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 nontopological insulator characterized 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.

Automated summary

Time-reversal symmetric topological insulators are generally stable against weak local interaction unless symmetry-breaking transitions occur. Using dynamical mean-field theory, we study an interacting model of quantum spin Hall insulators and find a first-order symmetry-breaking transition to a nontopological insulator with exciton condensation at intermediate coupling. With stronger interaction, the insulator becomes a Mott insulator. The transition is continuous in the absence of magnetic order and progresses through Mott localization before the condensate coherence is lost. We demonstrate that the correlated excitonic state corresponds to a magneto-electric insulator, which can be experimentally probed. Lastly, we discuss the fate of helical edge modes across the excitonic transition.