arXiv subjects

Motoi Kimata

Publications and source records attributed to Motoi Kimata.

At least 19 recordsLinked to original sources

arXiv2026

Electron-Hole Scattering Dichotomy and Anisotropic Warping in Quasi-Two-Dimensional Fermi Surfaces of UTe2

We present a combined experimental and theoretical study of the detailed Fermi-surface (FS) geometry of UTe2, a heavy-fermion superconductor that has recently attracted considerable attention as a promising candidate for spin-triplet pairing. Using angle-dependent magnetoresistance oscillations, a bulk- and low-energy-sensitive transport probe for quasi-two-dimensional (Q2D) electronic structures, we directly determine the in-plane FS geometry. We found that the Q2D FS exhibits a rectangular cross-sectional shape with strongly anisotropic warping, originating from the hybridization of two orthogonal quasi-one-dimensional bands. Through a quantitative comparison between experiment and theoretical calculations, we further reveal a large electron-hole scattering dichotomy: the quasiparticle lifetime on the electron FS is substantially shorter than that on the hole FS. This dichotomy is naturally explained by anisotropic, low-dimensional antiferromagnetic fluctuations, which selectively enhance scattering on the electron FS. This suggests a dominant role of the electron pockets for the emergence of superconductivity. Our results clarify a direct relation between FS geometry, magnetic fluctuations, and momentum-dependent quasiparticle lifetimes, and thus providing a crucial basis for the microscopic understanding of pairing mechanism, and impose stringent constraints on the gap symmetry of spin-triplet superconductivity in UTe2.

cond-mat.supr-con↗

arXiv2026

Hall effect on nontrivial quadrupole order in quasi-kagome compound URhSn

This study focuses on the transport properties of the quasi-kagome compound URhSn, which exhibits successive phase transitions at TC =16 K (ferromagnetic phase) and TO =54 K (intermediate phase). A large anomalous Hall component is present along the easy-magnetization axis (H|| [0001]), and the Hall resistivity shows very complex temperature- and field-dependence, with a sign reversal at low temperatures. The Hall resistivity exhibits a nonlinear and unusual field-dependence. Interestingly, there exists an unusual Hall component that is not proportional to the magnetic susceptibility for H || [0001] in both the intermediate and ferromagnetic states. These results reveal unconventional transport properties of URhSn, providing important insights into nontrivial multipolar phases in 5f- electron systems.

cond-mat.str-el↗

arXiv2026

Intimate relationship between spin configuration in the triplet pair and superconductivity in UTe$_2$

Spin-triplet superconductivity is an intriguing quantum coherent state with both spin and orbital degrees of freedom, which holds significant potential for future applications in quantum technology. However, how the spin of the triplet pairs responds to an external magnetic field remains poorly understood. This is mainly due to the absence of suitable spin-triplet superconductors. Here, we report results of Knight-shift and ac-susceptibility measurements on UTe$_2$. We demonstrate that the spin susceptibility, which slightly decreases compared to the normal-state value below the superconducting (SC) transition temperature $T_{\rm c}$, is rapidly restored and nearly recovers to the normal-state values around 5 T, well below the SC upper critical field $H_{c2}$ when the magnetic field is applied along the $c$ axis ($H \parallel c$). In addition, we found that $H_{\rm c2}$ of superconductivity becomes larger when the SC spin aligns with the magnetic field. By considering the results on $H \parallel b$, our results suggest the presence of a close relationship between the spin configuration of the triplet pair and $H_{\rm c2}$, as well as the anisotropic pinning interaction acting on the triplet pairs. These phenomena, which have never been observed in spin-singlet superconductors, represent characteristic features unique to spin-triplet superconductors. We discuss the similarities between superconductivity in UTe$_2$ and superfluid $^3$He, focusing on their spin-triplet pairing states.

cond-mat.supr-con↗

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

Large Spontaneous Nonreciprocal Charge Transport in a Zero-Magnetization Antiferromagnet

