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 effect 1 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′ MoS 2 , which we investigate from first principles using many-body perturbation theory 4 . 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. Topological insulators have been studied primarily with regard to the behaviour of electrons. A theoretical study now shows that a single layer of a metal dichalcogenide can become a topological insulator for excitons.