arXiv subjects

Shinichi Itoh

Publications and source records attributed to Shinichi Itoh.

At least 19 recordsLinked to original sources

arXiv2026

Anomalous metallic states at magnetic interfaces in an antiferromagnetic topological insulator candidate DyPtBi with ferroquadrupolar order

Antiferromagnetic topological insulators provide a fertile platform where symmetry-breaking magnetic order is intertwined with topological electronic states. In particular, magnetic domain walls have attracted much attention, as they can be easily controlled by external fields as in general magnets, and moreover, host nontrivial electronic states distinct from those in bulks and sample surfaces. Here, we report a new antiferromagnetic topological insulator candidate DyPtBi, which hosts conductive magnetic domain walls controllable by uniaxial stress and magnetic field. We find that the resistivity exhibits abrupt increase upon the magnetic and structural phase transition. Concomitantly, the transverse ultrasonic mode shows remarkable softening of 6 %, indicating that the Dy 4f ferroquadrupolar order plays a vital role in the phase transition. Furthermore, we reveal by neutron experiments that applying compressive uniaxial stress aligns the magnetic domain state, leading to the strong resistivity enhancement of 14 % while eliminating the conductive magnetic domain walls. These findings demonstrate that DyPtBi exhibits topological electronic states entangled with multipolar degrees of freedom, providing a promising route for in situ control of topological properties.

cond-mat.str-el↗

arXiv2026

Inelastic neutron scattering study on the AFM uniform spin-1/2 chain compound CuSb2O6

We carried out inelastic neutron scattering experiments on a powdered sample of the antiferromagnetic (AFM) uniform spin-1/2 chain compound CuSb2O6.The magnetic excitations appear in the energy range of 1.8 to 13 meV at 2.5 K below the AFM transition temperature (TN = 8.7 K).The gap value (1.8 meV) is close to that evaluated from the specific heat (1.51 meV). The excitations at 12.5 K (> TN) appear gapless. Thus, the 1.8 meV gap is caused by some anisotropy in spin-wave excitations. The gap excitations are strongest at 0.48 corresponding to a length of 0.66 nm. This result is consistent with the theoretical one that the interaction in a Cu pair with a length of 0.65562 nm (Jab) is strongest. The magnetic excitations can be explained by the AFM uniform XXZ chain with Jab = 6.437 meV and DJab = 0.063 meV. The 1.8 meV gap is caused by the small Ising anisotropy (DJab/Jab = 0.0098).

cond-mat.str-el↗

arXiv2025

Kitaev interaction and proximate higher-order skyrmion crystal in the triangular lattice van der Waals antiferromagnet NiI2

Topological spin textures, such as magnetic skyrmions, are a spectacular manifestation of magnetic frustration and anisotropy. Most known skyrmion systems are restricted to a topological charge of one, require an external magnetic field for stabilization, and are only reported in a few materials. Here, we investigate the possibility that the Kitaev anisotropic-exchange interaction stabilizes a higher-order skyrmion crystal in the insulating van der Waals magnet NiI2. We unveil and explain the incommensurate static and dynamic magnetic correlations across three temperature-driven magnetic phases of this compound using neutron scattering measurements, simulations, and modeling. Our parameter optimisation yields a minimal Kitaev-Heisenberg Hamiltonian for NiI2 which reproduces the experimentally observed magnetic excitations. Monte Carlo simulations for this model predict the emergence of the higher-order skyrmion crystal but neutron diffraction and optical experiments in the candidate intermediate temperature regime are inconclusive. We discuss possible deviations from the Kitaev-Heisenberg model that explains our results and conclude that NiI2, in addition to multiferroic properties in the bulk and few-layer limits, is a Kitaev bulk material proximate to the finite temperature higher-order skyrmion crystal phase.

cond-mat.mtrl-sci↗

arXiv2024

Chiral-Split Magnon in Altermagnetic MnTe

Altermagnetism is a newly discovered magnetic class named after the alternating spin polarizations in both real and reciprocal spaces. Like the spin-splitting of electronic bands, the magnon bands in altermagnets are predicted to exhibit alternating chiral splitting. In this work, by performing inelastic neutron scattering on $α$-MnTe, we directly verified the chiral splitting in altermagnetic magnon dispersions. The lifted degeneracy of chirality is further explained by a symmetric-exchange origin. In addition, the $g$-wave magnetism was identified in MnTe.

