arXiv subjects

Koji Kobayashi

Publications and source records attributed to Koji Kobayashi.

At least 19 recordsLinked to original sources

arXiv2026

Magnetic Weyl semimetals: Interplay of band topology and magnetism

We review recent theoretical and experimental developments in magnetic Weyl semimetals, focusing on the electromagnetic responses emerging from the interplay of their electronic band topology and magnetism. We begin by introducing the fundamental topological properties of the electrons in Weyl semimetals, and provide an overview of the characteristic phenomena arising from their band topology, such as the anomalous Hall effect and chiral magnetic effect. The materials exhibiting the magnetic Weyl semimetal state, with ferromagnetic ordering, antiferromagnetic ordering, etc., are listed. The possible mechanisms for their magnetism are discussed in connection with the Weyl electrons. Non-uniform magnetic textures and magnetization dynamics are expected to exhibit a topological interplay with the Weyl electrons, manifesting as spinmotive force and spin torques. We also review the magnetotransport phenomena such as domain wall magnetoresistance, studied by mesoscopic scale calculations. Finally, we mention the spin transport properties studied in magnetic Weyl semimetals. The topological nature of Weyl electrons reviewed here is important not only for fundamental physics, but also for the potential application to low-dissipative electronics and spintronics devices.

cond-mat.mes-hall↗

arXiv2025

Backsolution: A Framework for Solving Inverse Problems via Automatic Differentiation

We present a simple yet powerful framework for solving inverse problems by leveraging automatic differentiation. Our method is broadly applicable whenever a smooth cost function can be defined near the true solution, and a numerical simulator is available. As a concrete example, we demonstrate that our method can accurately reconstruct the spatial profiles in a conductor from magnetotransport measurements. Even if the given data are insufficient to uniquely determine the profiles, the same framework enables effective reverse modeling. This method is general, flexible, and readily adaptable to a broad class of inverse problems across condensed matter physics and beyond.

cond-mat.dis-nn↗

arXiv2024

Topological Spin-Orbit Torque in Ferrimagnetic Weyl Semimetal

The spin-orbit torque (SOT) in a compensated ferrimagnetic Weyl semimetal, ${\rm Ti}_{2}{\rm MnAl}$, is studied by the linear response theory. We elucidate that the SOT driven by all the occupied electronic states is present in magnetic Weyl semimetal, unlike in conventional metallic magnets. Around the energy of the Weyl points, we find that such an SOT is dominant and almost independent of the disorder. The emergence of the SOT in ${\rm Ti}_{2}{\rm MnAl}$ can be understood from the structure of the mixed Berry curvature around the Weyl points, which is similar to that of the ordinary Berry curvature.

cond-mat.mes-hall↗

arXiv2023

Interactions between atomic-scale skyrmions in 2D chiral magnets

To commercialize stable, highly-integrated, and low-power-consuming magnetic memories of skyrmions, stable control of their motion is crucial. Manipulating the inter-skyrmion interactions is key to achieving this goal. We find that distortion of the skyrmion shape can induce the interskyrmion attraction between atomic-scale small skyrmions, similar to larger skyrmions. Also, the interaction of smaller skyrmions reaches further, when scaled by the skyrmion size, than that of larger skyrmions. The continuum model is still capable of explaining the behavior. Moreover, the formation of the magnetic domains between skyrmions also induces attraction, which is again the same mechanism as larger skyrmions. This study contributes to better control of the skyrmion motion with a wide range of their sizes.

cond-mat.mes-hall↗

arXiv2023

Effective model analysis of intrinsic spin Hall effect with magnetism in stacked-kagome Weyl semimetal Co3Sn2S2

