Photo-induced spin-state change in itinerant correlated-electron system

Photo-induced spin-state change in itinerant correlated-electron system is studied. The model Hamiltonians before and after photon-pumping are derived from the two-orbital Hubbard model with crystalline field splitting. A photon introduced in the low-spin band insulator induces a bound state of the high-spin state and a photoexcited hole. This bound state brings a characteristic peak in the optical absorption spectra in the photo-excited state. The resent results well explain the recent experimental results of the ultrafast optical spectroscopy in perovskite cobaltites.