arXiv subjects

Hikaru Takeda

Publications and source records attributed to Hikaru Takeda.

16 recordsLinked to original sources

arXiv2026

Signatures of field-induced multi-color kagome spin liquids in the dipole-octupole pyrochlore $\mathrm{Ce_2Hf_2O_7}$

We report low-temperature magnetization and magnetostriction measurements on the dipole-octupole pyrochlore $\mathrm{Ce_2Hf_2O_7}$, revealing an unconventional field response for $\mathbf B\parallel[111]$. The magnetization shows no kagome-ice plateau; instead it evolves continuously and exhibits two rapid changes in slope near 0.35~T and 1.2~T, accompanied by magnetostriction features at the same field scales. Classical Monte Carlo simulations, exact diagonalization, and ground-state analysis of the Hamiltonian show that a representative QSI-compatible parameter set captures the data. For this parameter set, the lower-field anomaly marks a closely spaced transition sequence from a two-color kagome spin liquid (KSL) through a narrow three-color KSL into a mixed two-/three-color KSL, while the upper-field anomaly marks the transition from the mixed KSL into a nearly polarized state. These results identify $\mathrm{Ce_2Hf_2O_7}$, and dipole-octupole pyrochlore magnets more broadly, as promising platforms for exotic KSLs beyond conventional spin ice.

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

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

Observation of Anomalous Thermal Hall Effect in a Kagome Superconductor

Broken time-reversal symmetry (TRS) in superconductors can induce not only spontaneous magnetization by the finite angular momentum of Cooper pairs, but also anomalous thermal Hall effects (ATHEs), whose detection has been extremely challenging. Here we report the successful observation of an ATHE developing below the superconducting transition temperature at zero magnetic field in the kagome-lattice superconductor CsV3Sb5. This finding is verified by the absence of a signal in a conventional type-II superconductor using the same setup and by ruling out the trapped-vortex effects through micro-Hall array measurements. Remarkably, both the temperature dependence and the magnitude of the observed anomalous thermal Hall conductivity are quite different from those expected for the quantized thermal edge current of an intrinsic ATHE, but consistent with extrinsic impurity-induced ATHEs in chiral superconductivity. Our study of ATHE offers an alternative approach to probe TRS breaking in the superconducting states.

cond-mat.supr-con↗

arXiv2025

Huge anisotropic magneto-thermal switching in high-purity polycrystalline compensated metals

Magneto-thermal transport is a promising physical property for thermal management applications. Magneto-thermal switching enables active control of heat flows, and a high switching ratio is desirable for improving performance. Here, we report on the observation of a huge magneto-thermal switching (MTS) effect in high-purity (5N) Pb polycrystalline wires, where magnetic fields perpendicular to the heat current direction are applied at low temperatures. At T = 3 K and B = 0.1 T, the measured thermal conductivity (\k{appa}) of the Pb wire is about 2500 W m-1 K-1 but is reduced to ~150 and ~5 W m-1 K-1 at B = 1 and 9 T, respectively. This strong suppression is attributed to magnetoresistance in compensated metals. Although the huge magnetoresistance has been studied in single crystals with field along the selected orbitals, our results demonstrate that a huge MTS can similarly be realized even in flexible polycrystalline wires. This finding highlights the practical potential of magneto-thermal control in low-temperature thermal management, including applications in space environments where temperatures are around 3 K.

cond-mat.mtrl-sci↗

arXiv2024

Magnetic structure of polar magnet GaV$_4$Se$_8$ with Néel-type skyrmion lattice probed by $^{51}$V NMR

We report the magnetization and the $^{51}$V NMR measurements in the polar magnet GaV$_4$Se$_8$ in which a magnetic skyrmion lattice appears in the structural domain with the polar axis parallel to the magnetic field. Although we successfully separate the $^{51}$V NMR signals in the domain from those in the other structural domains, only the high-frequency region of NMR spectrum is observed due to a significant increase of the spin-echo decay rate in the low-frequency region. In $B_{\rm{ext}}\parallel[111]$, we find the NMR signals from the supermagnetized cycloidal state in the parallel domains as well as from the conical state in domains where the polar axis is tilted from the magnetic field. No NMR signal from the skyrmion lattice state is observed, suggesting a significant increase of the decay rate by additional low-energy excitations caused by dynamics of the skyrmions. In $B_{\rm{ext}}\parallel[001]$, where all the structural domains are magnetically equivalent, multiple NMR peaks converge into one peak at the saturation magnetic field. This field dependence is explained by the closing of magnetic cones as approaching the forced-ferromagnetic state.

cond-mat.str-el↗

arXiv2024

Incommensurate magnetic order in an axion insulator candidate EuIn$_2$As$_2$ investigated by NMR measurement

