arXiv subjects

Kenji Nakajima

Publications and source records attributed to Kenji Nakajima.

At least 19 recordsLinked to original sources

arXiv2024

Spin excitations arising from anisotropic Dirac spinons in YCu$_3$(OD)$_6$Br$_2$[Br$_{0.33}$(OD)$_{0.67}$]

A Dirac quantum spin liquid hosts Dirac spinons, which are low-energy fractionalized neutral quasiparticles with spin 1/2 that obey the Dirac equation. Recent inelastic neutron scattering studies have revealed a cone spin continuum in YCu$_3$(OD)$_6$Br$_2$[Br$_{x}$(OD)$_{1-x}$], consistent with the convolution of two Dirac spinons. In this work, we further studied spin excitations using the inelastic neutron scattering technique. The width of low-energy spin excitations shows a linear temperature dependence, which can be explained by spinon-spinon interactions with a Dirac dispersion. Polarized neutron scattering measurements reveal that in-plane magnetic fluctuations are about 1.5 times stronger than the out-of-plane ones, suggesting the presence of Dzyaloshinskii-Moriya interaction and consistent with our theoretical modeling and simulations. Moreover, the high-energy spin excitations around 14 meV agree with the one-pair spinon-antispinon excitations in Raman studies. The real part of the dynamical susceptibility derived from the Kramers-Kronig relationship also agrees with the Knight shift measured by nuclear magnetic resonance, clearly demonstrating the negligible effects of magnetic impurities on static susceptibility. These results provide a rare example in studying quantum-spin-liquid materials where different experimental techniques can be directly compared, and they give further insights for the possible Dirac quantum spin liquid in this system.

cond-mat.str-el↗

arXiv2024

Magnetic excitations in the noncentrosymmetric magnet Sr2MnSi2O7

Magnetic excitations in the noncentrosymmetric magnet Sr$_2$MnSi$_2$O$_7$ were investigated through inelastic neutron scattering measurements. Major magnetic excitations are limited up to the energy transfer of 0.5 meV, and two magnon branches under zero magnetic field were well explained in the framework of linear spin-wave theory. The magnitudes of the square-lattice in-plane and inter-plane nearest-neighbor interactions, spin anisotropy term, and the Dzyaloshinskii-Moriya interaction are respectively estimated to be $J_1=45.54(5)$ $\mu$eV, $J_2=0.52(1)$ $\mu$eV, $\Lambda=4.98(11)$ $\mu$eV, $D_{xy}=0.02(9)$ $\mu$eV, and $D_z=4.10(1)$ $\mu$eV, and calculations using these parameters reproduce experimental data quite well. Sr$_2$MnSi$_2$O$_7$ appears to have the smallest energy scale among the melilite-type compounds, and the small $J_2/J_1=0.0114(2)$ indicates the sufficient two-dimensionality.

cond-mat.str-el↗

arXiv2023

Spectral evidence for Dirac spinons in a kagome lattice antiferromagnet

Emergent quasiparticles with a Dirac dispersion in condensed matter systems can be described by the Dirac equation for relativistic electrons, in analogy with Dirac particles in high-energy physics. For example, electrons with a Dirac dispersion have been intensively studied in electronic systems such as graphene and topological insulators. However, charge is not a prerequisite for Dirac fermions, and the emergence of Dirac fermions without charge degree of freedom has been theoretically predicted to be realized in Dirac quantum spin liquids. These quasiparticles carry a spin of 1/2 but are charge-neutral, and so are called spinons. Here we show that the spin excitations of a kagome antiferromagnet, YCu$_3$(OD)$_6$Br$_2$[Br$_{0.33}$(OD)$_{0.67}$], are conical with a spin continuum inside, which is consistent with the convolution of two Dirac spinons. The predictions of a Dirac spin liquid model with a spinon velocity obtained from the spectral measurements are in agreement with the low-temperature specific heat of the sample. Our results thus provide spectral evidence for the Dirac quantum spin liquid state emerging in this kagome lattice antiferromagnet. However, the locations of the conical spin excitations differ from those calculated by the nearest neighbor Heisenberg model, suggesting the Dirac spinons have an unexpected origin.