Spontaneous breaking of time-reversal and spatial-inversion symmetries in solids triggers diverse intriguing phenomena. Although these phenomena have been extensively studied in insulators, similar investigations for metals remain limited. Herein, we report the observation and properties of spontaneous (i.e., zero-magnetic field) nonreciprocal charge transport in the zigzag intermetallic compound NdRu2Al10. This effect is attributed to the antiferromagnetic (AF) order, which can be interpreted as a magnetic toroidal dipole order. Our results reveal an excessively large nonreciprocal coefficient for this material, attributed to the strong effective magnetic field generated through c-f exchange interactions. The results also suggest that the nonreciprocal response of this material depends on the spin configurations of the AF domains. Overall, our findings are distinct from those previously reported for field-induced nonreciprocal charge transport and contribute to a comprehensive understanding of cross-correlations in symmetry-broken metals.

cond-mat.str-el↗

arXiv2025

Magneto-cubic and magneto-linear dependence observed in an in-plane anomalous Hall magnet

The Hall effect, particularly that arising from in-plane magnetic field, has recently emerged as a sensitive probe of quantum geometric properties in solids. Especially in trigonal systems, in-plane anomalous Hall effect (AHE) can be explicitly induced by nontrivial off-diagonal coupling between the magnetic field and the Hall vector on the principal plane. Here we elucidate multipolar dependence of the off-diagonal coupling in the in-plane AHE, by systematically measuring on the (001) principal plane of trigonal antiferromagnet EuCd2Sb2 thin films for each magnetic phase. Around zero field, magneto-cubic dependence of anomalous Hall resistivity is clearly observed not only in the paramagnetic phase but also even in the antiferromagnetic phase. An off-diagonal component of the octupolar tensor also exhibits unconventional decay above the magnetic ordering temperature, roughly depending on the inverse temperature to the third power. In the forced ferromagnetic phase, on the other hand, magneto-linear dependence dominantly appears and notably persists up to very high fields. Our findings clarify key aspects of the off-diagonal coupling in the in-plane AHE, paving the way for its future investigations and potential applications beyond conventional expectations about the Hall effect.

cond-mat.str-el↗

arXiv2025

Slow magnetic quantum oscillations in the c-axis magnetoresistance of UTe$_2$

Details of the electronic band structure in unconventional superconductors are key to the understanding of their fundamental ground state. The potential spin-triplet superconductor UTe$_2$, with $T_\mathrm{c}\approx 2.1\,$K, has attracted attention recently. Its main Fermi surface consists of weakly corrugated, two-dimensional Fermi-surface cylinders that run along the crystallographic $c$ axis. In addition, there is evidence for the presence of an additional small three-dimensional band. This has been discussed controversially as it may be essential for the realization of superconductivity in UTe$_2$. Here, we investigate the angle-resolved magnetoresistance and Hall effect in bulk crystalline samples with current along the $c$ axis in fields up to $60\,$T. We observe low-frequency magnetic quantum oscillations with light effective masses that are most pronounced for magnetic field applied along the $a$ axis. Two distinct frequencies indicate two separate changes in the Fermi-surface topology, likely connected with Lifshitz transitions. We discuss the origin of these oscillations in terms of magnetic breakdown, quantum interference, and other potential mechanisms.

cond-mat.str-el↗

arXiv2025

Electronic Structure of Kramers Nodal-Line Semimetal YAuGe and Anomalous Hall Effect Induced by Magnetic Rare-Earth Substitution

Nodal-line semimetals are a class of topological materials hosting one dimensional lines of band degeneracy. Kramers nodal-line (KNL) metals/semimetals have recently been theoretically recognized as a class of topological states inherent to all non-centrosymmetric achiral crystal lattices. We investigate the electronic structure of candidate KNL semimetal YAuGe by angle-resolved photoemission spectroscopy (ARPES) and quantum oscillations as well as by density functional theory (DFT) calculations. DFT has revealed that YAuGe hosts KNLs on the G-A-L-M plane of the Brillouin zone, that are protected by the time reversal and mirror-inversion symmetries. Through ARPES and quantum oscillations we identify signatures of hole bands enclosing the G point, and the observed splitting of quantum oscillation frequency with angle is attributed to spin-orbit-coupling-induced band splitting away from the KNLs. Furthermore, we show that the degeneracy of the nodal lines along the G-A line is lifted by the time-reversal-symmetry breaking when the Y is substituted by magnetic R ions (R = rare earth). This becomes a source of Berry curvature and contributes to the anomalous Hall effect in magnetic RAuGe. These findings establish RAuGe as a new class of KNL semimetals offering significant potential for engineering of anomalous magnetotransport properties via magnetic rare-earth substitution.

