Photo-induced phenomena in correlated electron system with spinstate degree of freedom

Photo-induced spin-state change in correlated electron systems is examined theoretically, motivated by the recent pump-probe experiments in perovskite cobaltites. Effective Hamiltonian for the photo-excited state is derived from the two-orbital Hubbard model. High-spin state is induced by the photo-irradiation in the low-spin ground state. We demonstrate that this photo-induced spin-state change is detected by the pump-probe spectroscopy.