arXiv subjects

Kosuke Fujiwara

Publications and source records attributed to Kosuke Fujiwara.

5 recordsLinked to original sources

arXiv2026

Interaction induced topological magnon in electron-magnon coupled systems

We theoretically study the emergence of topological magnons in electron-magnon coupled systems. The magnon dispersion in a ferromagnet usually possesses an effective time reversal symmetry in the absence of Dzyaloshinskii-Moriya (DM) interaction, preventing the appearance of topological magnons. When a spin system is coupled to itinerant electrons, we find that the magnon band structure of the spin system experiences time-reversal symmetry breaking with the electron-magnon interaction via the exchange coupling, where topological magnons arise without requiring strong DM. Specifically, we consider a heterostructure consisting of a ferromagnetic insulator and a transition metal dichalcogenide (TMD) monolayer and investigate topological gap opening in magnon bands. Our findings reveal that even trivial ferromagnets can host topological magnons via coupling to itinerant electronic systems.

cond-mat.str-el↗

arXiv2025

Topological phases of electrons induced by electron-magnon interactions

Topological phases of electrons such as topological insulators and quantum Hall states typically require strong spin-orbit coupling or magnetic fields. In this study, we consider an electron system coupled to a spin system, where electrons interact with magnons, quasiparticles of spin waves. We show that the interaction between electrons and magnons transfers the effect of symmetry breaking in the spin system to the electron system, whereby a non-trivial topological phase can be induced in the electron system that is otherwise topologically trivial. Through this ``topology transfer'' mechanism, we demonstrate the realization of various topological phases, including quantum Hall and quantum spin Hall insulators, in simple ferromagnetic spin systems, without requiring strong spin-orbit coupling or external magnetic field for electron systems.

cond-mat.str-el↗

arXiv2024

Interaction-induced nonlinear magnon transport in noncentrosymmetric magnets

We study the effect of the magnon-magnon interaction on the nonlinear magnon transport. The magnonmagnon interaction induces nonreciprocal magnon decay when the time-reversal symmetry is broken and leads to nonlinear thermal responses of magnons. We construct a theoretical framework to study the nonlinear thermal responses due to the nonreciprocal magnon decay by using the imaginary Dyson equation and quantum kinetic theory, which is then applied to models of one-dimensional (1D) antiferromagnets and 2D honeycomb ferromagnets with Dzyaloshinskii-Moriya interactions. An order estimate shows that the nonlinear thermal response from the present mechanism is feasible for experimental measurement.

cond-mat.str-el↗

arXiv2023

Nonlinear spin current of photoexcited magnons in collinear antiferromagnets

We study the nonlinear magnon spin current induced by an ac electric field under light irradiation in collinear antiferromagnets with broken inversion symmetry. For linearly polarized light, we find that a dc spin current appears through ``the magnon spin shift current" mechanism, which is driven by a spin polarization generation in the two magnon creation process and has a close relationship to the geometry of magnon bands through Berry connection. For circularly polarized light, a dc spin current appears through ``the spin injection current" mechanism, which is proportional to the relaxation time of magnons and can be large when the magnon lifetime is long. We demonstrate generation of the magnon spin shift and injection currents, based on a few toy models and a realistic model for a multiferroic material M$_2$Mo$_3$O$_8$.

cond-mat.str-el↗

arXiv2022

Thermal Hall responses in frustrated honeycomb spin systems

We study geometrical responses of magnons driven by a temperature gradient in frustrated spin systems. While Dzyaloshinskii-Moriya (DM) interactions are usually incorporated to obtain geometrically nontrivial magnon bands, here we investigate thermal Hall responses of magnons that do no rely on the DM interactions. Specifically, we focus on frustrated spin systems with sublattice degrees of freedom and show that a nonzero Berry curvature requires breaking of an effective $PT$ symmetry. According to this symmetry consideration, we study the $J_1$-$J_2$-$J_2^\prime$ Heisenberg models on a honeycomb lattice as a representative example, and demonstrate that magnons in the spiral phase support the thermal Hall effect once we introduce a magnetic field and asymmetry between the two sublattices. We also show that driving the magnons by a temperature gradient induces spin current generation (i.e., magnon spin Nernst effect) in the $J_1$-$J_2$-$J_2^\prime$ Heisenberg models.

cond-mat.str-el↗