arXiv subjects

Hitoshi Sugawara

Publications and source records attributed to Hitoshi Sugawara.

At least 19 recordsLinked to original sources

arXiv2026

Large spontaneous Hall effect arising from collinear antiferromagnetism in Ce$_2$PtGe$_6$

The spontaneous Hall effect, corresponding to a zero-field anomalous Hall effect (AHE), is induced by symmetry breaking associated with ferromagnetism. Studies in recent years, however, have revealed that antiferromagnetic (AFM) states characterized by magnetic point groups that allow ferromagnetism can also break the relevant symmetries and induce AHE without a large net magnetization. Here, we report that the AFM system Ce$_2$PtGe$_6$ exhibits a pronounced spontaneous Hall effect. Single-crystal neutron scattering experiments demonstrate that Ce$_2$PtGe$_6$ exhibits a collinear AFM structure with a propagation vector $q=0$. The small net magnetization of $\sim 10^{-3}$ $\mu_B$/Ce indicates that the observed AHE arises from symmetry breaking inherent to its AFM structure. The anomalous Hall conductivity (AHC) reaches $300$ $\Omega^{-1}$cm$^{-1}$, which exceeds the intrinsic AHC of related compounds such as Ce$_2$CuGe$_6$ and Ce$_2$PdGe$_6$. This large AHC, most likely attributed to the large spin-orbit coupling of the Pt atoms, provides a platform for understanding the interplay between the Berry curvatures and localized $f$-moments with an AFM configuration.

cond-mat.str-el↗

arXiv2026

Spontaneous Anomalous Hall Effect at Room Temperature in Antiferromagnetic Material NbMnAs

Recent studies have shown that certain antiferromagnetic (AFM) materials with the same symmetry breaking as ferromagnets can generate sufficiently large ferromagnetic (FM) responses. Here, we report that the new AFM material NbMnAs exhibits a large anomalous Hall effect (AHE) at zero field and at room temperature, despite having only a small net magnetization. A polycrystalline sample of NbMnAs, likely close to stoichiometric composition, exhibited an AFM state with a small spontaneous magnetization of approximately $6 \times 10^{-3} \mu_{\rm B}$/Mn and the AHE below $T_{\rm N}=354\,{\rm K}$. In contrast, single crystals of NbMnAs obtained by a flux method exhibited a deficiency at the As site, {which resulted} in a decrease in $T_{\rm N}$ and an increase in spontaneous magnetization. Although improvement of the single-crystal growth is still required, our study reveals that NbMnAs is a novel material capable of exhibiting significant FM responses derived from antiferromagnetism at room temperature.

cond-mat.str-el↗

arXiv2025

Magnetic Field Dependence of the Spin Susceptibility on Conventional s-wave Superconductor LaRu$_4$P$_{12}$ Revealed by $^{31}$P-NMR and $^{139}$La-NMR

The magnetic field dependence of the spin part of Knight shift, which is proportional to the superconducting-state spin susceptibility, was investigated at two nuclear sites, $^{31}$P and $^{139}$La in a conventional s-wave superconductor LaRu$_4$P$_{12}$. After the analyses, we confirmed that the superconducting-state spin susceptibility is proportional to magnetic field, and connects to the normal-state spin susceptibility smoothly. This is a textbook example, when the superconductivity is broken with the orbital pair-breaking effect.

cond-mat.supr-con↗

arXiv2025

Successive Antiferromagnetic Transition in the Frustrated Compound CeMgIn

We report on the magnetic, transport, and thermal properties of the hexagonal ZrNiAl-type compound CeMgIn with Ce atoms forming a distorted kagome network. This compound exhibits successive antiferromagnetic transition at $T_\text{N1} =$ 2.1 K, $T_\text{N2} =$ 1.7 K, and possibly $T_\text{N3} =$ 1.3 K. The electrical resistivity exhibits a minimum at 11 K and a nonlogarithmic increase with decreasing temperature down to $T_\text{N2}$. We found that CeMgIn is the first ZrNiAl-type compound whose resistivity increase can be well explained by considering a model in which the electron-spin scattering is enhanced by the magnetic frustration and the Ruderman-Kittel-Kasuya-Yosida interaction. These results suggest that CeMgIn is a notable compound whose physical properties are strongly affected by geometrical frustration. Since the Sommerfeld coefficient is 97 mJ/mol K$^2$, CeMgIn is classified as a moderate heavy-fermion compound.

cond-mat.str-el↗

arXiv2024

Large spontaneous Hall effect with flexible domain control in an antiferromagnetic material TaMnP