We theoretically study the spin Hall effect in a simple tight-binding model of stacked-kagome Weyl semimetal Co3Sn2S2 with ferromagnetic ordering. We focus on the two types of the spin Hall current: one flowing in the in-plane direction with respect to the kagome lattice (in-plane spin Hall current), and one flowing in the stacking direction (out-of-plane spin Hall current). We show the spin Hall conductivities for those spin currents drastically change depending on the direction of the magnetic moment. Especially, the out-of-plane spin Hall current may induce surface spin accumulation, which are useful for the perpendicular magnetization switching via spin-orbit torque.

cond-mat.mes-hall↗

arXiv2023

Effective Tight-Binding Model of Compensated Ferrimagnetic Weyl Semimetal with Spontaneous Orbital Magnetization

The effective tight-binding model with compensated ferrimagnetic inverse-Heusler lattice Ti$_{2}$MnAl, candidate material of magnetic Weyl semimetal, is proposed. The energy spectrum near the Fermi level, the configurations of the Weyl points, and the anomalous Hall conductivity are calculated. We found that the orbital magnetization is finite, while the total spin magnetization vanishes, at the energy of the Weyl points. The magnetic moments at each site are correlated with the orbital magnetization, and can be controlled by the external magnetic field.

cond-mat.mes-hall↗

arXiv2022

Magnetic orderings from spin-orbit coupled electrons on kagome lattice

We investigate magnetic orderings on kagome lattice numerically from the tight-binding Hamiltonian of electrons, governed by the filling factor and spin-orbit coupling (SOC) of electrons. We find that even a simple kagome lattice model can host both ferromagnetic and noncollinear antiferromagnetic orderings depending on the electron filling, reflecting gap structures in the Dirac and flat bands characteristic to the kagome lattice. Kane--Mele- or Rashba-type SOC tends to stabilize noncollinear orderings, such as magnetic spirals and 120-degree antiferromagnetic orderings, due to the effective Dzyaloshinskii--Moriya interaction from SOC. The obtained phase structure helps qualitative understanding of magnetic orderings in various kagome-layered materials with Weyl or Dirac electrons.

cond-mat.mes-hall↗

arXiv2021

Intrinsic and extrinsic anomalous Hall effects in disordered magnetic Weyl semimetal

We study the intrinsic and extrinsic Hall effects in disordered magnetic Weyl semimetals numerically. We show that in Weyl metals, where the Fermi energy deviates from the Weyl point, the Hall and longitudinal conductances exhibit a specific relation, which is distinguished from the well-known relation in integer quantum Hall systems. Around the Weyl point, the Hall conductance increases with increasing longitudinal conductance. This increasing behavior indicates the existence of additional contributions to the Hall conductance from the subbands of Weyl cones besides that from the bulk Berry curvature. We also show that the extrinsic anomalous Hall effect due to the spin scatterers (skew scattering) is significantly suppressed in Weyl metals.

cond-mat.mes-hall↗

arXiv2021

Wannier-based implementation of the coherent potential approximation with applications to Fe-based transition-metal alloys

We develop a formulation of the coherent potential approximation (CPA) on the basis of the Wannier representation to develop a computationally efficient method for the treatment of homogeneous random alloys that is independent on the applied first-principles electric structure code. To verify the performance of this CPA implementation within the Wannier representation, we examine the Bloch spectral function, the density of states (DOS), and the magnetic moment in Fe-based transition-metal alloys Fe-X (X = V, Co, Ni, and Cu), and compare the results with those of the well-established CPA implementation based on the KKR Green's function method. The Wannier-CPA and the KKR-CPA lead to results very close to each other. The presented Wannier-CPA method has a wide potential applicability to other physical quantities and large compound systems because of its low computational effort required.

