Multiple Floquet Chern insulator phases in the spin-charge coupled triangular-lattice ferrimagnet: Crucial role of higher-order terms in the high-frequency expansion

We study the effects of photoirradiation with circularly polarized light on the Dirac half-metal state induced by the ferrimagnetic order in a triangular Kondo-lattice model. Our analysis based on the Floquet theory reveals that two types of Floquet Chern insulator phases appear as photoinduced nonequilibrium steady states and that these two phases can be experimentally detected and distinguished by measurements of the Hall conductivity. It is elucidated that these rich nonequilibrium topological phases come from higher-order terms in the high-frequency expansion called Brillouin-Wigner expansion, which is in striking contrast to usually discussed Floquet Chern insulator phases originating from the lowest-order terms of the expansion. So far, the lattice electron models on simple nonmultipartite lattices such as triangular lattices and square lattices have not been regarded as targets of the Floquet engineering because the lowest-order terms of the high-frequency expansion for Floquet effective Hamiltonians cancel each other to vanish in these systems. Our findings of the Floquet Chern insulator phases in a triangular Kondo-lattice model are expected to expand the range of potential models and even materials targeted by the Floquet engineering.