arXiv subjects

Michihiro Hirata

Publications and source records attributed to Michihiro Hirata.

13 recordsLinked to original sources

arXiv2026

A continuous confinement-deconfinement transition in a triangular quantum magnet

A continuous transition between phases hosting distinct excitations---bosonic magnons versus fermionic spinons---is a long-sought phenomenon in quantum magnetism, analogous to the confinement--deconfinement transition in quantum chromodynamics. We report evidence for such a transition in the triangular-lattice antiferromagnet TlYbS$_2$. Antiferromagnetic order develops below $T_\mathrm{N} \approx 0.53\,\mathrm{K}$. A $c$-axis field suppresses this order, driving the system into a gapless quantum spin liquid with a spinon Fermi surface above $μ_0 H_\mathrm{c} \approx 3\,\mathrm{T}$, evidenced by a finite residual linear term in thermal conductivity, a Pauli-like susceptibility, and a temperature-independent NMR Knight shift. Approaching $H_\mathrm{c}$ from above, the scattering rate of itinerant excitations is strongly enhanced while their density of states shows no critical enhancement, atypical of conventional magnetic quantum criticality. These results point to a continuous confinement--deconfinement transition governed by fractionalized excitations beyond the Ginzburg--Landau paradigm.

cond-mat.str-el↗

arXiv2025

Evidence for easy-plane XY ferromagnetism in heavy-fermion quantum-critical CeRh6Ge4

We report $^{73}$Ge nuclear quadrupole resonance (NQR) and magnetic resonance (NMR) spectroscopy in the heavy-fermion quantum-critical ferromagnet CeRh$_6$Ge$_4$. NQR and NMR spectral measurements at the two non-equivalent Ge sites reveal electric field gradient tensors and the directions of their principal axes relative to the hexagonal basal plane. The spin-lattice relaxation rate $1/T_1$ experiments reveal a clear critical slowing down approaching the ferromagnetic transition. $1/T_1$ in the paramagnetic state is found to be predominantly caused by fluctuating 4$f$-local moments. The Knight shift shows Curie-Weiss behavior at high temperature and a deviation from this below $T^\ast$ $\approx$ 25 K possibly due to a mixture of crystalline electric field effects and Kondo screening. Order-parameter-like behavior of hyperfine fields at the Ge sites and ferromagnetic signal enhancement are observed in Zeeman-perturbed NQR, with uniform ferromagnetic order and a small ordered moment of $\approx$ 0.26 $μ_B$/Ce confined within the $ab$-plane. The ordered moment shows a notable in-plane magnetic stiffness against out-of-plane radiofrequency fields and has an (XY-type) in-plane isotropic nature. Our results reveal a strong easy-plane anisotropy of 4$f$-electron moment and suggest an involved interplay of hybridization and local moment physics in this quantum critical heavy-fermion ferromagnet.

cond-mat.str-el↗

arXiv2024

Local aging effects in PuB$_{4}$: Growing inhomogeneity and slow dynamics of local-field fluctuations probed by $^{239}$Pu NMR

The effect of self-irradiation damage can influence many properties of a radioactive material. Actinide materials involving the decay through alpha radiation have been frequently studied using techniques such as transport, thermodynamics, and x-ray diffraction. The use of nuclear magnetic resonance (NMR) spectroscopy to study such effects, however, has seen relatively little attention. Here, we use $^{239}$Pu NMR to study the local influence of self-damage in a single crystal of the candidate topological insulator plutonium tetraboride (PuB$_{4}$). We first characterize the anisotropy of the $^{239}$Pu resonance in a single crystal and confirm the local axial site symmetry inferred from previous polycrystalline measurements. Aging effects are then evaluated over the timeframe of six years. We find that, though the static NMR spectra may show a slight modulation in their shape, their field-rotation pattern reveals no change in Pu local site symmetry over time, suggesting that aging has a surprisingly small impact on the spatial distribution of the static hyperfine field. By contrast, aging has a prominent impact on the NMR relaxation processes and signal intensity. Specifically, aging-induced damage manifests itself as an increase in the spin-lattice relaxation time $T_{1}$, an increased distribution of $T_{1}$, and a signal intensity that decreases linearly by 20 % per year. The spin-spin relaxation time $T_{2}$ in the aged sample shows a strong variation across the spectrum as well as a drastic shortening towards lower temperature, suggesting growth of slow fluctuations of the hyperfine field that are linked to radiation-damage-induced inhomogeneity and could be responsible for the signal wipeout that develops over time.

cond-mat.mtrl-sci↗

arXiv2020

Magnetic-Field Dependence of Novel Gap Behavior Related to the Quantum-Size Effect