cond-mat.str-el↗

arXiv2023

Bond-dependent anisotropy and magnon decay in cobalt-based Kitaev triangular antiferromagnet

The Kitaev model, a honeycomb network of spins with bond-dependent anisotropic interactions, is a rare example of having a quantum spin liquid ground state. Although most Kitaev model candidate materials eventually order magnetically due to inevitable non-Kitaev terms, their bond-dependent anisotropy manifests in unusual spin dynamics. It has recently been suggested that bond-dependent anisotropy can stabilise novel magnetic phases and exotic spin dynamics on the geometrically frustrated triangular lattice. However, few materials have been identified with simultaneous geometric frustration and bond-dependent anisotropy. Here, we report a frustrated triangular lattice with bond-dependent anisotropy in the cobalt-based triangular van der Waals antiferromagnet CoI2. Its momentum and energy-resolved spin dynamics exhibit substantial magnon breakdown and complex level repulsion, as measured by inelastic neutron scattering. A thorough examination of excitations in both the paramagnetic and magnetically ordered states reveals that the bond-dependent anisotropy is the origin of the spiral order and the magnon breakdown found in CoI2. Our result paves the way toward a new research direction for the Kitaev model with geometrical frustration.

cond-mat.str-el↗

arXiv2022

Magnetic field effects in an octupolar quantum spin liquid candidate

Quantum spin liquid (QSL) is a disordered state of quantum-mechanically entangled spins commonly arising from frustrated magnetic dipolar interactions. However, QSL in some pyrochlore magnets can also come from frustrated magnetic octupolar interactions. Although the key signature for both dipolar and octupolar interaction-driven QSL is the presence of a spin excitation continuum (spinons) arising from the spin quantum number fractionalization, an external magnetic field-induced ferromagnetic order will transform the spinons into conventional spin waves in a dipolar QSL. By contrast, in an octupole QSL, the spin waves carry octupole moments that do not couple, in the leading order, to the external magnetic field or to neutron moments but will contribute to the field dependence of the heat capacity. Here we use neutron scattering to show that the application of a large external magnetic field to Ce2Zr2O7, an octupolar QSL candidate, induces an Anderson-Higgs transition by condensing the spinons into a static ferromagnetic ordered state with octupolar spin waves invisible to neutrons but contributing to the heat capacity. Our theoretical calculations also provide a microscopic, qualitative understanding for the presence of octupole scattering at large wavevectors in Ce2Sn2O7 pyrochlore, and its absence in Ce2Zr2O7. Therefore, our results identify Ce2Zr2O7 as a strong candidate for an octupolar U (1) QSL, establishing that frustrated magnetic octupolar interactions are responsible for QSL properties in Ce-based pyrochlore magnets.

cond-mat.str-el↗

arXiv2021

Magnons and Spinons in $\mathrm{Ba}_2\mathrm{CoTeO}_6 $: A Composite System of Isolated Spin-$1/2$ Triangular Heisenberg-like and Frustrated Honeycomb Ising-like Antiferromagnets

We report the neutron scattering results on magnetic orderings and excitations in $\mathrm{Ba}_2\mathrm{CoTeO}_6$ composed of two almost isolated subsystems A and B, which are described as an $S\,{=}\,1/2$ triangular Heisenberg-like antiferromagnet and a frustrated honeycomb Ising-like antiferromagnet, respectively. Stripy ordering of subsystem B was confirmed below $T_{\rm N1}\,{=}\,12.0$ K, whereas sharp streaks were observed along $(1/3, 1/3, L)$ and $(2/3, 2/3, L)$ at 0.3 K (${\ll}\,T_{\rm N2}\,{=}\,3.0$ K). This indicates the two-dimensional nature of ordering in subsystem A. It was found that the excitation spectra of both subsystems are well separated and independent of each other. The excitation spectrum of subsystem A is composed of two single-magnon branches with roton-like minima at the M point and a clearly structured intense continuum, as similarly observed in $\mathrm{Ba}_3\mathrm{CoSb}_2\mathrm{O}_9$, which is strongly indicative of spinon excitations. Dispersion curves for subsystem B can be described by linear spin wave theory within the third-neighbor exchange interaction.

