunity to investigate the potential electronic applications of the material.

cond-mat.mtrl-sci↗

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

Odd-parity multipole order in the spin-orbit coupled metallic pyrochlore Pb$_2$Re$_2$O$_{7-δ}$

The pyrochlore oxide Pb2Re2O7-δ (PRO) is a candidate spin-orbit-coupled metal (SOCM) that exhibits a structural phase transition with inversion symmetry breaking. In this study, we report the results of detailed X-ray diffraction (XRD) measurements on single crystals of PRO to clarify the crystal structure below the phase transition temperature at Ts = 300 K. In the XRD patterns, a clear peak splitting is observed below Ts, indicating a cubic to tetragonal transition. Based on the group-subgroup relationship and the observed reflection conditions, the space group of the low-temperature phase is proposed to be I4122, which agrees with optical second harmonic generation measurements. This space group is the same as that of the lowest temperature structure of the analogous SOCM Cd2Re2O7 (CRO), which is realized by the emergence of odd-parity multipole order. The comparison between PRO and CRO allows for advancing our understanding on the symmetry-lowering complex order exhibited by SOCMs.

cond-mat.str-el↗

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↗

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↗

arXiv2022

Electronic Properties of Pyrochlore-Type Ca2Ir2O7

The electrical resistivity, magnetic susceptibility, and heat capacity of sintered samples of Ca2Ir2O7, in which pentavalent Ir atoms with 5d4 electron configuration form a pyrochlore structure, have been studied. The obtained experimental data strongly suggest that Ca2Ir2O7 is metallic below room temperature and exhibits no electronic or magnetic phase transitions above 0.12 K.

cond-mat.str-el↗

arXiv2022

Large Magnetic-Field-Induced Strain at the Spin-Reorientation Transition in the A-Site Ordered Spinel Oxide LiFeCr4O8

Sintered samples of a spinel oxide LiFeCr4O8, where Cr3+ and Fe3+ ions have localized moments, were found to show a large magnetic-field-induced volume increase approaching 500 ppm by applying a magnetic field of 9 T. This large volume increase appeared only at around 30 K. At 30 K, a spin-reorientation transition from ferrimagnetic to conical order occurs, giving rise to this large volume increase. The coexistence of ferrimagnetic and conical phases at this transition was found to be important, suggesting that such a large magnetic-field-induced volume change can be realized at various magnetic transitions in localized magnets with strong spin-lattice coupling.

cond-mat.mtrl-sci↗

arXiv2021

Superconductivity in Nb2Pd3Te5 and Chemically-Doped Ta2Pd3Te5

We report on the superconductivity in ternary transition metal tellurides Ta2Pd3Te5 and Nb2Pd3Te5, which have a one-dimensional crystal structure. Single-crystalline and polycrystalline samples of Ta2Pd3Te5 show nonmetallic electron conduction, while Ti or W doping results in metallic behavior with a bulk superconducting transition at 2-4 K. In contrast, the polycrystalline samples of Nb2Pd3Te5, which are found to be isostructural to Ta2Pd3Te5, show a bulk superconducting transition at 3.3 K. The crystal structure and physical properties of Ta2Pd3Te5 and Nb2Pd3Te5 are also discussed in comparison with a candidate excitonic insulator Ta2NiSe5.

cond-mat.supr-con↗

arXiv2021

Large Magnetic-Field-Induced Strain at the Magnetic Order Transition in Triangular Antiferromagnet AgCrS2

Strain induced by a magnetic field is a common phenomenon for ferromagnets, but few antiferromagnets show large strain induced by a magnetic field. On the basis of linear strain measurements of sintered samples of triangular antiferromagnet ACrS2 (A = Cu, Ag, and Au) in magnetic fields up to 9 T, the AgCrS2 sample was found to show a large strain, yielding a large volume change over 700 ppm, which is one of the largest volume changes measured to date for an antiferromagnet. This large strain appeared only at the Néel temperature of 42 K and was not restored to its initial state when the applied magnetic field was decreased to zero; however, it was initialized by cooling the sample to far below the Néel temperature. These results suggest that the coexistence of magnetically ordered and paramagnetic phases at the first-order phase transition plays an important role. AuCrS2 showed a magnetic-field-induced strain with similar features, although it was smaller than that in AgCrS2.

cond-mat.mtrl-sci↗

arXiv2020

Large Thermoelectric Power Factor in Whisker Crystals of Solid Solutions of the One-Dimensional Tellurides Ta4SiTe4 and Nb4SiTe4

One-dimensional tellurides Ta4SiTe4 and Nb4SiTe4 were found to show high thermoelectric performance below room temperature. This study reported the synthesis and thermoelectric properties of whisker crystals of Ta4SiTe4-Nb4SiTe4 solid solutions and Mo- or Ti-doped (Ta0.5Nb0.5)4SiTe4. Thermoelectric power of the solid solutions systematically increased with increasing Ta content, while their electrical resistivity was unexpectedly small. Mo- and Ti-doped (Ta0.5Nb0.5)4SiTe4 showed n- and p-type thermoelectric properties with large power factors exceeding 40 microW cm-1 K-2, respectively. The fact that not only Ta4SiTe4 and Nb4SiTe4 but also their solid solutions showed high performance indicated that this system is a promising candidate for thermoelectric applications at low temperatures.

cond-mat.mtrl-sci↗