arXiv subjects

Wataru Koshibae

Publications and source records attributed to Wataru Koshibae.

11 recordsLinked to original sources

arXiv2026

Equivalent Circuit Representation and Thermodynamic Limitations of Non-Adiabatic Spin-Transfer Torque effect

We present an equivalent circuit representation of the Thiele equation for the current-driven domain walls. We show that the non-adiabatic spin-transfer torque (β-term) cannot be represented by passive elements alone (such as resistors, inductors, and capacitors) and necessarily requires active elements, corresponding to negative dissipation. Consequently, assuming that the driving current is the only power source for the texture dynamics, directly incorporating the β-term introduces a fundamental contradiction with thermodynamic principles. This equivalent circuit theory indicates that the interpretation of experiments solely through the β-term does not fully capture the underlying physics, highlighting the need to account for effects distinct from the spin dynamics.

cond-mat.mes-hall↗

arXiv2026

Visualizing Microwave-Driven Dynamics of Antiskyrmions and Surface Skyrmions

Microwaves provide coherent access to low-energy excitations and serve as effective probes of high-frequency spin dynamics in quantum and magnetic systems. For topological spin textures, microwave excitation is expected to generate rich collective responses, yet direct real-space observation of ultrafast dynamics remains limited. Here we use time-resolved Lorentz transmission electron microscopy to visualize microwave-driven dynamics in a hybrid antiskyrmion structure composed of a central antiskyrmion and surface skyrmions. We resolve the picosecond evolution of antiskyrmion area and second-harmonic signals, evidencing nonlinear responses of spin textures under microwave excitations. We track the core motions of the antiskyrmion and surface skyrmions, which follow distinct trajectories while sharing the same rotational sense. Micromagnetic simulations reproduce the key observations and associate the dynamic modes with the spatial modulation of the core profile along the thickness. These achievements establish ultrafast electron microscopy as a powerful real-space approach for probing high-frequency microwave-driven dynamics of topological magnetic solitons.

cond-mat.mtrl-sci↗

arXiv2026

Josephson diode effect via a non-equilibrium Rashba system

A non-equilibrium state in a Rashba system under an in-plane magnetic field is identified as the origin of the Josephson diode effect. This state is induced by a current bias--necessary for measuring the current-voltage characteristics--which shifts the Fermi momentum away from equilibrium. This essential mechanism has been overlooked in previous studies. This oversight stems from the implicit assumption that the equilibrium-based formulations are sufficient to describe Josephson effect. We formulate the Josephson coupling via the non-equilibrium Rashba system under current bias using a tunneling Hamiltonian, where the Rashba system is modeled as one-dimensional. When the magnetic field is applied perpendicular to the current, the Josephson coupling becomes asymmetric, giving rise to the diode effect. The magnitude and sign of this effect depend on the distance between the superconducting electrodes $d$, the in-plane magnetic field, and the spin-orbit coupling strength. Our results clarify the microscopic origin of the Josephson diode effect, which can be optimized by tuning $d$.

cond-mat.supr-con↗

arXiv2025

Reply to "Comment on "Reconsidering the nonlinear emergent inductance: time-varying Joule heating and its impact on the AC electrical response"" by Yokouch et al

This is a response to the comments [arXiv:2407.15682 and Phys.Rev.B 111, 146401 (2025)] by Yokouchi et al. on our paper in Phys.Rev.B 110, 174402 (2024). In this Reply, we note that (i) their arguments lack a discussion of whether the overall characteristics of the observed nonlinear impedance, including its magnitude and unphysical negative inductance interpretation, can be explained by the emergent induction scenario, whereas the Joule heating model can, (ii) they incorrectly refer to the Joule heating model, and (iii) their new data in the Comment are also quantitatively explained by the Joule heating model. These findings suggest that, contrary to the opinion by Yokouchi et al., the overall behavior of the observed impedance is irrelevant to emergent induction.

cond-mat.mtrl-sci↗

arXiv2024

Reconsidering the nonlinear emergent inductance: time-varying Joule heating and its impact on the AC electrical response

A nonlinearly enhanced electrical reactance, $\Im Z$, under a large AC current has been measured to explore emergent inductors, which constitute a new class of inductors based on the spin-transfer torque effect. A nonlinear $\Im Z$ has been observed in conducting magnets that contain noncollinear spin textures and interpreted as the realization of an inductance due to current-induced spin dynamics. However, curious behavior has concomitantly been observed. For instance, the nonlinear $\Im Z$ always has a cutoff frequency of $10^0$--$10^4$ Hz, which is much lower than the resonance frequency of a ferromagnetic domain wall, $\sim$10$^7$ Hz; furthermore, the temperature and magnetic field variations in $\Im Z$ appear to be considerably correlated with those in the temperature derivative of resistance. This behavior appears to be difficult to understand in terms of the current-induced spin dynamics, and therefore, the earlier interpretation of the nonlinear $\Im Z$ should be further verified. Here, we theoretically and experimentally show that time-varying Joule heating and its impact on the AC electrical response can naturally explain these observations. In the experimental approach, we study the nonlinear AC electrical response of two conducting materials that exhibit no magnetic order, CuIr$_2$S$_4$ and 1$T$'-MoTe$_2$. Under time-varying Joule heating, a nonlinearly enhanced $\Im Z$ with the curious behavior mentioned above is observed in both systems. Our study implies that the nonlinear $\Im Z$ previously observed in noncollinear magnets includes a considerable contribution of the Joule-heating-induced apparent AC impedance.

