arXiv subjects

Jun-ichi Yamaura

Publications and source records attributed to Jun-ichi Yamaura.

At least 19 recordsLinked to original sources

arXiv2026

Molecular-Orbital Degeneracy Lifting and Suppression of a Flat-Band Metal in a Tetrahedral Cluster System NbSeI

The lifting of degenerate electronic states has been extensively studied in transition metal compounds, where diverse quantum phenomena arise from the degrees of freedom of d electrons on individual atoms. In contrast, transition metal compounds containing high-symmetry clusters composed of multiple transition-metal atoms are expected to host a broader range of emergent phenomena due to the entanglement of the electronic degrees of freedom across multiple atoms. Here, we report the discovery of two distinct structural responses associated with molecular-orbital-degeneracy lifting in NbSeI, which comprises Nb4 tetrahedral clusters with molecular orbital degrees of freedom and whose cubic average crystal structure is predicted to host a flat-band metallic state. Below 106 K, NbSeI is a molecular-orbital-ordered nonmagnetic insulator with orthorhombic symmetry. Above this temperature, the average structure becomes face-centered cubic without superlattice formation, while local symmetry breaking associated with pronounced local distortions of the Nb4 tetrahedra lifts the molecular-orbital degeneracy. This local symmetry breaking occurs without cooperative long-range order and stabilizes a nonmagnetic insulating state instead of the flat-band metallic state predicted for the cubic average structure.

cond-mat.str-el↗

arXiv2026

Raman spectroscopy of van der Waals topological magnet GdGaI

We report polarization-resolved Raman spectroscopy of a van der Waals compound GdGaI that is a candidate for excitonic insulators. By combining the symmetry analysis with density functional theory calculations, we identify six Raman-active phonons. The spectra exhibit only the expected anharmonic hardening down to 4 K: no additional peaks, no soft modes, and no signatures of zone folding are observed. This result indicates that any lattice distortion is below our experimental sensitivity, supporting an electronically driven origin for the band reconstruction reported by angle-resolved photoemission spectroscopy rather than an electron-phonon-driven mechanism. Moreover, we observe a pronounced circular dichroism of the $A_{1g}$ modes under an out-of-plane magnetic field. Based on symmetry considerations, we attribute this dichroic response to chiral $A_{1g}$ phonons with opposite angular momenta generated by spin-phonon coupling in the time-reversal-broken state. The temperature evolution of the degree of circular polarization further suggests that circularly polarized Raman spectroscopy detects the emergence of short-range antiferromagnetic correlations. Our results highlight GdGaI as a promising platform in which excitonic order, magnetism, and circularly polarized phonons can be intertwined, and demonstrate that circular-polarization Raman provides a sensitive probe of spin-phonon coupling in excitonic systems.

cond-mat.mtrl-sci↗

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

Anomalies in the thermal conductivity of honeycomb antiferromagnet MnPS$_{3}$

Intrinsic two-dimensional magnets serve as a good platform to explore collective, charge-neutral and low-energy excitations. Distinguishing the crucial role of them in experimental aspect remains a challenge for decades. Here, we study the thermal transport in honeycomb antiferromagnet MnPS$_{3}$ with $T_N$=78 K down to very low temperatures (<0.01$T_N$). At high temperatures (>0.1$T_N$), the field dependence of the thermal Hall conductivity exhibits a linear phonon Hall effect and a peak associated with the spin-flop transition due to a strong spin-lattice coupling, well reproducing the previous report (Phys. Rev. B 110, 165147 (2024)). Notably, below 2 K, we find that the field dependence of the thermal Hall conductivity exhibits sign reversals within the spin-flop phase, at which the field dependence of the longitudinal thermal conductivity also shows multiple valleys. We suggest that these anomalies are caused by the redistribution of Berry curvature in magnon bands, demonstrating the superior performance of the thermal Hall measurements to detect the Berry curvature distributions in magnetic insulators.

