Excitonic instability in transition metal dichalcogenides

TL;DR

This paper investigates excitonic properties in non-inversion symmetric transition metal dichalcogenide monolayers, revealing potential for exciton insulator phases and analyzing their optical responses.

Contribution

It introduces a detailed analysis of excitons in Janus TMDC monolayers, highlighting their large binding energies and optical properties, which are novel insights in this material class.

Findings

Exciton binding energies can exceed the electronic band gap.

Inversion symmetry breaking influences excitonic optical selection rules.

Materials may host exciton insulator phases due to large exciton binding energies.

Abstract

When transition-metal dichalcogenide monolayers lack inversion symmetry, their low-energy single particle spectrum can described by tilted massive Dirac Hamiltonians. The so-called Janus materials fall into that category. Inversion symmetry can also be broken by the application of out-of-plane electric fields, or by the mere presence of a substrate. Here we explore the properties of excitons in TMDC monolayers lacking inversion symmetry. We find that exciton binding energies can be larger than the electronic band gap, making such materials promising candidates to host the elusive exciton insulator phase. We also investigate the excitonic contribution to their optical conductivity and discuss the associated optical selection rules.