cond-mat.mtrl-sci↗

arXiv2021

Ferromagnetic-electrodes-induced Hall effect in topological Dirac semimetals

We propose an unconventional type of Hall effect in a topological Dirac semimetal with ferromagnetic electrodes. The topological Dirac semimetal itself has time-reversal symmetry, whereas attached ferromagnetic electrodes break it, causing the large Hall response. This induced Hall effect is a characteristic of the helical surface/edge states that arise in topological materials, such as topological Dirac semimetals or quantum spin Hall insulators. We compute the Hall conductance/resistance and the Hall angle by using a lattice model with four-terminal geometry. For topological Dirac semimetals with four electrodes, the induced Hall effect occurs whether the current electrodes or the voltage electrodes are ferromagnetic. When the spins in electrodes are almost fully polarized, the Hall angle becomes as large as that of quantum Hall states or ideal magnetic Weyl semimetals. We show the robustness of the induced Hall effect against impurities and also discuss the spin injection and spin decay problems. This Hall response can be used to detect whether the magnetizations of the two ferromagnetic electrodes are parallel or antiparallel.

cond-mat.mes-hall↗

arXiv2021

Quantum Hall effect induced by chiral Landau levels in topological semimetal films

Motivated by recent transport experiments, we theoretically study the quantum Hall effect in topological semimetal films. Owing to the confinement effect, the bulk subbands originating from the chiral Landau levels establish energy gaps that have quantized Hall conductance and can be observed in relatively thick films. We find that the quantum Hall state is strongly anisotropic for different confinement directions not only due to the presence of the surface states but also because of the bulk chiral Landau levels. As a result, we re-examine the quantum Hall effect from the surface Fermi arcs and chiral modes in Weyl semimetals and give a more general view into this problem. Besides, we also find that when a topological Dirac semimetal is confined in its rotational symmetry axis, it hosts both quantum Hall and quantum spin Hall states, in which the helical edge states are protected by the conservation of the spin-$z$ component.

cond-mat.mes-hall↗

arXiv2020

Ballistic transport in disordered Dirac and Weyl semimetals

We study the dynamics of Dirac and Weyl electrons in disordered point-node semimetals. The ballistic feature of the transport is demonstrated by simulating the wave-packet dynamics on lattice models. We show that the ballistic transport survives under a considerable strength of disorder up to the semimetal-metal transition point, which indicates the robustness of point-node semimetals against disorder. We also visualize the robustness of the nodal points and linear dispersion under broken translational symmetry. The speed of the wave packets slows down with increasing disorder strength, and vanishes toward the critical strength of disorder, hence becoming the order parameter. The obtained critical behavior of the speed of the wave packets is consistent with that predicted by the scaling conjecture.

cond-mat.dis-nn↗

arXiv2019

Robust magnetotransport in disordered ferromagnetic kagome layers with quantum anomalous Hall effect

The magnetotransport properties of disordered ferromagnetic kagome layers are investigated numerically. We show that a large domain-wall magnetoresistance or negative magnetoresistance can be realized in kagome layered materials (e.g. Fe$_3$Sn$_2$, Co$_3$Sn$_2$S$_2$, and Mn$_3$Sn), which show the quantum anomalous Hall effect. The kagome layers show a strong magnetic anisotropy and a large magnetoresistance depending on their magnetic texture. These domain-wall magnetoresistances are expected to be robust against disorder and observed irrespective of the domain-wall thickness, in contrast to conventional domain-wall magnetoresistance in ferromagnetic metals.

cond-mat.mes-hall↗

arXiv2018

Helicity-protected domain-wall magnetoresistance in ferromagnetic Weyl semimetal

The magnetotransport properties of disordered ferromagnetic Weyl semimetals are investigated numerically. We found an extraordinarily stable and huge magnetoresistance effect in domain walls of Weyl semimetals. This effect originates from the helicity mismatch of Weyl fermions and is a specific property of Weyl semimetals. Although conventional magnetoresistance effects are strongly suppressed in domain walls where local magnetization varies gradually, the helicity-protected magnetoresistance in Weyl semimetals maintains almost $100\%$ of the magnetoresistance ratio for any kind of thick domain walls, even in the presence of disorder. The contribution of surface Fermi arcs to the magnetoresistance is also discussed.

