Hidden Spin-States in Cobalt Oxides: Photoinduced State and Excitonic Insulating State

Abstract

Recent theoretical researches in correlated electron systems with the spin-state degree of freedom are reviewed. Novel electronic states which appear under the competition between the low-spin and high-spin states in perovskite cobaltites are focused on. Calculated results obtained from the two-orbital Hubbard model, an effective model for the low-energy electronic structure, and the five-orbital Hubbard model are introduced. In particular, we pay our attention to the possibilities of the following three exotic states: (i) The excitonic insulating state, in which the electronic wave function is represented by the linear combination of the low-spin and high-spin states, is introduced. The ground state and finite-temperature phase diagrams and the elementary excitations are shown. We propose the possible experimental methods to identify the excitonic insulating state in cobaltates. (ii) Possibility of the electronic phase separation by hole-carrier doping is introduced. The ground state energy as a function of carrier number indicates that a homogeneous electronic state is unstable, and the electronic state is separated into the low-spin band insulator and the high-spin ferromagnetic metallic state. A microscopic mechanism of this phase separation phenomena is discussed. (iii) A bound state between the high-spin state and the hole state induced by the photoirradiation is proposed by the complement theoretical calculations. This is termed the photoinduced high-spin polaron state. Implications of this characteristic state to the optical pump-probe experiments are discussed.