cond-mat.str-el↗

arXiv2024

Spin dynamics in linear magnetoelectric material Mn$_{3}$Ta$_{2}$O$_{8}$

We performed inelastic neutron scattering experiments on single crystal samples of a linear magnetoelectric material Mn$_{3}$Ta$_{2}$O$_{8}$, which exhibits a collinear antiferromagnetic order, to reveal the spin dynamics. Numerous modes observed in the neutron spectra were reasonably reproduced by linear spin-wave theory on the basis of the spin Hamiltonian including eight Heisenberg interactions and an easy-plane type single-ion anisotropy. The presence of strong frustration was found in the identified spin Hamiltonian.

cond-mat.str-el↗

arXiv2023

In-plane anisotropy of the single-$q$ and multiple-$q$ ordered phases in the antiferromagnetic metal CeRh$_2$Si$_2$ unveiled by the bulk measurements under uniaxial stress and neutron scattering

We performed magnetization, resistivity, and neutron diffraction measurements under uniaxial stress applied along [1-10] direction on the tetragonal magnet CeRh$_2$Si$_2$ with commensurate magnetic orders. CeRh$_2$Si$_2$ has two successive antiferromagnetic (AF) orders in zero magnetic field. The high temperature phase (AF1 phase) has the magnetic modulation wave vector of $q = (\frac{1}{2}, \frac{1}{2}, 0)$, and the low temperature phase (AF2 phase) is characterized by the four $q$-vectors of $q = (\frac{1}{2}, \frac{1}{2}, 0), (\frac{1}{2}, -\frac{1}{2}, 0), (\frac{1}{2}, \frac{1}{2}, \frac{1}{2})$, and $(\frac{1}{2}, -\frac{1}{2}, \frac{1}{2})$. By measuring the uniaxial stress dependence of the magnetization, resistivity and the intensities of magnetic Bragg reflections, we confirmed that the AF1 phase has the single-$q$ magnetic order with two-fold rotational symmetry and the AF2 phase has the multi-$q$ magnetic order with four-fold rotational symmetry. In order to understand the origin of multi-$q$ order of CeRh$_2$Si$_2$, we also performed inelastic neutron scattering measurement on the single crystal samples. We found a magnetic excitation at the transfer energy $\hbar ω\sim$ 8 meV. By applying the linear spin-wave theory, we found that the nearest and the next-nearest neighbor exchange interactions on the $ab$-plane, $J_1$ and $J_2$, are dominant in the AF2 phase. However, the $J_1$-$J_2$ model cannot lift the degeneracy between the single-$q$ (AF1) and multi-$q$ (AF2) phases. We suggest that it can be lifted by taking into account the biquadratic interaction derived from the perturbative expansion for the Kondo lattice Hamiltonian. [S. Hayami et al., Phys. Rev. B 95, 224424 (2017).

cond-mat.str-el↗

arXiv2023

Direct observations of spin fluctuations in spin-hedgehog-anti-hedgehog lattice states in MnSi$_{1-x}$Ge$_x$ ($x=0.6$ and $0.8$) at zero magnetic field

The helimagnetic compounds MnSi$_{1-x}$Ge$_{x}$ show the three-dimensional multiple-$q$ order as referred to as spin-hedgehog-anti-hedgehog (SHAH) lattice. Two representative forms of SHAH are cubic-3$q$ lattice with $q \| \langle100\rangle$ and tetrahedral-4$q$ lattice with $q \| \langle111\rangle$, which show up typically for $x=1.0-~0.8$ and for $x=0.6$, respectively. Here, we have investigated the spin fluctuations in the MnSi$_{1-x}$Ge$_{x}$ polycrystalline samples with $x=0.6$ and $0.8$ by using the time-of-flight (TOF) neutron inelastic scattering and MIEZE-type neutron spin echo techniques to elucidate the microscopic origin of the unconventional Hall effect in the SHAH lattice states. This research is motivated by the observation of a sign change in the unconventional Hall resistivity as a function of temperature [Y. Fujishiro et al., Nat. Comm. $\textbf{10}$, 1059 (2019)]. The present results reveal the correspondences between the temperature ranges where the positive Hall resistivity and spin fluctuations are observed. These results agree well with the theoretical model of the conduction electrons scattered by the fluctuating spin clusters with a non-zero average of sign-biased scalar spin chirality as a mechanism of the positive Hall resistivity [H. Ishizuka and N. Nagaosa, Sci. Adv. $\textbf{4}$, eaap9962 (2018)].