cond-mat.mtrl-sci↗

arXiv2024

Giant Hall effect in a highly conductive frustrated magnet GdCu$_2$

The Hall effect is one of the most fundamental but elusive phenomena in condensed matter physics due to the rich variety of underlying mechanisms. Here we report an exceptionally large Hall effect in a frustrated magnet GdCu$_2$ with high conductivity. The Hall conductivity at the base temperature is as high as 4 x 10$^4$ $Ω^{-1}$cm$^{-1}$ and shows abrupt sign changes under magnetic fields. Remarkably, the giant Hall effect is rapidly suppressed as the longitudinal conductivity is lowered upon increasing temperature or introducing tiny amount of quenched disorder. Our systematic transport measurements together with neutron scattering measurements and ab initio band calculations indicate that the unusual Hall effect can be understood in terms of spin-splitting induced emergence/disappearance of Fermi pockets as well as skew scattering from spin-chiral cluster fluctuations in a field-polarized state. The present study demonstrates complex interplay among magnetization, spin-dependent electronic structure, and spin fluctuations in producing the giant Hall effect in highly conductive frustrated magnets.

cond-mat.str-el↗

arXiv2023

Field-induced compensation of magnetic exchange as the possible origin of reentrant superconductivity in UTe$_2$

The potential spin-triplet heavy-fermion superconductor UTe$_2$ exhibits signatures of multiple distinct superconducting phases. For field aligned along the $b$ axis, a metamagnetic transition occurs at $μ_0 H_\mathrm{m}\approx35\,$T. It is associated with magnetic fluctuations that may be beneficial for the field-reinforced superconductivity surviving up to $H_\mathrm{m}$. Once the field is tilted away from the $b$ towards the $c$ axis, a reentrant superconducting phase emerges just above $H_\mathrm{m}$. In order to better understand this remarkably field-resistant superconducting phase, we conducted magnetic-torque and magnetotransport measurements in pulsed magnetic fields. We determine the record-breaking upper critical field of $μ_0 H_\mathrm{c2}\approx 73\,$T and its evolution with angle. Furthermore, the normal-state Hall effect experiences a drastic suppression indicative of a reduced band polarization above $H_\mathrm{m}$ in the angular range around $30^\circ$ caused by a partial compensation between the applied field and an exchange field. This promotes the Jaccarino-Peter effect as a likely mechanism for the reentrant superconductivity above $H_\mathrm{m}$.

cond-mat.supr-con↗

arXiv2022

Magnetic-field-induced Anderson localization in orbital selective antiferromagnet BaMn$_2$Bi$_2$

We report a metal-insulator transition (MIT) in the half-filled multiorbital antiferromagnet (AF) BaMn$_2$Bi$_2$ that is tunable by a magnetic field perpendicular to the AF sublattices. Instead of an Anderson-Mott mechanism usually expected in strongly correlated systems, we find by scaling analyses that the MIT is driven by an Anderson localization. Electrical and thermoelectrical transport measurements in combination with electronic band calculations reveal a strong orbital-dependent correlation effect, where both weakly and strongly correlated $3d$-derived bands coexist with decoupled charge excitations. Weakly correlated holelike carriers in the $d_{xy}$-derived band dominate the transport properties and exhibit the Anderson localization, whereas other $3d$ bands show clear Mott-like behaviors with their spins ordered into AF sublattices. The tuning role played by the perpendicular magnetic field supports a strong spin-spin coupling between itinerant holelike carriers and the AF fluctuations, which is in sharp contrast to their weak charge coupling.

cond-mat.str-el↗

arXiv2021

Electron transport in folded bilayer-bilayer graphene/hexagonal boron nitride superlattices under high magnetic fields