cond-mat.str-el↗

arXiv2021

Dimensional reduction by geometrical frustration in a cubic antiferromagnet composed of tetrahedral clusters

Dimensionality is a critical factor in determining the properties of solids and is an apparent built-in character of the crystal structure. However, it can be an emergent and tunable property in geometrically frustrated spin systems. Here, we study the spin dynamics of the tetrahedral cluster antiferromagnet, pharmacosiderite, via muon spin resonance and neutron scattering. We find that the spin correlation exhibits a two-dimensional characteristic despite the isotropic connectivity of tetrahedral clusters made of spin 5/2 Fe3+ ions in the three-dimensional cubic crystal, which we ascribe to two-dimensionalisation by geometrical frustration based on spin wave calculations. Moreover, we suggest that even one-dimensionalisation occurs in the decoupled layers, generating low-energy and one-dimensional excitation modes, causing large spin fluctuation in the classical spin system. Pharmacosiderite facilitates studying the emergence of low-dimensionality and manipulating anisotropic responses arising from the dimensionality using an external magnetic field.

cond-mat.str-el↗

arXiv2021

Q-dependent Collective Relaxation Dynamics of Glass-Forming Liquid Ca0.4K0.6(NO3)1.4 Investigated by Wide-Angle Neutron Spin-Echo

Employing wide-angle neutron spin echo spectroscopy, we measured the Q-dependent coherent intermediate scattering function of the prototypical ionic glass former Ca0.4K0.6(NO3)1.4, in the equilibrium and supercooled liquid states beyond the hydrodynamic regime. The data reveal a clear two-step relaxation: an exponential fast process, and a stretched exponential slow alpha process. de Gennes narrowing is observed in all characteristic variables of the alpha process: the relaxation time, amplitude, and stretching exponent. At all length scales probed, the relative amplitude of the alpha-relaxation decreases with increasing temperature and levels off in the normal liquid state. The temperature dependence of the stretching exponent and the relaxation time at different Q's indicate that modifications of the relaxation mechanisms at the local length scales, manifested as temperature independent dynamic heterogeneity and smaller deviations from Arrhenius behavior, have occurred even above the alpha-beta (Johari-Goldstein) bifurcation temperature.

cond-mat.soft↗

arXiv2020

Emergent spin-$1$ Haldane gap and ferroelectricity in a frustrated spin-$1/2$ ladder

We report experimental and theoretical evidence that Rb$_2$Cu$_2$Mo$_3$O$_{12}$ has a nonmagnetic tetramer ground state of a two-leg ladder comprising antiferromagnetically coupled frustrated spin-$1/2$ chains and exhibits a Haldane spin gap of emergent spin-1 pairs. Three spin excitations split from the spin-1 triplet by a Dzyaloshinskii-Moriya interaction are identified in inelastic neutron-scattering and electron spin resonance spectra. A tiny magnetic field generates ferroelectricity without closing the spin gap, indicating a novel class of ferroelectricity induced by a vector spin chirality order.

cond-mat.str-el↗

arXiv2020

Ultralow thermal conductivity from transverse acoustic phonon suppression in distorted crystalline α-MgAgSb

Low thermal conductivity is favorable for preserving the temperature gradient between the two ends of a thermoelectric material in order to ensure continuous electron current generation. In high-performance thermoelectric materials, there are two main low thermal conductivity mechanisms: the phonon anharmonic in PbTe and SnSe and phonon scattering resulting from the dynamic disorder in AgCrSe2 and CuCrSe2, which have been successfully revealed by inelastic neutron scattering. Using neutron scattering and ab initio calculations, we report here a mechanism of static local structure distortion combined with phonon-anharmonic-induced ultralow lattice thermal conductivity in α-MgAgSb. Since the transverse acoustic phonons are almost fully scattered by the compound's intrinsic distorted rocksalt sublattice, the heat is mainly transported by the longitudinal acoustic phonons. The ultralow thermal conductivity in α-MgAgSb is attributed to its atomic dynamics being altered by the structure distortion, which presents a possible microscopic route to enhance the performance of similar thermoelectric materials.

