RKKY signatures of a topological phase transition in the α- T3 model irradiated by a circularly polarized light

Recently, the alpha-T3 model, whose specific lattice structure is determined by the parameter alpha, has attracted broad attention due to the intriguing flat band. Under irradiation of an off-resonant circularly polarized light (CPL), the Floquet alpha-T3 model experiences a topological phase transition, as alpha varies from 0 to 1, due to the distortion of the flat band. Interestingly, it is found that the phase boundary becomes tunable by the CPL strength if nonzero on-site energy is considered, not a constant alpha c = 1/ 2 as reported in previous literatures. To characterize the topological phase transition, we investigate the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction between magnetic impurities. It is interestingly found that a sharp dip structure in the RKKY magnitude can well ascertain the boundary of the topological phase transition, and different topological phases can be distinguished by the RKKY oscillation behaviors. Thus, the signatures of the RKKY interaction could provide us an alternative method to characterize the topological phase transition, different from the conventional electrical or optical means.