$^{195}$Pt-NMR measurements of Pt nanoparticles with a mean diameter of 4.0 nm were performed in a high magnetic field of approximately $μ_0 H = 23.3$ T to investigate the low-temperature electronic state of the nanoparticles. The characteristic temperature $T^*$, below which the nuclear spin-lattice relaxation rate $1/T_1$ deviates from the relaxation rate of the bulk, shows a magnetic-field dependence. This dependence supports the theoretical prediction of the appearance of discrete energy levels.

cond-mat.mtrl-sci↗

arXiv2019

Excitonic instability of two-dimensional tilted Dirac cones

The electron-electron Coulomb interaction in Dirac-Weyl semimetals harbours a novel paradigm of correlation effects that hybridizes diverse realms of solid-state physics with their relativistic counterpart. Driving spontaneous mass acquisition, the excitonic condensate of strongly-interacting massless Dirac fermions is one such example whose exact nature remains debated. Here, by focussing on the two-dimensional tilted Dirac cones in the organic salt $α$-(BEDT-TTF)$_2$I$_3$, we show that the excitonic instability is controlled by a small chemicalpotential shift and an in-plane magnetic field. In combined analyses based on renormalization-group approaches and ladder approximation, we demonstrate that the nuclear relaxation rate is an excellent probe of excitonic-spin fluctuations in an extended parameter region. Comparative nuclear magnetic resonance (NMR) experiments show good agreements with this result, jointly revealing the importance of intervalley nesting between field-induced, spin-split Fermi pockets of opposite charge polarities. Our work provides an accurate framework to search for excitonic instability of strongly-interacting massless fermions.

cond-mat.str-el↗

arXiv2017

Pseudogap Behavior of the Nuclear Spin-lattice Relaxation Rate in FeSe Probed by $^{77}$Se-NMR

We conducted $^{77}$Se-nuclear magnetic resonance studies of the iron-based superconductor FeSe in magnetic fields of 0.6 to 19 T to investigate the superconducting and normal-state properties. The nuclear spin-lattice relaxation rate divided by the temperature $(T_1T)^{-1}$ increases below the structural transition temperature $T_\mathrm{s}$ but starts to be suppressed below $T^*$, well above the superconducting transition temperature $T_\mathrm{c}(H)$, resulting in a broad maximum of $(T_1T)^{-1}$ at $T_\mathrm{p}(H)$. This is similar to the pseudogap behavior in optimally doped cuprate superconductors. Because $T^*$ and $T_\mathrm{p}(H)$ decrease in the same manner as $T_\mathrm{c}(H)$ with increasing $H$, the pseudogap behavior in FeSe is ascribed to superconducting fluctuations, which presumably originate from the theoretically predicted preformed pair above $T_\mathrm{c}(H)$.

cond-mat.supr-con↗

arXiv2017

Anomalous Spin Correlations and Mass-Generating Excitonic Instability of Interacting Weyl Fermions

Recent advances in the study of nodal Weyl fermions (WFs), quasi-relativistic massless particles, constitute a novel realm of quantum many-body phenomena. The Coulomb interaction in such systems, having a zero density of states at the Fermi level, is of particular interest, since in contrast to conventional correlated metals, its long-ranged component is unscreened. Here, through nuclear-magnetic-resonance (NMR) measurements, we unveil the exotic spin correlations of two-dimensional WFs in an organic material, causing a divergent increase of the Korringa ratio by a factor of 1000 upon cooling, in striking contrast with conventional metallic behaviors. Combined with model calculations, we show that this divergence stems from the interaction-driven velocity renormalization that almost exclusively suppresses the zero-momentum spin fluctuations. At low temperatures, the NMR rate shows a remarkable increase, which is shown by numerical analyses to correspond to inter-node excitonic fluctuations, precursor of a transition from massless to massive quasiparticles.

cond-mat.str-el↗

arXiv2016

Observation of an anisotropic Dirac cone reshaping and ferrimagnetic spin polarization in an organic conductor

The Coulomb interaction among massless Dirac fermions in graphene is unscreened around the isotropic Dirac points, causing a logarithmic velocity renormalization and a cone reshaping. In less symmetric Dirac materials possessing anisotropic cones with tilted axes, the Coulomb interaction can provide still more exotic phenomena which have not been experimentally unveiled yet. Here, using site-selective nuclear magnetic resonance, we find a non-uniform cone reshaping accompanied by a bandwidth reduction and an emergent ferrimagnetism in tilted Dirac cones that appear on the verge of charge ordering in an organic compound. Our theoretical analyses based on the renormalization-group approach and the Hubbard model show that these observations are the direct consequences of the long-range and short-range parts of the Coulomb interaction, respectively. The cone reshaping and the bandwidth renormalization, as well as the novel magnetism revealed here, can be ubiquitous and vital for many Dirac materials.

cond-mat.str-el↗

arXiv2016

Spin excitations in the quasi-two-dimensional charge-ordered insulator $α$-(BEDT-TTF)$_2$I$_3$ probed via $^{13}$C NMR

