arXiv subjects

Yuki Shiomi

Publications and source records attributed to Yuki Shiomi.

At least 19 recordsLinked to original sources

arXiv2026

Electrical detection of spin-flip transition in metal/\ce{Na_5Co_{15.5}Te_6O_{36}} heterostructure

We report on the longitudinal magnetoresistance (MR) in thin metal films on an Ising-type antiferromagnetic insulator, \ce{Na_5Co_{15.5}Te_6O_{36}} (\ce{NCTO}). Steep changes in the MR spectra with hysteresis were observed at spin-flip transitions driven by magnetic fields applied along the easy axis of the \ce{NCTO} crystal. The MR jumps almost follow step-like changes in magnetization at the spin-flip transition. At very low temperatures where \ce{Co} moments are partially frozen, the MR anomalies exhibit a tunnel-magnetoresistance-like shape. The observed MR anomalies at the spin-flip transition are attributed to strain effects via magnetostriction upon the magnetic-structure change of the \ce{Co} nets in \ce{NCTO}, because similar MR jumps are observed in \ce{Pt/NCTO}, \ce{Pt/SiO_x/NCTO}, and \ce{Cu/NCTO} heterostructures. Interestingly, we found that the high-field slopes of the MR spectra show opposite signs between \ce{Pt/NCTO} and \ce{Cu/NCTO} at low temperatures. Because similar MR spectra are observed for \ce{Pt/SiO_x/NCTO} in which the interface magnetic interaction is negligible, the weak antilocalization due to the strong spin-orbit interaction of the Pt films is likely to contribute to the low-temperature MR.

cond-mat.str-el↗

arXiv2026

Hydrogenation-induced gigantic resistance decrease of palladium films deposited by high pressure magnetron sputtering

We demonstrate a pronounced decrease in the electrical resistance of highly disordered palladium (Pd) films deposited under a high working Ar pressure using a compact film coating system. The resulting resistance change ratio of up to $1/335$ is predominant among those reported previously. Film characterization suggests two primary mechanisms responsible for this significant resistance reduction: atomic force microscopy observation indicates improved electrical contacts among Pd grains, and X-ray diffraction measurement demonstrates hydrogenation-induced crystallization of Pd. These findings offer a simple scheme to enhance hydrogen sensor performance and can contribute to a more comprehensive understanding of the hydrogenation process in Pd.

cond-mat.mtrl-sci↗

arXiv2025

Nonlinear planar Hall effect from superconducting vortex motion

We report the nonreciprocal charge transport along the longitudinal and transverse directions in the vortex flow regime of FeSe superconducting films. Clear nonreciprocal signals under an inplane magnetic field reveals symmetry breaking at the film surfaces since the crystal structure of FeSe is centrosymmetric. Although the symmetry in such polar superconductors allows the nonreciprocal transverse response under a magnetic field parallel to the electric current, its observation is physically counterintuitive because vortex motion is not expected in this configuration. We propose that thermally excited (anti)vortices due to the two-dimensional nature of FeSe give rise to the nonreciprocal transverse signals when the mirror symmetry is broken by the inplane magnetic field.

cond-mat.supr-con↗

arXiv2025

Signature of BKT-like spin transport in a quasi-2D antiferromagnet BaNi$_2$V$_2$O$_8$

In two-dimensional (2D) spin systems, the augmentation of spin fluctuations gives rise to quasi-long-range order; however, how they manifest in spin transport remains unclear. Here we investigate the spin Seebeck effect (SSE) in a quasi-2D antiferromagnet, BaNi$_2$V$_2$O$_8$, which has been reported to exhibit the Berezinskii-Kosterlitz-Thouless (BKT) transition owing to its distinct 2D nature. We found that the SSE in Pt / BaNi$_2$V$_2$O$_8$ persists well above the Néel temperature, significantly different from the behavior of 3D ordered magnets. Our numerical analysis for a 2D microscopic spin model supports the hypothesis that the observed SSE is linked to strong magnetic correlations in the BKT-like phase.

cond-mat.str-el↗

arXiv2024

Electric-field-enhanced second-harmonic domain contrast and nonreciprocity in a van der Waals antiferromagnet