cond-mat.mtrl-sci↗

arXiv2025

High-Field Torque Magnetometry on the Kagome Antiferromagnet Karpenkoite

We report torque magnetometry results on single crystals of karpenkoite Co3(V2O7)(OH)2 2H2O, a model candidate for the kagome antiferromagnet. No field-induced phase transition is detected up to 45 T for B||c and B||a. Instead, the torque reveals a continuous spin reorientation toward saturation, most likely governed by a dominant Dzyaloshinskii-Moriya interaction.

cond-mat.str-el↗

arXiv2025

Field-Angle Dependence of Phonon Thermal Hall Effect in Na2X2TeO6 (X = Co, Zn)

The mechanism behind thermal Hall effects by phonons, which are observed in various materials, is not clarified despite the dominant contribution as heat carriers. Theoretically, mechanisms based on the intrinsic Berry phase and those on extrinsic impurity-induced scatterings have been proposed, which can be distinguished by comparing the field-angle dependence of the thermal Hall effect and that of the magnetic anisotropy. Here, we investigate the field-angle dependence of the thermal Hall effects in the antiferromagnet Na2Co2TeO6 and its non-magnetic isostructural analogue Na2Zn2TeO6 in the ac plane. We find that the field-angle dependence of the thermal Hall conductivity in both materials well follows that of the out-of-plane magnetization, showing a common mechanism by extrinsic impurity-induced scatterings in both the phonon thermal Hall effect and that enhanced by a coupling with the magnetism.

cond-mat.mtrl-sci↗

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

One-third magnetization plateau in a spin-1 kagome magnet BaNi$_3$(AsO$_4$)$_2$(OH)$_2$

We investigate the structural and magnetic properties of BaNi$_3$(AsO$_4$)$_2$(OH)$_2$, focusing on its spin-1 kagome lattice and the intricate coexistence of ferromagnetic and antiferromagnetic interactions. Powder x-ray diffraction analysis confirms a highly crystalline trigonal structure. Detailed Rietveld refinement identifies a single crystallographic Ni site, indicative of a perfect kagome lattice. Magnetic susceptibility measurements suggest predominantly ferromagnetic interactions with an effective magnetic moment consistent with Ni$^{2+}$ spins, yet the system undergoes antiferromagnetic ordering at a N$é$el temperature of 5.8 K. Isothermal magnetization measurements reveal a series of metamagnetic transitions culminating in a plateau-like phase near one-third of the total saturation magnetization. Analysis of the phase boundaries shows that the antiferromagnetic phase supports a substantial net moment in each kagome layer, comparable to that of the one-third plateau. This observation challenges the conventional model-where a 120$^\circ$ ground state transitions to an up-up-down configuration-commonly assumed for kagome antiferromagnets. Instead, our findings indicate that both the zero-field ground state and the field-induced phases exhibit in-plane ferrimagnetic spin arrangements on the kagome lattice, with the metamagnetic transition corresponding to a shift from layer-by-layer antiferromagnetically aligned net moments to ferromagnetically aligned ones. This configuration is stabilized by bond frustration, a network of competing interactions that can favor both ferromagnetic and antiferromagnetic couplings, highlighting the essential role of frustration in governing the low-temperature magnetic behavior of spin-1 kagome systems.

cond-mat.str-el↗

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

$J_{\rm{eff}}$ = 1/2 Hyperoctagon Lattice in Cobalt Oxalate Metal-Organic-Framework

We report the magnetic properties of a cobalt oxalate metal-organic-framework featuring the hyperoctagon lattice. Our thermodynamic measurements reveal the $J_{\rm{eff}}$ = 1/2 state of the high-spin Co$^{2+}$ (3$\textit{d}^{7}$) ion and the two successive magnetic transitions at zero field with two-stage entropy release. $^{13}$C-NMR measurements reveal the absence of an internal magnetic field in the intermediate temperature phase. Multiple field-induced phases are observed before full saturation at around 40 T. We argue the unique cobalt oxalate network gives rise to the Kitaev interaction and/or a bond frustration effect, providing an unconventional platform for frustrated magnetism on the hyperoctagon lattice.