Employing graphene as a template, we fabricate moiré superlattices by stacking bilayer or folded bilayer-bilayer graphene (BLG or fBBLG) and hexagonal boron nitride (hBN), i.e., hBN/BLG/hBN or hBN/fBBLG/hBN stacks, with a small twist angle between the graphene and one of the two hBN layers. Because of the modulation due to the hBN, higher-generation Dirac points can emerge with a narrow bandwidth and van Hove singularities. In the moiré superlattice devices, we can therefore access the higher-generation Dirac points by in-situ gate tuning. This study is based on our previous paper (Appl. Phys. Express 13, 035003 (2020)). Here we show more extended data by applying high magnetic fields up to ~24 T. We also comment on the temperature dependence of the resistivity and magnetoresistance with reference to the 'plain' BLG data for a comparative study.

cond-mat.mes-hall↗

arXiv2021

Field-Induced Superconductivity near the Superconducting Critical Pressure in UTe2

We report the magnetoresistance in the novel spin-triplet superconductor UTe2 under pressure close to the critical pressure Pc, where the superconducting phase terminates, for field along the three a, b and c-axes in the orthorhombic structure. The superconducting phase for H // a-axis just below Pc shows a field-reentrant behavior due to the competition with the emergence of magnetic order at low fields. The upper critical field Hc2 for H // c-axis shows a quasi-vertical increase in the H-T phase diagram just below Pc, indicating that superconductivity is reinforced by the strong fluctuations which persist even at high fields above 20T. Increasing pressure leads to the disappearance of superconductivity at zero field with the emergence of magnetic order. Surprisingly, field-induced superconductivity is observed at high fields, where a spin-polarized state is realized due to the suppression of the magnetic ordered phases; the spin-polarized state is favorable for superconductivity, whereas the magnetic ordered phase at low field seems to be unfavorable. The huge Hc2 in the spin-polarized state seems to imply a spin-triplet state. Contrary to the a- and c-axes, no field-reinforcement of superconductivity occurs for magnetic field along the b-axis. We compare the results with the field-reentrant superconductivity above the metamagnetic field, Hm for the field direction tilted by about 30 deg. from b to c-axis at ambient pressure as well as the field-reentrant (-reinforced) superconductivity in ferromagnetic superconductors, URhGe and UCoGe.

cond-mat.str-el↗

arXiv2021

Large negative magnetoresistance in BaMn$_2$Bi$_2$ antiferromagnet

A very large negative magnetoresistance (LNMR) is observed in the insulating regime of the antiferromagnet BaMn$_2$Bi$_2$ when a magnetic field is applied perpendicular to the direction of the sublattice magnetization. High perpendicular magnetic field eventually suppresses the insulating behavior and allows BaMn$_2$Bi$_2$ to re-enter a metallic state. This effect is seemingly unrelated to any field induced magnetic phase transition, as measurements of magnetic susceptibility and specific heat did not find any anomaly as a function of magnetic fields at temperatures above $2\,\mathrm{K}$. The LNMR appears in both current-in-plane and current-out-of-plane settings, and Hall effects suggest that its origin lies in an extreme sensitivity of conduction processes of holelike carriers to the infinitesimal field-induced canting of the sublattice magnetization. The LNMR-induced metallic state may thus be associated with the breaking of the antiferromagnetic parity-time symmetry by perpendicular magnetic fields and/or the intricate multi-orbital electronic structure of BaMn$_2$Bi$_2$.

cond-mat.str-el↗

arXiv2020

Dichotomy Between Orbital and Magnetic Nematic Instabilities in BaFe2S3

Nematic orders emerge nearly universally in iron-based superconductors, but elucidating their origins is challenging because of intimate couplings between orbital and magnetic fluctuations. The iron-based ladder material BaFe2S3, which superconducts under pressure, exhibits antiferromagnetic order below TN ~ 117K and a weak resistivity anomaly at T* ~ 180K, whose nature remains elusive. Here we report angle-resolved magnetoresistance (MR) and elastoresistance (ER) measurements in BaFe2S3, which reveal distinct changes at T*. We find that MR anisotropy and ER nematic response are both suppressed near T*, implying that an orbital order promoting isotropic electronic states is stabilized at T*. Such an isotropic state below T* competes with the antiferromagnetic order, which is evidenced by the nonmonotonic temperature dependence of nematic fluctuations. In contrast to the cooperative nematic orders in spin and orbital channels in iron pnictides, the present competing orders can provide a new platform to identify the separate roles of orbital and magnetic fluctuations.

