Katsuhisa Taguchi

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

Ultrafast magnetic vortex core switching driven by the topological inverse Faraday effect.

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Photovoltaic Chiral Magnetic Effect

)/ topological insulator (TI)/ferromagnet (F

Photovoltaic anomalous Hall effect in line-node semimetals

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

) 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 nonequilibrium spin-polarized current, which plays the role of an effective electrochemical potential on the surface of the TI.

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

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

Active electromagnetic metamaterial based on spin torque oscillators

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.

Monopoles in ferromagnetic metals

We study theoretically 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 creates a gap in the line node in the bulk and leaves Weyl point nodes. The Hall conductivity is sensitive to the location of the Fermi level: When the Fermi level is located at the node, the Hall conductivity depends on the radius of the line node and is nearly independent of the intensity of the light. Far from the line node, the Hall conductivity is dependent on the intensity of the light. The sensitive Fermi-level dependence of the Hall conductivity in the presence of a laser of weak intensity can have applications in phototransistors based on thin films of line-node semimetals.

Theory of spin relaxation torque in metallic ferromagnets

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 by 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.

Axial current driven by magnetization dynamics in Dirac semimetals

We consider a superconducting nanowire proximity coupled to a superconductor/ferromagnet/superconductor (S/F/S) junction, where the magnetization penetrates into a superconducting segment in a nanowire decaying as ∼exp[−∣n∣ξ], where n is the site index and the ξ is the decay length. We tune chemical potential and spin-orbit coupling so that the topological superconducting regime hosting the Majorana fermion is realized for long ξ. We find that when ξ becomes shorter, zero energy state at the interface between a superconductor and a ferromagnet splits into two states at nonzero energy. Accordingly, the behavior of the Josephson current is drastically changed due to this “zero mode-nonzero mode crossover.” By tuning the model parameters, we find an almost second-harmonic current-phase relation sin2φ, where φ is the phase difference of the junction. Based on the analysis of Andreev bound state (ABS), we clarify that the 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 the current-phase relation. We further study both the energy and φ dependence of pair amplitudes in the ferromagnetic region. For large ξ, an 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.