cond-mat.mtrl-sci↗

arXiv2023

Symmetry of the emergent inductance tensor exhibited by magnetic textures

Metals hosting gradually varying spatial magnetic textures are attracting attention as a new class of inductor. Under the application of an alternating current, the spin-transfer-torque effect induces oscillating dynamics of the magnetic texture, which subsequently yields the spin-motive force as a back action, resulting in an inductive voltage response. In general, a second-order tensor representing a material's response can have an off-diagonal component. However, it is unclear what symmetries the emergent inductance tensor has and also which magnetic textures can exhibit a transverse inductance response. Here we reveal both analytically and numerically that the emergent inductance tensor should be a symmetric tensor in the so-called adiabatic limit. By considering this symmetric tensor in terms of symmetry operations that a magnetic texture has, we further characterize the magnetic textures in which the transverse inductance response can appear. This finding provides a basis for exploring the transverse response of emergent inductors, which has yet to be discovered.

cond-mat.mtrl-sci↗

arXiv2023

Electron Hydrodynamics by Spin Hall Effect

Electron hydrodynamics is currently known to emerge only when electron-electron interaction dominates over the momentum-nonconserving scatterings of electrons, where the electron transport is described by a hydrodynamic equation. Here we show that electron transport in electron systems with the spin Hall effect is also given by the hydrodynamic equation, whose kinetic viscosity is determined by the spin diffusion length and the transport lifetime. The electric current vorticity is proportional to the spin accumulation due to the spin Hall effect in two-dimensional systems. We demonstrate by solving the hydrodynamic equation in a two-dimensional system with a cavity, combined with micromagnetic simulation for an attached chiral magnetic insulator, that the spin accumulated near the boundary of the cavity creates a magnetic skyrmion. Our findings and demonstration shed light on a novel aspect of electron hydrodynamics and spin transport.

cond-mat.mes-hall↗

arXiv2020

Bulk and surface topological indices for a skyrmion string

The magnetic skyrmion is a topological magnetic vortex, and its topological nature is characterized by an index called skyrmion number which is a mapping of the magnetic moments defined on a two-dimensional space to a unit sphere. In three-dimensions, a skyrmion, i.e., a vortex penetrating though the magnet naturally forms a string, which terminates at the surfaces of the magnet or in the bulk. For such a string, the topological indices, which control its topological stability are less trivial. Here, we show theoretically, in terms of numerical simulation for the current-driven motion of a skyrmion string in a film sample with the step edges on the surface, that the topological indices relevant to the stability are the followings; (i) skyrmion number along the developed surface, and (ii) the monopole charge in the bulk defined as the integral over the surface enclosing a singular magnetic configuration. As long as the magnetic configuration is slowly varying, the former is conserved while its changes is associated with nonzero monopole charge. The skyrmion number and the monoplole charge offer a coherent understanding of the stability of the topological magnetic texture and the nontrivial dynamics of skyrmion strings.

cond-mat.mes-hall↗

arXiv2020

Slow steady flow of a skyrmion lattice in a confined geometry probed by resistance narrow-band noise

Using resistance fluctuation spectroscopy, we observe current-induced narrow-band noise (NBN) in the magnetic skyrmion-lattice phase of micrometer-sized MnSi. The NBN appears only when electric-current density exceeds a threshold value, indicating that the current-driven motion of the skyrmion lattice triggers the NBN. The observed NBN frequency is 10-10$^4$ Hz at $\sim$10$^{9}$ A/m$^{2}$, implying a skyrmion steady flow velocity of 1-100 $μ$m/s, 3-5 orders of magnitude slower than previously reported. The temperature evolution of the NBN frequency suggests that the steady flow entails thermally activated processes, which are most likely due to skyrmion creation and annihilation at the sample edges. This scenario is qualitatively supported by our numerical simulations considering boundary effects, which reveals that the edges limit the steady flow of skyrmions, especially at low temperatures. We discuss a mechanism that dramatically slows the skyrmion steady flow in a microfabricated specimen.

cond-mat.mes-hall↗

arXiv2012

Towards precise measurement of oscillatory domain wall by ferromagnetic Josephson junction

We theoretically propose a principle for precise measurement of oscillatory domain wall (DW) by a ferromagnetic Josephson junction, which is composed of a ferromagnetic wire with DW and two superconducting electrodes. The current-voltage curve exhibits stepwise structures, only when DW oscillates in the ferromagnetic wire. The voltage step appears at V = n(\hbar/2e)ω_DW with the fundamental constant \hbar/e, integer number n, and the DW frequency ω_DW. Since V can be determined in the order of 10^9 accuracy, the oscillatory DW will be measured more precisely than present status by conventional method.

cond-mat.supr-con↗

arXiv1998

``Flux'' state in double exchange model

We study the ground state properties of the double-exchange systems. The phase factor of the hopping matrix elements arises from $t_{2g}$ spin texture in two or more dimensions. A novel ``flux'' state is stabilized against the canted antiferromagnetic and spiral spin states. In a certain range of hole doping, the phase separation occurs between the ``flux'' state and antiferromagnetic states. Constructing a trial state which provides the rigorous upper bound on the ground state, we show that the metallic canted antiferromagnetic state is not stable in the double exchange model.

cond-mat.stat-mech↗