Antiferromagnets without parity-time ($\mathcal{PT}$) symmetry offer novel perspectives in the field of functional magnetic materials. Among them, those with ferromagnetic-like responses are promising candidates for future applications such as antiferromagnetic (AF) memory; however, examples showing large effects are extremely limited. In this study, we show that the orthorhombic system TaMnP exhibits a large anomalous Hall conductivity (AHC) $\sim360-370$ $\Omega^{-1}$cm$^{-1}$ in spite of the small net magnetization $\sim10^{-2}$ $\mu_B$/Mn. Our neutron scattering experiment and the observation of the AH effect indicated that a magnetic structure of TaMnP was dominated by an AF component represented by $B_{3g}$ with the propagation vector $q=0$. Furthermore, we confirmed that the obtained AHC is among the largest observed in AF materials at zero fields. Additionally, our first-principles calculations revealed that the spin-orbit interaction originating in the nonmagnetic Ta-$5d$ electrons significantly contributes to enhancing Berry curvatures in the momentum space. We found that the magnetic fields along all the crystal axes triggered the AF domain switching, indicating the possibility of controlling the AF domain using the small net magnetization, which is symmetrically different.

cond-mat.str-el↗

arXiv2024

Large Anomalous Hall Conductivity Derived from an $f$-Electron Collinear Antiferromagnetic Structure

Appropriate symmetry breaking generates an anomalous Hall (AH) effect, even in antiferromagnetic (AFM) materials. Itinerant magnets with $d$ electrons are typical examples that show a significant response. By contrast, the process by which a response emerges from $f$-electron AFM structures remains unclear. In this study, we show that an AFM material, Ce$_2$CuGe$_6$, yields a large AH conductivity (AHC) of $550$ $Ω^{-1}$cm$^{-1}$, which exceeds the values previously reported in $d$-electron AFM materials. Observed features, including the scaling relation against electrical conductivity, suggest that this AH transport is induced cooperatively by both intrinsic and extrinsic mechanisms derived from the AFM structure.

cond-mat.str-el↗

arXiv2024

Intrinsic anomalous Hall effect arising from antiferromagnetic structure revealed by high-quality NbMnP

The large anomalous Hall effect (AHE) in antiferromagnetic (AF) materials arises from symmetry breaking equivalent to a ferromagnetic (FM) state. Consequently, this suggests that the observed AHE is induced by the intrinsic mechanism of the band structure effect, which in turn induces dissipationless transverse conductivity. Confirmation of impurity-insensitive anomalous Hall conductivity (AHC) is crucial to conclude this interpretation; however, experimental investigations in AF materials are limited by the lack of high quality systems. In this study, we show that the AF material NbMnP, which exhibits a large AHE, offers a high quality single crystal. Our findings clearly revealed that the large AHC and the tiny net magnetization of $\sim10^{-3} μ_{\mathrm{B}}$/Mn are inherent in this material, irrespective of disorder. NbMnP is a novel AF material that generates FM responses in the regime where there is less impurity scattering.

cond-mat.mes-hall↗

arXiv2023

Large anomalous Hall effect and unusual domain switching in an orthorhombic antiferromagnetic material NbMnP

Specific antiferromagnetic (AF) spin configurations generate large anomalous Hall effects (AHEs) even at zero magnetic field through nonvanishing Berry curvature in momentum space. In addition to restrictions on AF structures, suitable control of AF domains is essential to observe this effect without cancellations among its domains; therefore, compatible materials remain limited. Here we show that an orthorhombic noncollinear AF material, NbMnP, acquired AF structure-based AHE and controllability of the AF domains. Theoretical calculations indicated that a large Hall conductivity of $\sim230$ $Ω^{-1}$cm$^{-1}$ originated from the AF structure of NbMnP. Symmetry considerations explained the production of a small net magnetization, whose anisotropy enabled the generation and cancellation of the Hall responses using magnetic fields in different directions. Finally, asymmetric hysteresis in NbMnP shows potential for development of controllability of responses in AF materials.

cond-mat.str-el↗

arXiv2023

CeFe$_2$Al$_{10}$: a Correlated Metal with a Fermi Surface Exhibiting Nonmetallic Conduction

