Excitonic Correlations, Spin-State Ordering, and Magnetic-Field Effects in One-Dimensional Two-Orbital Hubbard Model for Spin-Crossover Region

The electronic properties of excitonic insulators have been examined precisely in recent years. Pictures of exciton condensation may be applied to the spin-state transition observed in perovskite cobalt oxides. We examine the crystal-field and magnetic-field dependences of spatial spin structures on the basis of the density matrix renormalization group method using a low-energy effective model for the one-dimensional two-orbital Hubbard model. We find an excitonic insulating (EI) phase and a spin-state ordering (SSO) phase in the intermediate region between low-spin and high-spin phases. In the EI phase, an excitonic correlation function follows a power law, and an incommensurate spin correlation realizes. In the SSO phase, incommensurate or 3-fold SSO structures realize depending on the crystal-field splitting. these structures are stabilized as a result of the competition of exchange interactions between spin states.Comment: 10 pages, 6 figure