Research

Research Highlights

The magnetic skyrmion is the most representative two-dimensional topological spin texture. The in-plane spin current gives rise to a transverse motion of the magnetic skyrmion, resulting in the skyrmion Hall effect. Due to their topological stability and ability to be driven at low current densities, magnetic skyrmions are expected for applications such as nonvolatile racetrack memory.

We have investigated the influence of spin currents flowing parallel to the magnetic skyrmion string extended in the third direction in the three-dimensional system. We clarified that such a spin current immediately destabilizes the magnetic skyrmion string in a clean system. This instability is caused by the translational Goldstone modes leading to the helix-shaped deformation, whose amplitudes grow with time and eventually break the magnetic skyrmion string [1]. We also demonstrated that the magnetic skyrmion lattice is destabilized through the same mechanism.

A magnetic monopole is one of the elementary particles that have yet to be discovered in a vacuum, but can be realized in quantum materials. A magnetic (anti)hedgehog is a three-dimensional topological defect regarded as an emergent magnetic (anti)monopole. The magnetic monopole and antimonopole are connected by a magnetic skyrmion string as a Dirac string. This composite structure is also called a magnetic tron. 

We have theoretically explored the physics of magnetic hedgehog lattices, i.e., magnetic monopole crystals. We first revealed the stabilization mechanism of hedgehog lattices at zero fields in the ground state in itinerant magnets [1, 2]. Also, we found the multiple topological transitions accompanying pair annihilations of monopoles and antimonopoles in a magnetic field, which nonmonotonically change in the topological Hall effect. 

A Weyl state is one of the three-dimensional (3D) topological electronic structures with a linear energy dispersion and chirality. This Weyl quasiparticle, regarded as a magnetic monopole in the momentum space, leads to anomalous transport. In the aspect of material design, it is a great challenge to create and control the Weyl state through the light-matter coupling.

We have investigated the Floquet engineering of the 3D Dirac semimetals irradiated by a circularly polarized light (CPL). We found that there are two distinct mechanisms for the CPL-induced Dirac-Weyl transitions: 1. chiral gauge field and 2. 1-photon resonance [1]. We also examined the light-induced anomalous Hall conductivity for both mechanisms 1 and 2 in comparison with the experimental results in the 3D Dirac semimetals Co₃Sn₂S₂ [2] and Bi [3], respectively.

Chiral magnets breaking both spatial inversion and time reversal symmetries carry a potential for novel nonlinear responses. In particular, nonlinear optical responses have attracted much attention in the aspect of the application for next-generation optoelectronic devices, for instance, unconventional solar cells and optical sensors. 

We have proposed the potential usage of a magnetic conical state for quadratic optical responses, such as the photovoltaic effect and second-harmonic generation, by nonlinear response theory. We found the flexible controllability of the giant photovoltaic effect changing in not only the magnitude but also the sign, which can be tuned by the magnetization and frequency of light [1].