# A monolayer transition metal dichalcogenide as a topological excitonic insulator

## TL;DR

This paper predicts that monolayer T' phase transition metal dichalcogenides can form a topological excitonic insulator state, where strong electron-hole interactions induce a new ground state with enhanced topological and excitonic properties.

## Contribution

It demonstrates, through first-principles calculations, the emergence of a topological excitonic insulator phase in monolayer T' transition metal dichalcogenides, revealing a novel interplay of topology and excitonic order.

## Key findings

- Electron-hole binding energy exceeds the band gap in MoS2.
- The excitonic insulator state enhances the bulk gap and breaks inversion symmetry.
- Strain induces additional ferroelectric excitonic phases with distinct topological properties.

## Abstract

Monolayer transition metal dichalcogenides in the T' phase promise to realize the quantum spin Hall (QSH) effect at room temperature, because they exhibit a prominent spin-orbit gap between inverted bands in the bulk. Here we show that the binding energy of electron-hole pairs excited through this gap is larger than the gap itself in MoS2, a paradigmatic material that we investigate from first principles by many-body perturbation theory (MBPT). This paradoxical result hints at the instability of the T' phase against the spontaneous generation of excitons, and indeed we find that it gives rise to a recostructed `excitonic insulator' ground state. Importantly, we show that in this system topological and excitonic order cooperatively enhance the bulk gap by breaking the crystal inversion symmetry, in contrast to the case of bilayers where the frustration between the two orders is relieved by breaking time reversal symmetry. The excitonic topological insulator departs distinctively from the bare topological phase as 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 as an effect of electron-electron interactions.