Optical Response in Excitonic Insulating State: Variational Cluster Approach

TL;DR

This paper investigates the optical properties of an excitonic insulating state in a strongly correlated electron system using a variational cluster approach, revealing characteristic spectral features and phase transitions.

Contribution

It introduces a variational cluster method with vertex corrections to analyze optical responses in an excitonic insulator within a two-orbital Hubbard model.

Findings

Identification of optical spectral features characteristic of the EI phase

Finite-temperature phase diagram showing EI between band and Mott insulators

Correlation between optical intensity and EI order parameter

Abstract

Optical responses in an excitonic insulating (EI) system with strong electron correlation are studied. We adopt the two-orbital Hubbard model with a finite energy difference between the two orbitals where the spin state degree of freedom exists. This model is analyzed by the variational cluster approach. In order to include the local electron correlation effect, the vertex correction is taken into account in the formulation of the optical conductivity spectra. We calculate a finite-temperature phase diagram, in which an EI phase appears between a low-spin band insulating state and a high-spin Mott insulating state. Characteristic components of the optical conductivity spectra consisting of a sharp peak and continuum appear in the EI phase. Integrated intensity almost follows the order parameter of the EI state, suggesting that this component is available to identify the EI phases and transitions.