Metals can be defined as materials with a Fermi surface or as materials exhibiting metallic conduction (i.e., $\mathrm{d} ρ/ \mathrm{d}T > 0$). Usually, these definitions both hold at low temperatures, such as liquid-helium temperatures, as the Fermi energy is sufficiently larger than the thermal energy. However, they may not both hold in correlated electron systems where the Fermi energy is reduced by renormalization. In this paper, we demonstrate that although the resistivity of CeFe$_2$Al$_{10}$ increases with decreasing temperature below $\sim20$ K, CeFe$_2$Al$_{10}$ is a metal with a Fermi surface. This assertion is based on the observation of Shubnikov--de Haas oscillations and a Hall resistivity that changes sign with the magnetic field, which requires the coexistence of electron and hole carriers. Our analysis of Shubnikov--de Haas and magnetotransport data indicates that the Fermi energies are as small as $\sim$30 K and that, despite the increasing carrier mobility with decreasing temperature as in conventional metals, the loss of thermally excited carriers leads to nonmetallic conduction ($\mathrm{d} ρ/ \mathrm{d}T < 0$) below $\sim20$ K. Furthermore, we investigate how this anomalous metal transforms to a more conventional metal with metallic conduction by the application of high pressure and a high magnetic field. This study illustrates the subtle distinction between semimetals and semiconductors in correlated electron systems. This distinction is relevant to investigations of correlated topological insulators and semimetals.

cond-mat.str-el↗

arXiv2023

Observation of multigap and coherence peak in the noncentrosymmetric superconductor CaPtAs: $^{75}$As nuclear quadrupole resonance measurement

We present synthesis and $^{75}$As-nuclear quadrupole resonance (NQR) measurements for the noncentrosymmetric superconductor CaPtAs with a superconducting transition temperature $T_c$ of $\sim 1.5$ K. We discovered two different forms of CaPtAs during synthesis; one is a high-temperature tetragonal form that was previously reported, and the other is a low-temperature form consistent with the orthorhombic structure of CaPtP. According to the $^{75}$As-NQR measurement for superconducting tetragonal CaPtAs, the nuclear spin-lattice relaxation rate $1/T_1$ has an obvious coherence peak below $T_c$ and does not follow a simple exponential variation at low temperatures. These findings indicate that CaPtAs is a multigap superconductor and a large $s$-wave component.

cond-mat.supr-con↗

arXiv2022

Avoided ferromagnetic quantum critical point in CeZn

Cubic CeZn shows a structural phase transition under pressure, and it modifies the ground state from an antiferromagnetic (AFM) state to a ferromagnetic (FM) state. To investigate how the FM state terminates at a quantum phase transition, we measured the electrical resistivity under pressure for a single crystal CeZn. The transition temperature into the FM state decreases monotonously with increasing pressure, accompanied by the pronounced Kondo effect, but a drastic change in the field response occurs before the ordered phase terminates. This result suggests that the FM quantum critical point is avoided by the appearance of an AFM-like state.

cond-mat.str-el↗

arXiv2020

First-order phase transition to a nonmagnetic ground state in nonsymmorphic NbCrP

We report the discovery of a first-order phase transition at around 125 K in NbCrP, which is a nonsymmorphic crystal with Pnma space group. From the resistivity, magnetic susceptibility, and nuclear magnetic resonance measurements using the crystals made by the Sn-flux method, the high-temperature (HT) phase is characterized to be metallic with a non-negligible magnetic anisotropy. The low-temperature (LT) phase is also found to be a nonmagnetic metallic state with a crystal of lower symmetry. In the LT phase, the spin susceptibility is reduced by ~30 % from that in the HT phase, suggesting that the phase transition is triggered by the electronic instability. The possible origin of the phase transition in NbCrP is discussed based on the electronic structure by comparing with those in other nonsymmorphic compounds RuP and RuAs.

cond-mat.str-el↗

arXiv2019

Magnetic correlations in the pressure-induced superconductor CrAs investigated by $^{75}$As nuclear magnetic resonance

We report $^{75}$As-NMR results for CrAs under pressure, which shows superconductivity adjoining a helimagnetically ordered state. We successfully evaluated the Knight shift from the spectrum, which is strongly affected by the quadrupole interaction. The Knight shift shows the remarkable feature that the uniform spin susceptibility increases toward low temperatures in the paramagnetic state. This is in sharp contrast to CrAs at ambient pressure, and also to cuprates and Fe pnictides, where antiferromagnetic correlations are dominant. Superconductivity emerges in CrAs under unique magnetic correlations, which probably originate in the three-dimensional zigzag structure of its nonsymmorphic symmetry.

cond-mat.supr-con↗

arXiv2019

Optical and photoelectrical studies on anisotropic metal-insulator transition of RuAs

The anisotropic changes in the electronic structure of a metal-to-insulator transition (MIT) material, RuAs, with two-step phase transition are reported by using polarized optical conductivity [$σ(ω)$] spectra, angle-integrated photoelectron (PE) spectra, and band calculations based on local density approximation (LDA). Both the PE and $σ(ω)$ spectra not only in the high-temperature (HT) phase but also in the low-temperature (LT) phase as well as the energy gap formation owing to the MIT were almost consistent with those derived from the LDA band calculations, so the fundamental electronic structure in the HT and LT phases can be explained without electron correlations. However, the electronic structure in the middle phase between the HT and LT phases has not been clarified. The polarized $σ(ω)$ spectra revealed not only the anisotropic energy gap formation but also the anisotropic gap-opening temperature, i.e., the energy gap along the $c$ axis in the HT phase starts to open near the higher transition temperature, but that along the $b$ axis opens below the lower transition temperature. The finding suggests that the two-step MIT originates from the anisotropic energy gap formation.