cond-mat.str-el↗

arXiv2022

Inelastic Neutron Scattering in Weakly Coupled Triangular Spin Tubes CsCrF$_4$

We performed inelastic neutron scattering (INS) experiments to measure spin dynamics on a polycrystalline sample of a spin tube candidate CsCrF$_{4}$. The compound exhibits a successive phase transition from a paramagnetic phase through an intermediate temperature (IT) phase of a 120$^{\circ}$ structure to a low temperature (LT) phase of another 120$^{\circ}$ structure. Elaborate comparison between observed and calculated neutron spectra in LT phase reveals that the spin Hamiltonian is identified as antiferromagnetic spin tubes including perturbative terms of intertube interaction, Dzyaloshinskii-Moriya interaction, and single ion anisotropy. A phase diagram for the ground state is classically calculated. A set of parameters in the spin Hamiltonian obtained from the INS spectra measured in LT phase is quite close to a boundary to the phase of the 120$^{\circ}$ structure of IT phase. The INS spectra measured in IT phase is, surprisingly, the same as those in LT phase in the level of powder averaged spectra, even though the magnetic structures in IT and LT phases are different. Identical dynamical structures compatible with two different static structures are observed. No difference in the observed spectra indicates no change of the spin Hamiltonian with the temperature, suggesting that the origin of the successive phase transition being order-by-disorder mechanism.

cond-mat.other↗

arXiv2022

Spin Excitation in Coupled Honeycomb Lattice Ni$_2$InSbO$_6$

We performed an inelastic neutron scattering experiment on a polycrystalline sample of a helimagnet Ni$_2$InSbO$_6$ to construct the spin Hamiltonian. Well-defined spin-wave excitation with a band energy of 20 meV was observed below $T_{N} = 76$ K. Using the linear spin-wave theory, the spectrum was reasonably reproduced with honeycomb spin layers coupled along the stacking axis (the $c$ axis). The proposed spin model reproduces the soliton lattice induced by a magnetic field applied perpendicular to the $c$ axis.

cond-mat.str-el↗

arXiv2022

Antiferromagnetic Kitaev interaction in $J_\rm{eff}=1/2$ cobalt honeycomb materials Na$_3$Co$_2$SbO$_6$ and Na$_2$Co$_2$TeO$_6$

Finding new materials with antiferromagnetic (AFM) Kitaev interaction is an urgent issue to broaden and enrich the quantum magnetism research significantly. By carrying out inelastic neutron scattering experiments and subsequent analysis, we conclude that Na$_3$Co$_2$SbO$_6$ and Na$_2$Co$_2$TeO$_6$ are new honeycomb cobalt-based AFM Kitaev systems. The spin-orbit excitons at 20-28~meV in both compounds strongly supports the idea that Co$^{2+}$ ions of both compounds have a spin-orbital entangled $J_\rm{eff}=1/2$ state. Furthermore, we found that a generalized Kitaev-Heisenberg Hamiltonian can well describe the spin-wave excitations of both compounds with additional 3rd nearest-neighbor interaction. Our best-fit parameters show large AFM Kitaev terms and off-diagonal symmetric anisotropy terms of a similar magnitude in both compounds. We should stress that our parameters' optimized magnetic structures are consistent with the magnetic structures reported from neutron diffraction studies. Moreover, there is also the magnon-damping effect at the higher energy part of the spin waves, as usually observed in other Kitaev magnets. We demonstrate that Na$_3$Co$_2$SbO$_6$ and Na$_2$Co$_2$TeO$_6$ are the first experimental realization of AFM Kitaev magnets based on the systematic studies of the spin waves and analysis.

cond-mat.str-el↗

arXiv2021

Field-induced quantum spin disordered state in spin-1/2 honeycomb magnet Na2Co2TeO6

