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Katsuhisa Taguchi

Publications and source records attributed to Katsuhisa Taguchi.

At least 19 recordsLinked to original sources
arXiv2019

Spin-orbit coupling induced valley Hall effects in transition-metal dichalcogenides

In transition-metal dichalcogenides, electrons in the K-valleys can experience both Ising and Rashba spin-orbit couplings. In this work, we show that the coexistence of Ising and Rashba spin-orbit couplings leads to a special type of valley Hall effect, which we call spin-orbit coupling induced valley Hall effect. Importantly, near the conduction band edge, the valley-dependent Berry curvatures generated by spin-orbit couplings are highly tunable by external gates and dominate over the intrinsic Berry curvatures originating from orbital degrees of freedom under accessible experimental conditions. We show that the spin-orbit coupling induced valley Hall effect is manifested in the gate dependence of the valley Hall conductivity, which can be detected by Kerr effect experiments.

cond-mat.mes-hall↗
arXiv2018

Electromagnetic effects induced by time-dependent axion field

We studied the dynamics of the so-called $θ$-term, which exists in topological materials and is related to a hypothetical field predicted by Peccei-Quinn in particle physics, in a magnetic superlattice constructed using a topological insulator and two ferromagnetic insulators, where the ferromagnetic insulators had perpendicular magnetic anisotropies and different magnetic coercive fields. We examined a way to drive the dynamics of the $θ$-term in the magnetic superlattice through changing the inversion symmetry (from an anti-parallel to a parallel magnetic configuration) using an external magnetic field. As a result, we found that unconventional electromagnetic fields, which are magnetic field-induced charge currents and vice versa, are generated by the nonzero dynamics of the $θ$-term.

cond-mat.mes-hall↗
arXiv2018

Spin-Dependent Conductance in a Junction with Dresselhaus Spin-Orbit Coupling

We studied spin-dependent conductance in a normal metal (NM)/NM junction with Dresselhaus spin-orbit coupling (DSOC) and magnetization. As a reference, we also studied the spin-dependent conductance in such a junction with Rashba spin-orbit coupling (RSOC). Using a standard scattering method, we calculated the gate-voltage dependence of the spin-dependent conductances in DSOC and RSOC. In addition, we calculated the gate-voltage dependence of the conductances in a ferromagnetic metal (FM)/NM junction with spin-orbit coupling and magnetization, which we call ferromagnetic spin-orbit metal (FSOM). From these results, we discuss the relation between these conductance in the presence of DSOC and that in the presence of RSOC. We found that conductance in DSOC is the same as that in RSOC for the NM/FSOM junction. In addition, we found that in the FM/FSOM junction, the conductance in DSOC is the same as that in RSOC only when the FM magnetization is along the out-of-plane direction.

cond-mat.mes-hall↗
arXiv2018

Tunneling conductance in two-dimensional junctions between a normal metal and a ferromagnetic Rashba metal

We have studied charge transport in ferromagnetic Rashba metal (FRM), where both Rashba type spin-orbit coupling (RSOC) and exchange coupling coexist. It has nontrivial metallic states, i.e., normal Rashba metal (NRM), anomalous Rashba metal (ARM), and Rashba ring metal (RRM), and they are manipulated by tuning the Fermi level with an applied gate voltage. We theoretically studied tunneling conductance (G) in a normal metal / FRM junction by changing the Fermi level via an applied gate voltage (Vg) on the FRM. We found a wide variation in the Vg dependence of G, which depends on the metallic states. In NRM, the Vg dependence of G is the same as that in a conventional two-dimensional system. However, in ARM, the Vg dependence of G is similar to that in a conventional one (two)-dimensional system for a large (small) RSOC. Furthermore, in RRM, which is generated by a large RSOC, the Vg dependence of the $G$ is similar to that in the one-dimensional system. In addition, these anomalous properties stem from the spin-momentum locking of RSOC rather than the density of states in ARM and RRM because of the large RSOC and exchange coupling.

cond-mat.mes-hall↗
arXiv2017

Valley Edelstein Effect in Monolayer Transition Metal Dichalcogenides

In this work, we predict the emergence of the valley Edelstein Effect (VEE), which is an electric-field-induced spin polarization effect, in gated monolayer transition metal dichalcogenides (MTMDs). We found an unconventional valley-dependent response in which the spin-polarization is parallel to the applied electric field with opposite spin-polarization generated by opposite valleys. This is in sharp contrast to the conventional Edelstein effect in which the induced spin-polarization is perpendicular to the applied electric field. We identify the origin of VEE as combined effects of conventional Edelstein effect and valley-dependent Berry curvatures induced by coexisting Rashba and Ising SOCs in gated MTMDs. Experimental schemes to detect the VEE are also considered.

