Identifying topological excitonic insulators via bulk-edge correspondence

TL;DR

This paper proposes a method to identify topological excitonic insulators through their unique bulk-edge correspondence, supported by first-principles calculations and effective Hamiltonian analysis, with potential experimental verification.

Contribution

It introduces a novel identification approach for topological excitonic insulators based on bulk-edge correspondence and demonstrates its feasibility in LiFeX compounds.

Findings

Excitonic instabilities found in LiFeX compounds.

Bulk-edge correspondence is preserved despite exciton condensation.

Critical temperature of topological phase can exceed room temperature.

Abstract

Excitonic insulator remains elusive and there has been a lack of reliable identification methods. In this work, we demonstrate the promise of topological excitonic insulators for identification due to their unique bulk-edge correspondence, as illustrated by the LiFe$X$ ($X$ = S, Se, and Te) family. First-principles Bethe-Salpeter equation calculations reveal excitonic instabilities in these spin-orbit coupling quantum anomalous Hall insulators. Effective Hamiltonian analyses indicate that spontaneous exciton condensation does not disrupt the gapless edge state but reconstructs the bulk-gap to be almost independent of the spin-orbit coupling strength. This change in the bulk-edge correspondence can be experimentally inspected by angle-resolved photoelectron spectroscopy or electron compressibility measurements, providing observational evidence for the identification of topological excitonic insulators. Moreover, exciton condensation raises the critical temperature of the topological nontrivial phase above room temperature.