Slave-boson field fluctuation approach to the extended Falicov-Kimball model: charge, orbital, and excitonic susceptibilities

TL;DR

This paper employs a slave-boson approach to analyze susceptibilities in the extended Falicov-Kimball model, revealing phase instabilities and exciton behaviors that support a crossover between BCS and Bose-Einstein condensation.

Contribution

It introduces a SO(2)-invariant slave-boson scheme to calculate susceptibilities and explore phase instabilities and excitonic properties in the model.

Findings

Identifies instabilities towards charge, orbital, and excitonic phases.

Finds qualitative differences in exciton dispersion at different phase transitions.

Supports a crossover scenario between BCS and Bose-Einstein condensation.

Abstract

Based on the SO(2)-invariant slave-boson scheme, the static charge, orbital, and excitonic susceptibilities in the extended Falicov-Kimball model are calculated. Analyzing the phase without long-range order we find instabilities towards charge order, orbital order, and the excitonic insulator (EI) phase. The instability towards the EI is in agreement with the saddle-point phase diagram. We also evaluate the dynamic excitonic susceptibility, which allows the investigation of uncondensed excitons. We find qualitatively different features of the exciton dispersion at the semimetal-EI and at the semiconductor-EI transition supporting a crossover scenario between a BCS-type electron-hole condensation and a Bose-Einstein condensation of preformed bound electron-hole pairs.