cond-mat.mtrl-sci↗

arXiv2019

Evidence for singular-phonon-induced nematic superconductivity in a topological superconductor candidate Sr$_{0.1}$Bi$_2$Se$_3$

Superconductivity mediated by phonons is typically conventional, exhibiting a momentum-independent s-wave pairing function, due to the isotropic interactions between electrons and phonons along different crystalline directions. Here, by performing inelastic neutron scattering measurements on a superconducting single crystal of Sr0.1Bi2Se3, a prime candidate for realizing topological superconductivity by doping the topological insulator Bi2Se3, we find that there exist highly anisotropic phonons, with the linewidths of the acoustic phonons increasing substantially at long wavelengths, but only for those along the [001] direction. This observation indicates a large and singular electron-phonon coupling at small momenta, which we propose to give rise to the exotic p-wave nematic superconducting pairing in the MxBi2Se3 (M = Cu, Sr, Nb) superconductor family. Therefore, we show these superconductors to be example systems where electron-phonon interaction can induce more exotic superconducting pairing than the s-wave, consistent with the topological superconductivity.

cond-mat.supr-con↗

arXiv2019

Localized magnetic excitations in the fully frustrated dimerized magnet Ba$_2$CoSi$_2$O$_6$Cl$_2$

Magnetic excitations of the effective spin $S$=1/2 dimerized magnet Ba$_2$CoSi$_2$O$_6$Cl$_2$ have been probed directly via inelastic neutron scattering experiments at temperatures down to 4 K. We observed five types of excitation at 4.8, 5.8, 6.6, 11.4, and 14.0 meV, which are all dispersionless within the resolution limits. The scattering intensities of the three low-lying excitations were found to exhibit different $Q$-dependences. Detailed analysis has demonstrated that Ba$_2$CoSi$_2$O$_6$Cl$_2$ is a two-dimensional spin dimer system described only by a single dimer site, where the triplet excitations are localized owing to the almost perfect frustration of the interdimer exchange interactions and the undimerized spins, even in small concentration, make an essential contribution to the excitation spectrum.

cond-mat.str-el↗

arXiv2019

Magnetic excitation emergence relevant to carrier scattering in semimetal Yb$_3$Ir$_4$Ge$_{13}$

Inelastic neutron scattering experiments have been conducted to investigate the semimetal system Yb$_3$Ir$_4$Ge$_{13}$. No clear crystal-field-split levels were observed up to an excitation energy of 60 meV, while a magnetic excitation appears at low energy below approximately 3 meV. This excitation shows a short spatial correlation, and an energy spectrum is reproduced by an imaginary part of generalized magnetic susceptibility on the basis of a damped harmonic oscillator model. The magnetic excitation emerges simultaneously with an anomalous electrical-resistivity enhancement below 20 K. The close relationship between the magnetic and transport behaviors evidences that the semimetal carriers are scattered by magnetic fluctuation dominated by the Yb $4f$ state.

cond-mat.str-el↗

arXiv2018

Spin correlations of quantum-spin-liquid and quadrupole-ordered states of Tb$_{2+x}$Ti$_{2-x}$O$_{7+y}$

Spin correlations of the frustrated pyrochlore oxide Tb$_{2+x}$Ti$_{2-x}$O$_{7+y}$ have been investigated by using inelastic neutron scattering on single crystalline samples ($x=-0.007, 0.000,$ and $0.003$), which have the putative quantum-spin-liquid (QSL) or electric-quadrupolar ground states. Spin correlations, which are notably observed in nominally elastic scattering, show short-ranged correlations around $L$ points [$q = (\tfrac{1}{2},\tfrac{1}{2},\tfrac{1}{2})$], tiny antiferromagnetic Bragg scattering at $L$ and $Γ$ points, and pinch-point type structures around $Γ$ points. The short-ranged spin correlations were analyzed using a random phase approximation (RPA) assuming the paramagnetic state and two-spin interactions among Ising spins. These analyses have shown that the RPA scattering intensity well reproduces the experimental data using temperature and $x$ dependent coupling constants of up to 10-th neighbor site pairs. This suggests that no symmetry breaking occurs in the QSL sample, and that a quantum treatment beyond the semi-classical RPA approach is required. Implications of the experimental data and the RPA analyses are discussed.