cond-mat.str-el↗

arXiv2023

Distinctive Doping Dependence of Upper Critical Field in Iron-Based Superconductor LaFeAsO$_{1-x}$H$_{x}$

High magnetic fields up to 105 T have been utilized in deriving the upper critical field $B_{\rm c2}$ of LaFeAsO$_{1-x}$H$_x$ throughout whole temperatures below $T_{\rm c}$. Resistivity measurements demonstrate that $B_{\rm c2}$ behaves differently in samples with $x = 0.12$ (SC1) from those with 0.32 (SC2). In SC1, the two-band model assuming the $s$-wave pairing gives a good fitting with repulsive intraband interaction and dominant interband coupling. In SC2, we have to assume attractive intraband interaction with weak interband coupling, which in fact suggests a non-$s$-wave pairing in view of the strong Coulomb repulsion. These results support the possibility that SC1 and SC2 have different pairing symmetries.

cond-mat.supr-con↗

arXiv2020

Crystal structure built from a GeO$_6$-GeO$_5$ polyhedra network with high thermal stability: $β$-SrGe$_2$O$_5$

By tackling the challenge of extending transparent oxide semiconductors to Ge based oxides, we have found a not-yet-reported crystal structure, named $β$-SrGe$_2$O$_5$, which is composed of edge-sharing GeO$_6$ octahedra interconnected by GeO$_5$ bipyramids. Single crystals were successfully grown by the high-pressure flux method. $β$-SrGe$_2$O$_5$ has a band gap of 5.2 eV and a dispersive conduction band with an effective mass as small as 0.34 times the electron rest mass, which originates from the edge-sharing GeO$_6$ octahedra network. Although known compounds with octahedral GeO$_6$ coordination are commonly unstable at atmospheric pressure and elevated temperatures, $β$-SrGe$_2$O$_5$ exhibits thermal stability up to 700 $^\circ$C.

cond-mat.mtrl-sci↗

arXiv2019

Direct coupling of ferromagnetic moment and ferroelectric polarization in BiFeO$_3$

The spin-driven component of electric polarization in a single crystal of multiferroic BiFeO$_{3}$ was experimentally investigated in pulsed high magnetic fields up to 41 T. Sequential measurements of electric polarization for various magnetic field directions provide clear evidence of electric polarization normal to the hexagonal $c$ axis (${\bf P}_{\rm t}$) in not only the cycloidal phase, but also the field-induced canted antiferromagnetic phase. The direction of ${\bf P}_{\rm t}$ is directly coupled with the ferromagnetic moment in the canted antiferromagnetic phase, and thus controlled by changing the direction of the applied magnetic field. This magnetoelectric coupling is reasonably reproduced by the metal-ligand hybridization model.

cond-mat.str-el↗

arXiv2019

Quantum dynamics of hydrogen in iron-based superconductor LaFeAsO0.9D0.1 measured with inelastic neutron spectroscopy

Inelastic neutron scattering was performed for an iron-based superconductor LaFeAsO0.9D0.1, where most of D (deuterium) replaces oxygen, while a tiny amount goes into interstitial sites. By first-principle calculation, we characterize the interstitial sites for D (and for H slightly mixed) with four equivalent potential minima. Below the superconducting transition temperature Tc = 26 K,new excitations emerge in the range 5-15 meV, while they are absent in the reference system LaFeAsO0.9F0.1. The strong excitations at 14.5 meV and 11.1 meV broaden rapidly around 15 K and 20 K, respectively, where each energy becomes comparable to twice of the superconducting gap. The strong excitations are ascribed to a quantum rattling, or a band motion of hydrogen, which arises only if the number of potential minima is larger than two.

cond-mat.supr-con↗

arXiv2019

Crystal structure and the magnetic properties of the 5d transition metal oxide AOsO4 (A = K, Rb, Cs)