Spin-orbit coupled honeycomb magnets with the Kitaev interaction have received a lot of attention due to their potential of hosting exotic quantum states including quantum spin liquids. Thus far, the most studied Kitaev systems are 4d/5d-based honeycomb magnets. Recent theoretical studies predicted that 3d-based honeycomb magnets, including Na2Co2TeO6 (NCTO), could also be a potential Kitaev system. Here, we have used a combination of heat capacity, magnetization, electron spin resonance measurements alongside inelastic neutron scattering (INS) to study NCTO's quantum magnetism, and we have found a field-induced spin disordered state in an applied magnetic field range of 7.5 T < B (vertical to b-axis) < 10.5 T. The INS spectra were also simulated to tentatively extract the exchange interactions. As a 3d-magnet with a field-induced disordered state on an effective spin-1/2 honeycomb lattice, NCTO expands the Kitaev model to 3d compounds, promoting further interests on the spin-orbital effect in quantum magnets.

cond-mat.str-el↗

arXiv2021

Damped Dirac magnon in a metallic kagome antiferromagnet FeSn

The kagome lattice is a fertile platform to explore topological excitations with both Fermi-Dirac and Bose-Einstein statistics. While relativistic Dirac Fermions and flat-bands have been discovered in the electronic structure of kagome metals, the spin excitations have received less attention. Here we report inelastic neutron scattering studies of the prototypical kagome magnetic metal FeSn. The spectra display well-defined spin waves extending up to 120 meV. Above this energy, the spin waves become progressively broadened, reflecting interactions with the Stoner continuum. Using linear spin wave theory, we determine an effective spin Hamiltonian that reproduces the measured dispersion. This analysis indicates that the Dirac magnon at the K-point remarkably occurs on the brink of a region where well-defined spin waves become unobservable. Our results emphasize the influential role of itinerant carriers on the topological spin excitations of metallic kagome magnets.

cond-mat.str-el↗

arXiv2020

Spin-waves in 2D honeycomb lattice $XXZ$-type van der Waals antiferromagnet CoPS$_3$

The magnetic excitations in CoPS$_3$, a two-dimensional van der Waals (vdW) antiferromagnet with spin $S=3/2$ on a honeycomb lattice, has been measured using powder inelastic neutron scattering. Clear dispersive spin waves are observed with a large spin gap of ~13 meV. The magnon spectra were fitted using an $XXZ$-type $J_1-J_2-J_3$ Heisenberg Hamiltonian with a single-ion anisotropy assuming no magnetic exchange between the honeycomb layers. The best-fit parameters show ferromagnetic exchange $J_1=-2.08$ meV and $J_2=-0.26$ meV for the nearest and second-nearest neighbors and a sizeable antiferromagnetic exchange $J_3=4.21$ meV for the third-nearest neighbor with the strong easy-axis anisotropy $K=-2.06$ meV. The suitable fitting could only be achieved by the anisotropic $XXZ$-type Hamiltonian, in which the exchange interaction for the out-of-plane component is smaller than that for the in-plane one by a ratio $α=J_z/J_x=0.6$. Moreover, the absence of spin-orbit exciton around 30 meV indicates that Co$^{2+}$ ions in CoPS$_3$ have a $S=3/2$ state rather than a spin-orbital entangled $J_\rm{eff}=1/2$ ground state. Our result directly shows that CoPS$_3$ is an experimental realization of the $XXZ$ model with a honeycomb lattice in 2D vdW magnets.

cond-mat.mtrl-sci↗

arXiv2020

Origin of magnetovolume effect in a cobaltite

The layered perovskite PrBaCo2O5.5+x demonstrates a strong negative thermal expansion (NTE) which holds potential for being fabricated into composites with zero thermal expansion. The NTE was found to be intimately associated with the spontaneous magnetic ordering, known as magnetovolume effect (MVE). Here we report with compelling evidences that the continuous-like MVE in PrBaCo2O5.5+x is intrinsically of discontinuous character, originating from an magnetoelectric transition from an antiferromagnetic insulating large-volume (AFILV) phase to a ferromagnetic metallic small-volume (FMSV) phase. Furthermore, the magnetoelectric effect (ME) shows high sensitivity to multiple external stimuli such as temperature, carrier doping, hydrostatic pressure, magnetic field etc. In contrast to the well-known ME such as colossal magnetoresistance and multiferroic effect which involve symmetry breaking of crystal structure, the ME in the cobaltite is purely isostructural. Our discovery provides a new pathway to realizing the ME as well as the NTE, which may find applications in new techniques.

