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

TL;DR

This paper investigates excitonic insulator and spin-state order phases in a one-dimensional two-orbital Hubbard model, revealing quantum fluctuations, incommensurate correlations, and edge states influenced by magnetic and crystal-field effects.

Contribution

It introduces a detailed analysis of excitonic and spin-state phases in a 1D Hubbard model, highlighting quantum effects and edge states in the spin-crossover region.

Findings

Discovery of an excitonic insulator phase with quantum-fluctuating spin-triplet excitons.

Identification of spin-state ordering structures stabilized by exchange interactions.

Evidence of Haldane-like edge states in the excitonic phase.

Abstract

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 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-spin 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.