We synthesized the 5d1-transition metal oxides AOsO4 (A = K, Rb, Cs) by solid-state reaction, and performed structure determination and magnetic and heat capacity measurements. It was found that they crystallize in a scheelite (A = K and Rb) or a quasi-scheelite structure (A = Cs) comprising of distorted diamond lattices of septivalent Os (d1) ions tetrahedrally coordinated by four oxide ions without local inversion symmetry; hence an antisymmetric spin-orbit coupling is expected in the crystals. The K and Rb compounds have Weiss temperatures of theta = -66 and -18 K, effective magnetic moments of mu_eff = 1.44 and 1.45 mu_B/Os, and antiferromagnetic transition temperatures of T_N = 36.9 and 21.0 K, respectively. In contrast, the Cs compound has theta = 12 K and mu_eff = 0.8 mu_B/Os without magnetic transition above 2 K, instead exhibiting a first-order structural transition at T_s = 152.5 K. The decline of the Os moment from 1.73 mu_B/Os for the simple d1 spin, particularly for Cs, is likely to originate from the antiparallel orbital moment, although the spin-orbit coupling is generally quenched in the low-lying e orbitals.

cond-mat.str-el↗

arXiv2018

High-pressure Raman study on the superconducting pyrochlore oxide Cd2Re2O7

The superconducting pyrochlore oxide Cd2Re2O7 (Tc = 1 K), which is now considered as a candidate of the spin-orbit-coupled metal, shows an inversion-symmetry-breaking structural transition at Ts1 = 200 K. Ts1 decreases with increasing pressure and disappears at around Pc = 4:2GPa, where at least four high-pressure phases with tiny structural distortions are suggested by means of powder X-ray diffraction [Yamaura PRB 2017]. We have carried out Raman scattering experiments to investigate changes in the crystal symmetry under high pressures up to 4.8 GPa. A structural transition at 1.9-3.0 GPa and the recovery of inversion symmetry above Pc are observed at 12 K.

cond-mat.str-el↗

arXiv2017

Pyrochlore Oxide Superconductor Cd2Re2O7 Revisited

The superconducting pyrochlore oxide Cd2Re2O7 is revisited with a particular emphasis on the sample-quality issue. The compound has drawn attention as the only superconductor (Tc = 1.0 K) that has been found in the family of α-pyrochlore oxides since its discovery in 2001. Moreover, it exhibits two characteristic structural transitions from the cubic pyrochlore structure, with the inversion symmetry broken at the first one at 200 K. Recently, it has attracted increasing attention as a candidate spin-orbit coupled metal (SOCM), in which specific Fermi liquid instability is expected to lead to an odd-parity order with spontaneous inversion-symmetry breaking [L. Fu, Phys. Rev. Lett. 115, 026401 (2015)] and parity-mixing superconductivity [V. Kozii and L. Fu: Phys. Rev. Lett. 115 (2015) 207002; Y. Wang et al., Phys. Rev. B 93 (2016) 134512]. We review our previous experimental results in comparison with those of other groups in the light of the theoretical prediction of the SOCM, which we consider meaningful and helpful for future progress in understanding this unique compound.

cond-mat.supr-con↗

arXiv2017

Successive spatial symmetry breaking under high pressure in the spin-orbit-coupled metal Cd2Re2O7

The 5d-transition metal pyrochlore oxide Cd2Re2O7, which was recently suggested to be a prototype of the spin-orbit-coupled metal [Phys. Rev. Lett. 115, 026401 (2015)], exhibits an inversion-symmetry breaking (ISB) transition at 200 K and a subsequent superconductivity below 1 K at ambient pressure. We study the crystal structure at high pressures up to 5 GPa by means of synchrotron X-ray powder diffraction. A rich structural phase diagram is obtained, which contains at least seven phases and is almost consistent with the electronic phase diagram determined by previous resistivity measurements. Interestingly, the ISB transition vanishes at ~4 GPa, where the enhancement of the upper critical field was observed in resistivity. Moreover, it is shown that the point groups at 8 K, probably kept in the superconducting phases, sequentially transform into piezoelectric, ferroelectric, and centrosymmetric structures on the application of pressure.

cond-mat.str-el↗