The spin excitations from the nonmagnetic charge-ordered insulating state of $α$-(BEDT-TTF)$_2$I$_3$ at ambient pressure have been investigated by probing the static and low-frequency dynamic spin susceptibilities via site-selective nuclear magnetic resonance at $^{13}$C sites. The site-dependent values of the shift and the spin-lattice relaxation rate $1/T_1$ below the charge-ordering transition temperature ($T_{CO} \approx$ 135 K) demonstrate a spin density imbalance in the unit cell, in accord with the charge-density ratio reported earlier. The shift and $1/T_1$ show activated temperature dependence with a static (shift) gap $Δ_S \approx$ 47-52 meV and a dynamic ($1/T_1$) gap $Δ_R \approx$ 40 meV. The sizes of the gaps are well described in terms of a localized spin model, where spin one-half antiferromagnetic dimer chains are weakly coupled with each other.

cond-mat.str-el↗

arXiv2012

Electron Correlations in the Quasi-Two-Dimensional Organic Conductor $θ$-(BEDT-TTF)$_{2}$I$_{3}$ investigated by $^{13}$C NMR

We report a $^{13}$C-NMR study on the ambient-pressure metallic phase of the layered organic conductor $θ$-(BEDT-TTF)$_{2}$I$_{3}$ [BEDT-TTF: bisethylenedithio-tetrathiafulvalene], which is expected to connect the physics of correlated electrons and Dirac electrons under pressure. The orientation dependence of the NMR spectra shows that all BEDT-TTF molecules in the unit cell are to be seen equivalent from a microscopic point of view. This feature is consistent with the orthorhombic symmetry of the BEDT-TTF sublattice and also indicates that the monoclinic $I_{3}$ sublattice, which should make three molecules in the unit cell nonequivalent, is not practically influential on the electronic state in the conducting BEDT-TTF layers at ambient pressure. There is no signature of charge disproportionation in opposition to most of the $θ$-type BEDT-TTF salts. The analyses of NMR Knight shift, $K$, and the nuclear spin-lattice relaxation rate, $1/T_{1}$, revealed that the degree of electron correlation, evaluated by the Korringa ratio [$\varpropto 1/(T_{1}TK^{2}$)], is in an intermediate regime. However, NMR relaxation rate $1/T_{1}$ is enhanced above $\sim$ 200K, which possibly indicates that the system enters into a quantum critical regime of charge-order fluctuations as suggested theoretically.

cond-mat.mtrl-sci↗

arXiv2011

$^{13}$C NMR Study on the Charge-Disproportionated Conducting State in the Quasi-Two-Dimensional Organic Conductor $α$-(BEDT-TTF)$_2$I$_3$

The conducting state of the quasi-two-dimensional organic conductor, $α$-(BEDT-TTF)$_2$I$_3$, at ambient pressure is investigated with $^{13}$C NMR measurements, which separate the local electronic states at three nonequivalent molecular sites (A, B, and C). The spin susceptibility and electron correlation effect are revealed in a locally resolved manner. While there is no remarkable site-dependence around room temperature, the local spin susceptibility gradually disproportionates among the nonequivalent sites with decreasing temperature. The disproportionation-ratio yields 5:4:6 for A:B:C molecules at 140 K. Distinct site- and temperature-dependences are also observed in the Korringa ratio, $\mathcal{K}_i \propto (1/T_{1}T)_iK^{-2}_i$ ($i$ = A, B, and C), which is a measure of the strength and the type of electron correlations. The values of $\mathcal{K}_i$ point to sizable antiferromagnetic spin correlation. We argue the present results in terms of the theoretical prediction of the peculiar site-specific reciprocal-space ($\bm{k}$-space) anisotropy on the tilted Dirac cone, and discuss the $\bm{k}$-dependent profiles of the spin susceptibility and electron correlation on the cone.

cond-mat.mtrl-sci↗

arXiv1997

Photoabsorption on a Nucleon in the D_{13} Resonance Energy Region

We present a simple model for the $γN \to ππN$ reaction which reproduces the cross sections of the $π^+ π^- p, π^+ π^- n, π^+ π^0 n$ and $π^- π^0 p$ channels over the range of the energies 0.41-0.85 GeV. We use the dynamical model for the resonances, Δ(1232), N^*(1520) and ρ-meson. The total photoabsorption off a nucleon is also discussed.

nucl-th↗

arXiv1997

Phenomenological interaction between current quarks

We construct a phenomenological model which describes the dynamical chiral symmetry breaking (DCSB) of QCD vacuum and reproduces meson spectra. Quark condensates, the pion decay constant, and meson spectra are well reproduced by phenomenological interaction which consists of a linear confining potential, a Coulombic potential, and the 't Hooft determinant interaction. In this model, the 't Hooft determinant interaction plays a important role not to only η,η' mass difference, but other meson masses through DCSB.

hep-ph↗