cond-mat.str-el↗

arXiv2018

Triplon band splitting and topologically protected edge states in the dimerized antiferromagnet

The search for topological insulators has been actively promoted in the field of condensed matter physics for further development in energy-efficient information transmission and processing. In this context, recent studies have revealed that not only electrons but also bosonic particles such as magnons can construct edge states carrying nontrivial topological invariants. Here we demonstrate topological triplon bands in the spin-1/2 two-dimensional dimerized quantum antiferromagnet Ba$_2$CuSi$_2$O$_6$Cl$_2$, which is closely related to a pseudo-one-dimensional variant of the Su-Schrieffer-Heeger (SSH) model, through inelastic neutron scattering experiments. The excitation spectrum exhibits two triplon bands and a clear band gap between them due to a small alternation in interdimer exchange interactions along the $a$-direction, which is consistent with the crystal structure. The presence of topologically protected edge states is indicated by a bipartite nature of the lattice.

cond-mat.str-el↗

arXiv2018

Colossal barocaloric effects in plastic crystals

Solid-state refrigeration technology based on caloric effects are promising to replace the currently used vapor compression cycles. However, their application is restricted due to limited performances of caloric materials. Here, we have identified colossal barocaloric effects (CBCEs) in a class of disordered solids called plastic crystals. The obtained entropy changes are about 380 J kg-1 K-1 in the representative neopentylglycol around room temperature. Inelastic neutron scattering reveals that the CBCEs in plastic crystals are attributed to the combination of the vast molecular orientational disorder, giant compressibility, and high anharmonic lattice dynamics. Our study establishes the microscopic scenario for CBCEs in plastic crystals and paves a new route to the next-generation solid-state refrigeration technology.

cond-mat.mtrl-sci↗

arXiv2018

Continuum excitation and pseudospin wave in quantum spin-liquid and quadrupole ordered states of Tb$_{2+x}$Ti$_{2-x}$O$_{7+y}$

The ground states of the frustrated pyrochlore oxide Tb$_{2+x}$Ti$_{2-x}$O$_{7+y}$ have been studied by inelastic neutron scattering experiments. Three single-crystal samples are investigated; one shows no phase transition ($x=-0.007<x_{\text{c}}\sim -0.0025$), being a putative quantum spin-liquid (QSL), and the other two ($x=0.000, 0.003$) show electric quadrupole ordering (QO) below $T_{\text{c}} \sim 0.5$ K. The QSL sample shows continuum excitation spectra with an energy scale 0.1 meV as well as energy-resolution limited (nominally) elastic scattering. As $x$ is increased, pseudospin wave of the QO state emerges from this continuum excitation, which agrees with that of powder samples and consequently verifies good $x$ control for the present single crystal samples.

cond-mat.str-el↗

arXiv2017

Magnetic Properties of One-Dimensional Quantum Spin System Rb2Cu2Mo3O12 Studied by Muon Spin Relaxation

Magnetic properties of a one-dimensional frustrated quantum spin system Rb2Cu2Mo3O12 is investigated by the muon spin relaxation (muSR) technique. Although this system shows an incommensurate spin correlation in neutron scattering, it has not yet been determined whether the ground state reaches to a nonmagnetic spin-singlet state or not. In this study, our zero-field (ZF) muSR result undoubtedly reveals a nonmagnetic ground state, which establishes formation of the nonmagnetic incommensurate spin-singlet state in combination of the previous neutron scattering study. Furthermore, we found that the internal field is slightly enhanced below ~7 K. This temperature dependence resembles that of electric polarization under the magnetic field reported recently, indicating a possibility that a small change occurs even under ZF. In longitudinal-field measurements, a fast fluctuation around a narrowing region is observed. This relaxation is expected to reflect characteristics of formation of the spin-singlet state.

cond-mat.str-el↗