cond-mat.mes-hall↗
arXiv2016

Photovoltaic anomalous Hall effect in line-node semimetals

We theoretically study the circularly polarized light-induced Floquet state in line-node semimetals with time-reversal symmetry and inversion symmetry. It is found that the Floquet state can show the photovoltaic anomalous Hall effect when an applied circularly polarized light gaps out the line node in the bulk and leave Weyl point nodes. The Hall conductivity is sensitive to the location of Fermi level: When the Fermi level locates at the node, the Hall conductivity depends on the radius of line node and is nearly independent of the intensity of light. Away from the line node, the Hall conductivity is dependent on the intensity of light. Such a sensitive Fermi-level dependence of the Hall conductivity in the presence of a weak laser intensity can have applications in phototransistors based on thin films of line-node semimetals.

cond-mat.mes-hall↗
arXiv2016

Photovoltaic Chiral Magnetic Effect

We theoretically predict a generation of a current in Weyl semimetals by applying circularly polarized light. The electric field of the light can drive an effective magnetic field of order of ten Tesla. For lower frequency light, a non-equilibrium spin distribution is formed near the Fermi surface. Due to the spin-momentum locking, a giant electric current proportional to the effective magnetic field is induced. On the other hand, higher frequency light realizes a quasi-static Floquet state with no induced electric current. We discuss relevant materials and estimate order of magnitude of the induced current.

cond-mat.mes-hall↗
arXiv2015

Spin and charge transport induced by a twisted light beam on a surface of a topological insulator

We theoretically study spin and charge transport induced by a twisted light beam irradiated on a disordered surface of a doped three dimensional topological insulator (TI). We find that various types of spin vortices are imprinted on the surface of the TI depending on the spin and orbital angular momentum of the incident light. The key mechanism for the appearance of the unconventional spin structure is the spin-momentum locking in the surface state of the TI. Besides, the diffusive transport of electrons under an inhomogeneous electric field causes a gradient of the charge density, which then induces nonlocal charge current and spin density as well as the spin current. We discuss the relation between these quantities within the linear response to the applied electric field using the Keldysh-Green's function method.

cond-mat.mes-hall↗
arXiv2015

Josephson current in a normal-metal nanowire coupled to superconductor/ferromagnet/superconductor junction

We consider superconducting nanowire proximity coupled to superconductor / ferromagnet / superconductor junction, where the magnetization penetrates into superconducting segment in nanowire decaying as $\sim\exp[-\frac{\mid n \mid}ξ]$ with site index $n$ and the decay length $ξ$. We tune chemical potential and spin-orbit coupling so that topological superconducting regime hosting Majorana fermion is realized for long $ξ$. We find that when $ξ$ becomes shorter, zero energy state at the interface between superconductor and ferromagnet splits into two away from zero energy. Accordingly, the behavior of Josephson current is drastically changed due to this "zero mode-non-zero mode crossover". By tuning the model parameters, we find an almost second-harmonic current-phase relation, $\sin2φ$, with phase difference $φ$. Based on the analysis of Andreev bound state (ABS), we clarify that current-phase relation is determined by coupling of the states within the energy gap. We find that the emergence of crossing points of ABS is a key ingredient to generate $\sin2φ$ dependence in current-phase relation. We further study both the energy and $φ$ dependence of pair amplitudes in the ferromagnetic region. For long $ξ$, odd-frequency spin-triplet $s$-wave component is dominant. The magnitude of the odd-frequency pair amplitude is enhanced at the energy level of ABS.

cond-mat.supr-con↗
arXiv2015

Theory of tunneling conductance of anomalous Rashba metal / superconductor junctions

We theoretically study the charge conductance in anomalous Rashba metal (ARM)/superconductor junctions for various types of the pairing symmetries in the superconductor. The exotic state dubbed ARM, where one of the spin resolved Fermi surface is absent, is realized when the chemical potential is tuned both in the presence of Rashba spin-orbit interaction (RSOI) and an exchange field. Although a fully polarized ferromagnet metal (FPFM) is also a system where the electron's spin degrees of a freedom is reduced to be half, the electrons in an ARM have distinct features from those in FPFM. For the ARM/spin-singlet superconductor junctions, the obtained tunneling conductance within the bulk energy gap is enhanced with the increase in the magnitude of the RSOI. In particular, in ARM/$d_{xy}$-wave superconductor junctions, the zero bias conductance peak is enhanced owing to the presence of the RSOI. For ARM/$p_{x}$-wave superconductor junctions, the condition of the existence of the zero bias conductance peak is significantly sensitive to the direction of the d-vector of the $p_{x}$-wave superconductor. Furthermore, the obtained conductance in ARM/chiral $p$-wave superconductor junctions shows different behaviors as compared to those in ARM/helical $p$-wave superconductor junctions. This feature gives a guide to determine the spin structure of the Cooper pair in spin-triplet superconductor Sr$_{2}$RuO$_{4}$.