Magnetic topological insulators exhibit unique electronic states due to the interplay between the electronic topology and the spin structure. The antiferromagnetic metal $\rm{EuIn_2As_2}$ is a prominent candidate material in which exotic topological phases, including an axion insulating state, are theoretically predicted depending on the magnetic structure of the $\rm{Eu}^{2+}$ moments. Here, we report experimental results of the nuclear magnetic resonance (NMR) measurements of all the nuclei in $\rm{EuIn_2As_2}$ to investigate the coupling between the magnetic moments in the Eu ions and the conduction electrons in $\rm{In_2As_2}$ layers and the magnetic structure. The $^{75}\rm{As}$ and $^{115}\rm{In}$ NMR spectra observed at zero external magnetic field reveal the appearance of internal fields of $4.9$ and $3.6\ \rm{T}$ respectively at the lowest temperature, suggesting a strong coupling between the conduction electrons in the $\rm{In_2As_2}$ layer and the ordered magnetic moments in the Eu ions. The $^{75}\rm{As}$ NMR spectra under in-plane external magnetic fields show broad distributions of the internal fields produced by an incommensurate fan-like spin structure which turns into a forced ferromagnetic state above $0.7\ \rm{T}$. We propose a spin reorientation process that an incommensurate helical state at zero external magnetic field quickly changes into a fan state by applying a slight magnetic field.

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

Emergent SU(3) magnons and thermal Hall effect in the antiferromagnetic skyrmion lattice

Complexity of quantum phases of matter is often understood by the underlying gauge structures, as was recognized by the $\mathbb{Z}_2$ and U(1) gauge theory description of spin liquid in frustrated magnets. Anomalous Hall effect of conducting electrons can intrisically arise from U(1) gauges expressing the spatial modulation of ferromagnetic moments or from SU(2) gauges representing the spin-orbit coupling effect. Similarly, in insulating ferro and antiferromagnets, the magnon excitations can contribute to anomalous transports by feeling the U(1) and SU(2) gauges arising from the features of ordered moments or interactions. In this work, we report the emergent higher rank SU(3) gauge structure in the magnon transport based on the thermal conductivity measurements of MnSc$_2$S$_4$ in an applied field up to 14\,T. The thermal Hall coefficient takes a substantial value when the material enters a three-sublattice antiferromagnetic skyrmion phase, which is confirmed by the large-scale spin wave theory. The excited magnons are dressed with SU(3) gauge field, which is a mixture of three species of U(1) gauge fields originating from the slowly varying magnetic moments on these sublattices.

cond-mat.str-el↗

arXiv2022

Planar Thermal Hall Effects in Kitaev Spin Liquid Candidate Na2Co2TeO6

We investigate both the longitudinal thermal conductivity ($κ_{xx}$) and the planar thermal Hall conductivity ($κ_{xy}$) in the Kitaev spin liquid candidate of Co-based honeycomb antiferromagnet Na$_2$Co$_2$TeO$_6$ in a magnetic field ($B$) applied along the $a$ and $a^*$ axes. A finite $κ_{xy}$ is resolved for both field directions in the antiferromagnetic (AFM) phase below the Néel temperature of 27 K. The temperature dependence of $κ_{xy}/T$ shows the emergence of topological bosonic excitations. In addition, the field dependence of $κ_{xy}$ shows sign reversals at the critical fields in the AFM phase, suggesting the changes in the Chern number distribution of the topological magnons. Remarkably, a finite $κ_{xy}$ is observed in $B \parallel a^*$ between the first-order transition field in the AFM phase and the saturation field, which is prohibited in a disordered state by the two-fold rotation symmetry around the $a^*$ axis of the honeycomb lattice, showing the presence of a magnetically ordered state that breaks the two-fold rotation symmetry. Our results demonstrate the presence of topological magnons in this compound in the whole field range below the saturation field.

cond-mat.str-el↗

arXiv2022

Topological Thermal Hall Effect of Magnons in Magnetic Skyrmion Lattice

Topological transports of fermions are governed by the Chern numbers of the energy bands lying below the Fermi energy. For bosons, e.g. phonons and magnons in a crystal, topological transport is dominated by the Chern number of the lowest energy band when the band gap is comparable to the thermal energy. Here, we demonstrate the presence of topological transport by bosonic magnons in a lattice of magnetic skyrmions - topological defects formed by a vortex-like texture of spins. We find a distinct thermal Hall signal in the magnetic skyrmion phase of an insulating polar magnet GaV4Se8, identified as the topological thermal Hall effect of magnons governed by the Chern number of the lowest energy band of the magnons in a triangular lattice of magnetic skyrmions. Our findings lay a foundation for studying topological phenomena of other bosonic excitations through thermal Hall probe.

cond-mat.str-el↗

arXiv2021

Pressure-induced phase transition in the $J_1$-$J_2$ square lattice antiferromagnet RbMoOPO$_4$Cl