Imaging antiferromagnetic 180° domains with actively controlled visibility is vital for both fundamental science and sophisticated applications. While optical second-harmonic generation (SHG) is a well-known technique for distinguishing such domains in non-centrosymmetric antiferromagnets, a general material-based strategy to control domain contrast remains elusive. Using van der Waals antiferromagnet MnPS$_3$ as a proof of concept, we demonstrate the tuning of nonreciprocity-induced domain contrast in SHG through applying an in-plane electric field that transforms the magnetic point group to its unitary subgroup. The interference among intrinsic electric-dipole, magnetic-dipole, and field-induced electric-dipole transitions, each carrying distinct characters under space-inversion ($\mathcal{P}$) and time-reversal ($\mathcal{T}$) operations, enables large tuning of domain contrast and nonreciprocity in a broad spectral range. This strategy, generically applicable to systems characterized by $\mathcal{PT}$-symmetric magnetic groups with a polar unitary subgroup, offers a path to fast electrical modulation of nonlinear nonreciprocal photonic behaviors using antiferromagnets.

cond-mat.str-el↗

arXiv2023

Emergent electric field induced by current-driven domain wall motion in a room-temperature antiferromagnet FeSn2

Antiferromagnets have attracted extensive interest as platforms for nanoscale spintronic devices owing to ultrafast spin dynamics and lack of a stray field. One of the crucial missing pieces in antiferromagnets is a quantum-mechanical electric field known as an emergent electric field, which has been observed for the motion of ferromagnetic spin texture. Since this phenomenon allows for the development of novel spintronic devices such as quantum inductors, its identification in antiferromagnets is vital for developing nanoscale spintronic devices. Here, we demonstrate that the motion of antiferromagnetic spin textures generates an emergent electric field. In a room-temperature antiferromagnet FeSn2, we observed large current-nonlinear responses in the imaginary part of the complex impedance at and well above room temperature. This signal is attributed to an emergent electric field resulting from the nonadiabatic electron spin dynamics during the current-induced motion of antiferromagnetic domain walls. Notably, the observed electric response is strongly enhanced as the sample size decreases and robust against magnetic fields. Our finding may pave the way for novel nanoscale quantum spintronic devices.

cond-mat.mes-hall↗

arXiv2023

Exciton-magnon splitting in van der Waals antiferromagnet MnPS$_3$ unveiled by second-harmonic generation

Ultrafast and coherent generation of magnons is of great significance for high-speed antiferromagnetic spintronics. One possible route is by exciton-magnon pairwise optical excitation. To date, such exciton-magnon transitions have been studied mostly by linear optical means in a limited number of conventional three-dimensional antiferromagnets. Here we investigate this correlated transition in van der Waals antiferromagnet MnPS$_3$ by using resonant second-harmonic generation spectroscopy, a nonlinear optical probe sensitive to the symmetry of electronic and magnetic excitations. Two exciton-magnon peaks are observed, in line with the exciton-induced splitting of magnon density of states predicted by a Koster-Slater type theory, indicating significant exciton-magnon interactions. In addition, a large linear magnetoelectric effect of excitons is observed. These findings provide renewed understandings on the correlation effects in two-dimensional magnets with enhanced quasiparticle scattering cross-sections, and point to potential coherent control among charge, spin and orbital degrees of freedom in two-dimensional magnets by optical means.

cond-mat.str-el↗

arXiv2022

Giant Magnetochiral Anisotropy in Weyl-semimetal WTe2 Induced by Diverging Berry Curvature

The concept of Berry curvature is essential for various transport phenomena. However, an effect of the Berry curvature on magnetochiral anisotropy, i.e. nonreciprocal magneto-transport, is still elusive. Here, we report the Berry curvature originates the large magnetochiral anisotropy. In Weyl-semimetal WTe2, we observed the strong enhancement of the magnetochiral anisotropy when the Fermi level is located near the Weyl points. Notably, the maximal figure of merit $\barγ$ reaches $1.2\,{\times}10^{-6} \rm{m^2T^{-1}A^{-1}}$, which is the largest ever reported in bulk materials. Our semiclassical calculation shows that the diverging Berry curvature at the Weyl points strongly enhances the magnetochiral anisotropy.

cond-mat.mtrl-sci↗

arXiv2022

Pattern recognition with neuromorphic computing using magnetic-field induced dynamics of skyrmions