cond-mat.supr-con↗
arXiv2015

Spin-charge transport driven by magnetization dynamics on disordered surface of doped topological insulators

We theoretically study the spin and charge generation along with the electron transport on a disordered surface of a doped three-dimensional topological insulator/magnetic insulator junction by using Green's function techniques. We find that the spin and charge current are induced by not only local but also nonlocal magnetization dynamics through nonmagnetic impurity scattering on the disordered surface of the doped topological insulator. We also clarify that the spin current as well as charge density are induced by spatially inhomogeneous magnetization dynamics, and the spin current diffusively propagates on the disordered surface. Using these results, we discuss both local and nonlocal spin torques before and after the spin and spin current generation on the surface, and provide a procedure to detect the spin current.

cond-mat.mes-hall↗
arXiv2014

Axial Current driven by Magnetization Dynamics in Weyl Semimetals

We theoretically study the axial current $\bm{j}_5$ (defined as the difference between the charge current with opposite chirality) in doped Weyl semimetal using a Green's function technique. We show that the axial current is controlled by the magnetization dynamics in a magnetic insulator attached to a Weyl semimetal. We find that the induced axial current can be detected by using ferromagnetic resonance or the inverse spin Hall effect and can be converted into charge current with no accompanying energy loss. These properties make Weyl semimetal advantageous for application to low-consumption electronics with new functionality.

cond-mat.mes-hall↗
arXiv2013

Giant magnetoresistance in the junction of two ferromagnets on the surface of diffusive topological insulators

We reveal the giant magnetoresistance induced by the spin-polarized current in the ferromagnet (F_1)/topological insulator (TI)/ferromagnet (F_2) junction, where two ferromagnets are deposited on the diffusive surface of the TI. We can increase and reduce the value of the giant magnetoresistance by tuning the spin-polarized current, which is controlled by the magnetization configurations. The property is intuitively understood by the non-equilibrium spin-polarized current, which plays the role of an effective electrochemical potential on the surface of the TI.

cond-mat.mes-hall↗
arXiv2012

Active electromagnetic metamaterial based on spin torque oscillators

We propose theoretically an active material for electromagnetic radiation with frequency of GHz by use of spin-torque oscillators. The origin of the amplification is the energy supplied to the magnetization by the injected current. We show that close to a resonance with current-driven magnetization, the imaginary part of magnetic permeability becomes indeed negative for either of the two circular polarizations, resulting in negative imaginary part of refractive index. Besides, the real part of the refractive index is also manipulated by the current. Our system thus realizes an active filter to obtain circular polarized radiation and/or an electromagnetic metamaterial having negative refractive index, both controlled electrically.

cond-mat.mes-hall↗
arXiv2012

Ultrafast magnetic vortex core switching driven by topological inverse Faraday effect

We present a theoretical discovery of an unconventional mechanism of inverse Faraday effect (IFE) which acts selectively on topological magnetic structures. The effect, topological inverse Faraday effect (TIFE), is induced by spin Berry's phase of the magnetic structure when a circularly polarized light is applied. Thus a spin-orbit interaction is not necessary unlike in the conventional IFE. We demonstrate by numerical simulation that TIFE realizes ultrafast switching of a magnetic vortex within a switching time of 150 ps without magnetic field.

cond-mat.mes-hall↗
arXiv2012

Monopoles in ferromagnetic metals

The aim of this short review is to give an introduction to monopoles and to present theoretical derivation of two particular monopoles in ferromagnetic metals, a hedgehog monopole and a spin damping monopole. Spin damping monopoles can be generated in simple systems such as a junction of a ferromagnet and a heavy element with strong spin-orbit interaction such as Pt. This monopole is essential in coupling electronics with magnetism, and is thus expected to play an essential role in spintronics.

cond-mat.mes-hall↗
arXiv2010

Spin relaxation torque in metallic ferromagnets

Spin relaxation torque ${\cal T}$ in uniform metallic ferromagnets with the spin-orbit interaction arising from random impurities is calculated microscopically. ${\cal T}$ is shown to be written by spatial derivatives of the electric field, but is not always given as a divergence of a torque dipole density. The result is due to the fast dissipation of the orbital angular momentum, and thus a conserving spin current cannot be defined. It is discussed that the $β$ term in the spin transfer torque can also be asymmetric.

cond-mat.mes-hall↗