cond-mat.mes-hall↗

arXiv2018

Circuit complexity and Problem structure in Hamming space

This paper describes about relation between circuit complexity and accept inputs structure in Hamming space by using almost all monotone circuit that emulate deterministic Turing machine (DTM). Circuit family that emulate DTM are almost all monotone circuit family except some NOT-gate which connect input variables (like negation normal form (NNF)). Therefore, we can analyze DTM limitation by using this NNF Circuit family. NNF circuit have symmetry of OR-gate input line, so NNF circuit cannot identify from OR-gate output line which of OR-gate input line is 1. So NNF circuit family cannot compute sandwich structure effectively (Sandwich structure is two accept inputs that sandwich reject inputs in Hamming space). NNF circuit have to use unique AND-gate to identify each different vector of sandwich structure. That is, we can measure problem complexity by counting different vectors. Some decision problem have characteristic in sandwich structure. Different vectors of Negate HornSAT problem are at most constant length because we can delete constant part of each negative literal in Horn clauses by using definite clauses. Therefore, number of these different vector is at most polynomial size. The other hand, we can design high complexity problem with almost perfct nonlinear (APN) function.

cs.CC↗

arXiv2016

Anisotropic magnetotransport in Dirac-Weyl magnetic junctions

We theoretically study the anisotropic magnetotransport in Dirac-Weyl magnetic junctions where a doped ferromagnetic Weyl semimetal is sandwiched between doped Dirac semimetals. We calculate the conductance using the Landauer formula and find that the system exhibits extraordinarily large anisotropic magnetoresistance (AMR). The AMR depends on the ratio of the Fermi energy and the strength of the exchange interaction. The origin of the AMR is the shift of the Fermi surface in the Weyl semimetal and the mechanism is completely different from the conventional AMR originating from the spin dependent scattering and the spin-orbit interaction.

cond-mat.mes-hall↗

arXiv2016

Comparative study of Weyl semimetal, topological and Chern insulators: thin-film point of view

Regarding three-dimensional (3D) topological insulators and semimetals as a stack of constituent 2D topological (or sometimes non-topological) layers is a useful viewpoint. Primarily, concrete theoretical models of the paradigmatic 3D topological phases such as Weyl semimetal (WSM), strong and weak topological insulators (STI/WTI), and Chern insulator (CI), are often constructed in that way. Secondarily, fabrication of the corresponding 3D topological material is also done in the same spirit; epitaxial growth technique is employed, making the resulting sample in the form of a thin film. Here, in this paper we calculate $\mathbb{Z}$- and $\mathbb{Z}_2$-indices and study evolution of the topological properties of such thin films of 3D topological systems, making also a comparative study of CI- vs. TI-type models belonging to different symmetry classes in this respect. Through this comparative study we suggest that WSM is to CI as STI is to WTI. Finally, to test the robustness of our scenario against disorder and relevance to experiments we have also studied numerically the two-terminal conductance of the system using transfer matrix method.

cond-mat.mes-hall↗

arXiv2015

Dimensional crossover of transport characteristics in topological insulator nanofilms

We show how the two-dimensional (2D) topological insulator evolves, by stacking, into a strong or weak topological insulator with different topological indices, proposing a new conjecture that goes beyond an intuitive picture of the crossover from quantum spin Hall to the weak topological insulator. Studying the conductance under different boundary conditions, we demonstrate the existence of two conduction regimes in which conduction happens through either surface- or edge-conduction channels. We show that the two conduction regimes are complementary and exclusive. Conductance maps in the presence and absence of disorder are introduced, together with 2D $\mathbb{Z}_2$-index maps, describing the dimensional crossover of the conductance from the 2D to the 3D limit. Stacking layers is an effective way to invert the gap, an alternative to controlling the strength of spin-orbit coupling. The emerging quantum spin Hall insulator phase is not restricted to the case of odd numbers of layers.

cond-mat.mes-hall↗