cond-mat.str-el↗

arXiv2019

Noncoplanar ferrimagnetism and local crystalline-electric-field anisotropy in the quasicrystal approximant Au$_{70}$Si$_{17}$Tb$_{13}$

Neutron scattering experiments have been performed to elucidate magnetic properties of the quasicrystal approximant Au$_{70}$Si$_{17}$Tb$_{13}$, consisting of icosahedral spin clusters in a body-centered-cubic lattice. Bulk magnetic measurements performed on the single crystalline sample unambiguously confirm long-range ordering at $T_{\rm C} = 11.6 \pm 1$ K. In contrast to the simple ferromagnetic response in the bulk measurements, single crystal neutron diffraction confirms a formation of intriguing non-collinear and non-coplanar magnetic order. The magnetic moment direction was found to be nearly tangential to the icosahedral cluster surface in the local mirror plane, which is quite similar to that recently found in the antiferromagnetic quasicrystal approximant Au$_{72}$Al$_{14}$Tb$_{14}$. Inelastic neutron scattering on the powdered sample exhibits a very broad peak centered at $\hbar ω\simeq 4$ meV. The observed inelastic spectrum was explained by the crystalline-electric-field model taking account of the chemical disorder at the fractional Au/Si sites. The resulting averaged anisotropy axis for the crystalline-electric-field ground state is consistent with the ordered moment direction determined in the magnetic structure analysis, confirming that the non-coplanar magnetic order is stabilized by the local uniaxial anisotropy.

cond-mat.mtrl-sci↗

arXiv2019

Novel Excitations near Quantum Criticality in Geometrically Frustrated Antiferromagnet CsFeCl$_{3}$

Investigation of materials that exhibit quantum phase transition provides valuable insights into fundamental problems in physics. We present neutron scattering under pressure in a triangular-lattice antiferromagnet which has a quantum disorder in the low-pressure phase and a noncollinear structure in the high-pressure phase. The neutron spectrum continuously evolves through the critical pressure; a single mode in the disordered state becomes soft with the pressure, and it splits into gapless and gapped modes in the ordered phase. Extended spin-wave theory reveals that the longitudinal and transverse fluctuations of spins are hybridized in the modes because of the noncollinearity, and novel magnetic excitations are formed. We report a new hybridization of the phase and amplitude fluctuations of the order parameter in a spontaneously symmetry-broken state.

cond-mat.str-el↗

arXiv2018

Neutron spin resonance in the 112-type iron-based superconductor

We use inelastic neutron scattering to study the low-energy spin excitations of 112-type iron pnictide Ca$_{0.82}$La$_{0.18}$Fe$_{0.96}$Ni$_{0.04}$As$_{2}$ with bulk superconductivity below $T_c=22$ K. A two-dimensional spin resonance mode is found around $E=$ 11 meV, where the resonance energy is almost temperature independent and linearly scales with $T_c$ along with other iron-based superconductors. Polarized neutron analysis reveals the resonance is nearly isotropic in spin space without any $L$ modulations. Due to the unique monoclinic structure with additional zigzag arsenic chains, the As $4p$ orbitals contribute to a three-dimensional hole pocket around $Γ$ point and an extra electron pocket at $X$ point. Our results suggest that the energy and momentum distribution of spin resonance does not directly response to the $k_z$ dependence of fermiology, and the spin resonance intrinsically is a spin-1 mode from singlet-triplet excitations of the Cooper pairs in the case of weak spin-orbital coupling.

cond-mat.supr-con↗

arXiv2017

YUI and HANA: Control and Visualization Programs for HRC in J-PARC

We developed control and visualization programs, YUI and HANA, for High- Resolution Chopper spectrometer (HRC) installed at BL12 in MLF, J-PARC. YUI is a comprehensive program to control DAQ-middleware, the accessories, and sample environment devices. HANA is a program for the data transformation and visualization of inelastic neutron scattering spectra. In this paper, we describe the basic system structures and unique functions of these programs from the viewpoint of users.

physics.ins-det↗