Exciton condensation in strongly correlated quantum spin Hall insulators

TL;DR

This paper investigates how strong electron interactions in quantum spin Hall insulators can induce a first-order transition to an exciton condensate, revealing a new correlated insulating phase with potential experimental signatures.

Contribution

It demonstrates the existence of a symmetry-breaking exciton condensate in strongly correlated quantum spin Hall insulators using dynamical mean-field theory, a novel insight into their phase diagram.

Findings

Existence of a first-order transition to an excitonic insulator at intermediate coupling.

Transition to a Mott insulator at larger interactions.

Correlated excitonic state as a magneto-electric insulator.

Abstract

Time reversal symmetric topological insulators are generically robust with respect to weak local interaction, unless symmetry breaking transitions take place. Using dynamical mean-field theory we solve an interacting model of quantum spin Hall insulators and show the existence, at intermediate coupling, of a symmetry breaking transition to a non-topological insulator characterised by exciton condensation. This transition is of first order. For a larger interaction strength the insulator evolves into a Mott one. The transition is continuous if magnetic order is prevented, and notably, for any finite Hund's exchange it progresses through a Mott localization before the condensate coherence is lost. We show that the correlated excitonic state corresponds to a magneto-electric insulator which allows for direct experimental probing. Finally, we discuss the fate of the helical edge modes across the excitonic transition.