A monolayer transition-metal dichalcogenide as a topological excitonic insulator

Monolayer transition-metal dichalcogenides in the T' phase could enable the realization of the quantum spin Hall effect¹ at room temperature, because they exhibit a prominent spin-orbit gap between inverted bands in the bulk^2,3. Here we show that the binding energy of electron-hole pairs excited through this gap is larger than the gap itself in the paradigmatic case of monolayer T' MoS₂, which we investigate from first principles using many-body perturbation theory⁴. This paradoxical result hints at the instability of the T' phase in the presence of spontaneous generation of excitons, and we predict that it will give rise to a reconstructed 'excitonic insulator' ground state^5-7. Importantly, we show that in this monolayer system, topological and excitonic order cooperatively enhance the bulk gap by breaking the crystal inversion symmetry, in contrast to the case of bilayers^8-16 where the frustration between the two orders is relieved by breaking time reversal symmetry^13,15,16. The excitonic topological insulator is distinct from the bare topological phase because it lifts the band spin degeneracy, which results in circular dichroism. A moderate biaxial strain applied to the system leads to two additional excitonic phases, different in their topological character but both ferroelectric^17,18 as an effect of electron-electron interaction.