Nonlinear phenomena in physical systems can be used for brain-inspired computing with low energy consumption. Response from the dynamics of a topological spin structure called skyrmion is one of the candidates for such a neuromorphic computing. However, its ability has not been well explored experimentally. Here, we experimentally demonstrate neuromorphic computing using nonlinear response originating from magnetic-field induced dynamics of skyrmions. We designed a simple-structured skyrmion-based neuromorphic device and succeeded in handwritten digit recognition with the accuracy as large as 94.7 % and waveform recognition. Notably, there exists a positive correlation between the recognition accuracy and the number of skyrmions in the devices. The large degree of freedoms of skyrmion systems, such as the position and the size, originate the more complex nonlinear mapping and the larger output dimension, and thus high accuracy. Our results provide a guideline for developing energy-saving and high-performance skyrmion neuromorphic computing devices.

cs.ET↗

arXiv2021

Triplon current generation in solids

A triplon refers to a fictitious particle that carries angular momentum $S = 1$ corresponding to the elementary excitation in a broad class of quantum dimerized spin systems. Such systems without magnetic order have long been studied as a testing ground for quantum properties of spins. Although triplons have been found to play a central role in thermal and magnetic properties in dimerized magnets with singlet correlation, a spin angular momentum flow carried by triplons, a triplon current, has not been detected yet. Here we report spin Seebeck effects induced by a triplon current: triplon spin Seebeck effect, using a spin-Peierls system CuGeO$_3$. The result shows that the heating-driven triplon transport induces spin current whose sign is positive, opposite to the spin-wave cases in magnets. The triplon spin Seebeck effect persists far below the spin-Peierls transition temperature, being consistent with a theoretical calculation for triplon spin Seebeck effects.

cond-mat.mtrl-sci↗

arXiv2020

Modulation of Dirac electrons in epitaxial Bi2Se3 ultrathin films on van-der-Waals ferromagnet Cr2Si2Te6

We investigated the Dirac-cone state and its modulation when an ultrathin film of topological insulator Bi2Se3 was epitaxially grown on a van-der-Waals ferromagnet Cr2Si2Te6 (CST) by angle-resolved photoemission spectroscopy. We observed a gapless Dirac-cone surface state in 6 quintuple-layer (QL) Bi2Se3 on CST, whereas the Dirac cone exhibits a gap of 0.37 eV in 2QL counterpart. Intriguingly, this gap is much larger than those for Bi2Se3 films on Si(111). We also revealed no discernible change in the gap magnitude across the ferromagnetic transition of CST, suggesting the very small characteristic length and energy scale of the magnetic proximity effect. The present results suggest a crucial role of interfacial coupling for modulating Dirac electrons in topological-insulator hybrids.

cond-mat.mtrl-sci↗

arXiv2019

Magnon Pairs and Spin-Nematic Correlation in the Spin-Seebeck Effect

Investigating exotic magnetic materials with spintronic techniques is effective at advancing magnetism as well as spintronics. In this work, we report unusual field-induced suppression of the spin-Seebeck effect (SSE) in a quasi one-dimensional frustrated spin-$\frac{1}{2}$ magnet LiCuVO$_4$, known to exhibit spin-nematic correlation in a wide range of external magnetic field $B$. The suppression takes place above $|B| > 2$ T in spite of the $B$-linear isothermal magnetization curves in the same $B$ range. The result can be attributed to the growth of the spin-nematic correlation while increasing $B$. The correlation stabilizes magnon pairs carrying spin-2, thereby suppressing the interfacial spin injection of SSE by preventing the spin-1 exchange between single magnons and conduction electrons at the interface. This interpretation is supported by integrating thermodynamic measurements and theoretical analysis on the SSE.

cond-mat.str-el↗

arXiv2019

Spin Seebeck effect in the layered ferromagnetic insulators CrSiTe$_3$ and CrGeTe$_3$

We have studied the longitudinal spin Seebeck effect (LSSE) in the layered ferromagnetic insulators CrSiTe$_3$ and CrGeTe$_3$ covered by Pt films in the measurement configuration where spin current traverses the ferromagnetic Cr layers. The LSSE response is clearly observed in the ferromagnetic phase and, in contrast to a standard LSSE magnet Y$_3$Fe$_5$O$_{12}$, persists above the critical temperatures in both CrSiTe$_3$/Pt and CrGeTe$_3$/Pt samples. With the help of a numerical calculation, we attribute the LSSE signals observed in the paramagnetic regime to exchange-dominated interlayer transport of in-plane paramagnetic moments reinforced by short-range ferromagnetic correlations and strong Zeeman effects.