cond-mat.str-el↗

arXiv2019

Indication of Ferromagnetic Quantum Critical Point in Kondo Lattice CeRh$_6$Ge$_4$

We report resistivity measurements under pressure for Kondo-lattice ferromagnet CeRh$_6$Ge$_4$, and present that a quantum ferromagnetic (FM) phase transition is easily achieved. In most clean metallic ferromagnets, a quantum critical point (QCP) at zero field is avoided by changing the FM transition to a discontinuous transition or to an antiferromagnetic transition. In CeRh$_6$Ge$_4$, to the contrary, the Curie temperature of 2.5 K decreases continuously as increasing pressure without any clear signature that the transition changes to first order. The obvious non Fermi liquid behavior is observed in the vicinity of the quantum FM phase transition. The experimental data do not contradict a picture in which CeRh$_6$Ge$_4$ shows the FM QCP at zero field. Band structure calculation suggests the unusual electronic state of CeRh$_6$Ge$_4$ among Ce-based Kondo lattices. CeRh$_6$Ge$_4$ deserves further investigations and will be a key material to understand the matter of the FM QCP.

cond-mat.str-el↗

arXiv2019

Magnetic Field Effect on s-wave Superconductor LaRu4P12 Studied by 31P-NMR

We have performed 31P-NMR measurements on the s-wave superconductor LaRu4P12 to investigate the magnetic field effect of the nuclear spin-lattice relaxation rate 1/T1 on a conventional full-gap superconductor. With increasing magnetic field, the Hebel-Slichter peak immediately below Tc in 1=T1 was suppressed, and the magnetic field dependence of 1/T1 at 0.8 K, well below Tc, was proportional to H2. These behaviors can be fully understood by the orbital pair-breaking effect in a single-band s-wave superconductor

cond-mat.supr-con↗

arXiv2018

Superlattice formation lifting degeneracy protected by non-symmorphic symmetry through a metal-insulator transition in RuAs

The single crystal of RuAs obtained by Bi-flux method shows obvious successive metal-insulator transitions at T_MI1~255 K and T_MI2~195$ K. The X-ray diffraction measurement reveals a formation of superlattice of 3x3x3 of the original unit cell below T_MI2, accompanied by a change of the crystal system from the orthorhombic structure to the monoclinic one. Simple dimerization of the Ru ions is nor seen in the ground state. The multiple As sites observed in nuclear quadrupole resonance (NQR) spectrum also demonstrate the formation of the superlattice in the ground state, which is clarified to be nonmagnetic. The divergence in 1/T_1 at T_MI1 shows that a symmetry lowering by the metal-insulator transition is accompanied by strong critical fluctuations of some degrees of freedom. Using the structural parameters in the insulating state, the first principle calculation reproduces successfully the reasonable size of nuclear quadrupole frequencies for the multiple As sites, ensuring the high validity of the structural parameters. The calculation also gives a remarkable suppression in the density of states (DOS) near the Fermi level, although the gap opening is insufficient. A coupled modulation of the calculated Ru d electron numbers and the crystal structure proposes a formation of charge density wave (CDW) in RuAs. Some lacking factors remain, but it shows that a lifting of degeneracy protected by the non-symmorphic symmetry through the superlattice formation is a key ingredient for the metal-insulator transition in RuAs.

cond-mat.str-el↗

arXiv2015

Detection of an Unconventional Superconducting Phase in the Vicinity of the Strong First-Order Magnetic Transition in CrAs Using ^75As-Nuclear Quadrupole Resonance

Pressure-induced superconductivity was recently discovered in the binary helimagnet CrAs. We report the results of measurements of nuclear quadrupole resonance for CrAs under pressure. In the vicinity of the critical pressure P_c between the helimagnetic (HM) and paramagnetic (PM) phases, a phase separation is observed. The large internal field remaining in the phase-separated HM state indicates that the HM phase disappears through a strong first-order transition. This indicates the absence of a quantum critical point in CrAs; however, the nuclear spin-lattice relaxation rate 1/T_1 reveals that substantial magnetic fluctuations are present in the PM state. The absence of a coherence effect in 1/T_1 in the superconducting state provides evidence that CrAs is the first Cr-based unconventional superconductor.

cond-mat.supr-con↗