A monolayer transition metal dichalcogenide as a topological excitonic insulator

Monolayer transition metal dichalcogenides in the T’ phase might realize the quantum spin Hall effect at room temperature, since they have a large bulk hybridization gap between the inverted valence bands that provides topological order with robustness. Here we demonstrate that T’-MoS₂ is unstable against the spontaneous generation of excitons by using first-principles many-body perturbation theory, as the computed exciton binding energy is larger than the quasiparticle gap. We predict that the true ground state of T’-MoS2 is a novel correlated insulator in which both excitonic and topological orders coexist by reducing the spatial point group symmetry, whereas typically interactions tend to disrupt the topological order. A self-consistent calculation provides us with clear-cut signatures of this excitonic topological insulator, such as an enhanced bulk quasiparticle gap (and hence increased topological robustness), spontaneous inversion symmetry breaking, spin-splitting of quasiparticle bands. The phase diagram, in the space whose axes are temperature and strain, includes a second---topologically trivial---excitonic phase that spontaneously breaks mirror symmetry while changing discontinuously the quasiparticle gap, which surprisingly never closes.