Exciton condensation in strongly correlated quantum spin Hall insulators

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.