Analyzing Photoinduced Phase Transitions in an Organic Salt Using Floquet Theory

Research on photoinduced phase transitions has progressed recently accelerated because of the rapid development of laser technology. Irradiation by circularly polarized light was theoretically proven to induce a photoinduced topological phase transition to the Chern insulator phase in a tight-binding model on the honeycomb lattice via a special kind of band structure resembling those predicted by Haldane. According to this prediction, the possible emergence of the photoinduced topological phase in graphene has been explored, and an observation of photoinduced Hall currents in graphene was argued in this context.

Since these pioneering studies, photoinduced topological phase transitions have undergone extensive theoretical investigations. However, the further development of this growing research field requires proposals of novel target materials and theoretical predictions of interesting physical phenomena. In this study, we theoretically predicted the occurrence of photoinduced topological phase transitions and the emergence of the topologically nontrivial Chern insulator phase as a nonequilibrium steady state in the organic salt α-(BEDT-TTF)₂I₃ under irradiation with elliptically polarized light.

We constructed rich nonequilibrium phase diagrams in the plane of the x-axis and y-axis components of the amplitude of elliptically polarized light by calculating the band structures, Chern numbers, and Hall conductivity in a photodriven α-(BEDT-TTF)₂I₃ system using the Floquet theory. These include the Chern insulator phases, non-topological insulator phases, and semimetal phases. In addition, calculations of the Hall conductivity using the Floquet–Keldysh scheme predicted that the quantization of Hall conductivity can be observed in this nonequilibrium Chern insulator phase at low temperatures, just as it is observed in equilibrium Chern insulators. Furthermore, we revealed that the present photoinduced Chern insulator phase possesses another feature of the equilibrium Chern insulators, namely, the gapless state localized at the edges. The predicted quantized Hall conductivity and edge current owing to the predicted edge states in the photoinduced Chern insulator phase are expected to be observed in future experiments for α-(BEDT-TTF)₂I₃. Our results expand a range of target materials and contribute to the research on the optical manipulation of electronic states in matter.

(Written by M. Mochizuki on behalf of all authors)