cond-mat.mtrl-sci↗

arXiv2018

Nonreciprocal spin Seebeck effect in antiferromagnets

We theoretically propose a nonreciprocal spin Seebeck effect, i.e., nonreciprocal spin transport generated by a temperature gradient, in antiferromagnetic insulators with broken inversion symmetry. We find that nonreciprocity in antiferromagnets has rich properties not expected in ferromagnets. In particular, we show that polar antiferromagnets, in which the crystal lacks the spatial inversion symmetry, exhibit perfect nonreciprocity --- one-way spin current flow irrespective of the direction of the temperature gradient. We also show that nonpolar centrosymmetric crystals can exhibit nonreciprocity when a magnetic order breaks the inversion symmetry, and in this case, the direction of the nonreciprocal flow can be controlled by reversing the magnetic domain. As their representatives, we calculate the nonreciprocal spin Seebeck voltages for the polar antiferromagnet $α$-Cu$_2$V$_2$O$_7$ and the honeycomb antiferromagnet MnPS$_3$, while varying temperature and magnetic field.

cond-mat.mes-hall↗

arXiv2018

Spin-current coherence peak in superconductor/magnet junctions

Coherence peak effects in a superconductor induced by a thermal spin current are reported. We measured inverse spin Hall effects induced by spin injection from a ferrimagnetic insulator Y$_3$Fe$_5$O$_{12}$ into a superconductor NbN using longitudinal spin Seebeck effects. In the vicinity of the superconducting transition temperature of the NbN, a large enhancement of the spin Seebeck voltage is observed, whose sign is opposite to that for the vortex Nernst effect, but is consistent with a calculation for a coherence peak effect in the superconductor NbN.

cond-mat.mes-hall↗

arXiv2017

Magnetic thermal conductivity far above the Néel temperatures in the Kitaev-magnet candidate $α$-RuCl$_{3}$

We have investigated the longitudinal thermal conductivity of $α$-RuCl$_{3}$, the magnetic state of which is considered to be proximate to a Kitaev honeycomb model, along with the spin susceptibility and magnetic specific heat. We found that the temperature dependence of the thermal conductivity exhibits an additional peak around 100 K, which is well above the phonon peak temperature ($\sim$ 50 K). The higher-temperature peak position is comparable to the temperature scale of the Kitaev couplings rather than the Néel temperatures below 15 K. The additional heat conduction was observed for all five samples used in this study, and was found to be rather immune to a structural phase transition of $α$-RuCl$_{3}$, which suggests its different origin from phonons. Combined with experimental results of the magnetic specific heat, our transport measurement suggests strongly that the higher-temperature peak in the thermal conductivity is attributed to itinerant spin excitations associated with the Kitaev couplings of $α$-RuCl$_{3}$. A kinetic approximation of the magnetic thermal conductivity yields a mean free path of $\sim$ 20 nm at 100 K, which is well longer than the nearest Ru-Ru distance ($\sim$ 3 Å), suggesting the long-distance coherent propagation of magnetic excitations driven by the Kitaev couplings.

cond-mat.str-el↗

arXiv2017

Fabrication and magnetic control of Y3Fe5O12 cantilevers

We have fabricated ferrite cantilevers in which their vibrational properties can be controlled by external magnetic fields. Submicron-scale cantilever structures were made from Y3Fe5O12 (YIG) films by physical etching combined with use of a focused ion beam milling technique. We found that the cantilevers exhibit two resonance modes which correspond to horizontal and vertical vibrations. Under external magnetic fields, the resonance frequency of the horizontal mode increases, while that of the vertical mode decreases, quantitatively consistent with our numerical simulation for magnetic forces. The changes in resonance frequencies with magnetic fields reach a few percent, showing that efficient magnetic control of resonance frequencies was achieved.

cond-mat.mes-hall↗

arXiv2016

Observation of spin current in quantum spin liquid

Spin liquid is a state of electron spins in which quantum fluctuation breaks magnetic ordering while maintaining spin correlation. It has been a central topic in magnetism because of its relevance to high-Tc superconductivity and topological states. However, utilizing spin liquid has been quite difficult. Typical spin liquid states are realized in one-dimensional spin systems, called quantum spin chains. Here, we show that a spin liquid in a spin-1/2 quantum chain generates and carries spin current via its long-range spin fluctuation. This is demonstrated by observing an anisotropic negative spin Seebeck effect along the spin chains in Sr2CuO3. The results show that spin current can flow even in an atomic channel owing the spin liquid state, which can be used for atomic spin-current wiring.

cond-mat.str-el↗