cond-mat.supr-con↗

arXiv2020

Investigation of in-plane anisotropy of c-axis magnetoresistance for BiCh2-based layered superconductor NdO0.7F0.3BiS2

We have investigated the in-plane anisotropy of the c-axis magnetoresistance (MR) in both superconducting and normal states of single crystals of NdO0.7F0.3BiS2 under in-plane magnetic fields. In the superconducting states of NdO0.7F0.3BiS2, four-fold-symmetric in-plane anisotropy of the c-axis MR was observed below the superconducting transition temperature. Since the crystal structure of NdO0.7F0.3BiS2 is tetragonal, the rotational symmetry in the superconducting state is preserved in the present compound. This result is clearly different from the previous report observed in LaO0.5F0.5BiSSe single crystals, where the in-plane MR in the superconducting state shows two-fold symmetry. On the other hand, in the normal states of NdO0.7F0.3BiS2, two-fold symmetric MR with a small amplitude was observed. The possible origin of the two-fold-symmetric behavior was discussed with the presence of local structural disorder in the conducting plane of BiCh2-based compounds.

cond-mat.supr-con↗

arXiv2020

Two-fold symmetry of in-plane magnetoresistance anisotropy in the superconducting states of BiCh2-based LaO0.9F0.1BiSSe single crystal

Recently, two-fold symmetric in-plane anisotropy of the superconducting properties have been observed in a single crystal of BiCh2-based (Ch: S, Se) layered superconductor LaO0.5F0.5BiSSe having a tetragonal (four-fold-symmetric) in-plane structure; the phenomena are very similar to those observed in nematic superconductors. To explore the origin of the two-fold symmetric anisotropy in the BiCh2-based system, we have investigated the electron-doping dependence on the anisotropy by examining the in-plane anisotropy of the magnetoresistance in the superconducting states for a single crystal of LaO0.9F0.1BiSSe under high magnetic fields up to 15 T. We observed a two-fold symmetry of in-plane anisotropy of magnetoresistance for LaO0.9F0.1BiSSe. The results obtained for LaO0.9F0.1BiSSe are quite similar to those observed for LaO0.5F0.5BiSSe, which has a higher electron doping concentration than LaO0.9F0.1BiSSe. Our present finding suggests that the emergence of the in-plane symmetry breaking in the superconducting state is robust to the carrier concentration in the series of LaO1-xFxBiSSe.

cond-mat.supr-con↗

arXiv2020

Anisotropy of the Upper Critical Field in the Heavy-Fermion Superconductor UTe2 under Pressure

We studied the anisotropy of the superconducting upper critical field $H_{\rm c2}$ in the heavy-fermion superconductor UTe$_2$ under hydrostatic pressure by magnetoresistivity measurements. In agreement with previous experiments we confirm that superconductivity disappears near a critical pressure $p_{\rm c} \approx 1.5$~GPa, and a magnetically ordered state appears. The unusual $H_{\rm c2}(T)$ at low temperatures for $H \parallel a$ suggests that the multiple superconducting phases which appear under pressure have quite different $H_{\rm c2}$. For a field applied along the hard magnetization $b$ axis $H_{\rm c2} (0)$ is glued to the metamagnetic transition $H_{\rm m}$ which is suppressed near $p_{\rm c}$. The suppression of $H_{\rm m}$ with pressure follows the decrease of temperature $T_χ^{\rm max}$, at the maximum in the susceptibility along $b$. The strong reinforcement of $H_{\rm c2}$ at ambient pressure for $H \parallel b$ above 16~T is rapidly suppressed under pressure due to the increase of $T_{\rm sc}$ and the decrease of $H_{\rm m}$. The change in the hierarchy of the anisotropy of $H_{\rm c2}(0)$ on approaching $p_{\rm c}$ points out that the $c$ axis becomes the hard magnetization axis.

cond-mat.str-el↗