We report results of magnetization and $^{31}$P NMR measurements under high pressure up to 6.4~GPa on RbMoOPO$_4$Cl, which is a frustrated square-lattice antiferromagnet with competing nearest-neighbor and next-nearest-neighbor interactions. Anomalies in the pressure dependences of the NMR shift and the transferred hyperfine coupling constants indicate a structural phase transition at 2.6~GPa, which is likely to break mirror symmetry and triggers significant change of the exchange interactions. In fact, the NMR spectra in magnetically ordered states reveal a change from the columnar antiferromagnetic (CAF) order below 3.3~GPa to the Néel antiferromagnetic (NAF) order above 3.9~GPa. The spin lattice relaxation rate $1/T_1$ also indicates a change of dominant magnetic fluctuations from CAF-type to NAF-type with pressure. Although the NMR spectra in the intermediate pressure region between 3.3 and 3.9 GPa show coexistence of the CAF and NAF phases, a certain component of $1/T_1$ shows paramagnetic behavior with persistent spin fluctuations, leaving possibility for a quantum disordered phase. The easy-plane anisotropy of spin fluctuations with unusual nonmonotonic temperature dependence at ambient pressure gets reversed to the Ising anisotropy at high pressures. This unexpected anisotropic behavior for a spin 1/2 system may be ascribed to the strong spin-orbit coupling of Mo-4$d$ electrons.

cond-mat.str-el↗

arXiv2020

Dynamic Spin Fluctuations in the Frustrated A-site Spinel CuAl2O4

We performed nuclear magnetic resonance (NMR) and muon spin relaxation (μSR) experiments to identify the magnetic ground state of the frustrated quantum A-site spinel, CuAl2O4. Our results verify that the ground state does not exhibit a long-range magnetic ordering, but a glass-like transition manifests at T*=2.3 K. However, the Gaussian shape and the weak longitudinal field dependence of μSR spectra below T* show that the ground state has dynamic spin fluctuations, distinct from those of conventional spin-glasses.

cond-mat.str-el↗

arXiv2019

Field-Induced Switching of Ferro-Quadrupole Order Parameter in PrTi2Al20

We report magnetic-field-induced first-order phase transitions in the ferro-quadrupole (FQ) ordered state of PrTi2Al20, in which non-Kramers Pr3+ ions with two 4f electrons have a non-magnetic Gamma3 doublet ground state in the cubic Td crystalline electric field. For magnetic fields along [111], 27Al-NMR and magnetization experiments reveal Qz \propto 3z2-r2 type FQ order below 2 K independent of field strength. Magnetic fields along [001] or [110], however, induce discontinuous switching of order parameters within the two dimensional space spanned by Qz and Qx \propto x2-y2 at small field values less than a few tesla. A symmetry-based theoretical analysis shows that the transitions can be caused by competition between the magnetic Zeeman interaction and anisotropy in the quadrupole-quadrupole interactions, if the latter dominates over the former in low fields and vice versa in high fields. Furthermore, striking violation of proportionality between NMR Knight shift and magnetic susceptibility is observed in the symmetry-broken FQ phases, indicating significant influence of FQ order on the hybridization between conduction and f electrons, which in turn mediates the RKKY-type quadrupole interaction causing the FQ order. This feedback effect may be a specific feature of quadrupole orders not commonly observed in magnetic phase transitions and play a key role for inducing the discontinuous transitions.

cond-mat.str-el↗

arXiv2016

NMR Observation of Ferro-quadrupole Order in PrTi2Al20

We report the results of 27Al-NMR measurements on a single crystal of PrTi2Al20. This compound shows a phase transition near 2 K associated with the quadrupole degree of freedom of Pr ions, for which the ground state of the crystalline electric field is a nonmagnetic doublet. When a magnetic field is applied along the <111> direction, the NMR lines split upon entering the low-temperature phase, indicating the breaking of the three fold rotation symmetry due to the field-induced magnetic dipole perpendicular to the field. This provides microscopic evidence for a ferro order of the O20 quadrupole. Although a first-order transition is expected theoretically, the line splitting evolves continuously within the experimental resolution.

cond-mat.str-el↗

arXiv2012

An orbital-selective spin liquid in a frustrated heavy fermion spinel LiV$_2$O$_4$

The pronounced enhancement of the effective mass is the primary phenomenon associated with strongly correlated electrons. In the presence of local moments, the large effective mass is thought to arise from Kondo coupling, the interaction between itinerant and localised electrons. However, in d electron systems, the origin is not clear because of the competing Hund's rule coupling. Here we experimentally address the microscopic origin for the heaviest d fermion in a vanadium spinel LiV2O4 having geometrical frustration. Utilising orbital-selective 51V NMR, we elucidate the orbital-dependent local moment that exhibits no long-range magnetic order despite persistent antiferromagnetic correlations. A frustrated spin liquid, Hund-coupled to itinerant electrons, has a crucial role in forming heavy fermions with large residual entropy. Our method is important for the microscopic observation of the orbital-selective localisation in a wide range of materials including iron pnictides, cobaltates, manganites, and ruthnates.

cond-mat.str-el↗