Exciton topology and condensation in a model quantum spin Hall insulator

We study by a consistent mean-field scheme the role on the single- and two-particle properties of a local electron-electron repulsion in the Bernevig, Hughes, and Zhang model of a quantum spin Hall insulator. We find that the interaction fosters the intrusion between the topological and nontopological insulators of an insulating and magnetoelectric phase that breaks spontaneously inversion and time-reversal symmetries but not their product. The approach to this phase from both topological and nontopological sides is signaled by the softening of two exciton branches, i.e., whose binding energy reaches the gap value, that possess, in most cases, finite and opposite Chern numbers, thus allowing this phase to be regarded as a condensate of topological excitons. We also discuss how those excitons, and especially their surface counterparts, may influence the physical observables.