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 -and magnetic -field dependences of spatial spin structures on the basis of the density matrix renormalization group method using an effective model for the one-dimensional two-orbital Hubbard model in strong-coupling limit. We find an excitonic insulating (EI) phase and a spin-state ordering (SSO) phase in the intermediate region between low-and high-spin phases. In the EI phase, spin-triplet excitons are spatially fluctuating due to quantum effects, and an incommensurate spin correlation realizes. The analyses of a spin gap and degeneracy of entanglement spectra suggest the realization of the Haldane-like edge state in the EI phase. In the SSO phase, 3-fold or incommensurate 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.

Automated summary

The electronic properties of excitonic insulators and spin-state transitions in perovskite cobalt oxides have been examined. By using the density matrix renormalization group method and an effective model for the one-dimensional two-orbital Hubbard model, the spatial spin structures depending on crystal and magnetic fields were investigated. Excitonic insulating and spin-state ordering phases were found in the intermediate region between low and high-spin phases. In the excitonic insulating phase, spatially fluctuating spin-triplet excitons and incommensurate spin correlations were observed. The spin gap and entanglement spectra suggest the presence of Haldane-like edge states. In the spin-state ordering phase, different spin-state structures were stabilized depending on the crystal field splitting.