Can Janus Monolayer Transition-Metal Dihalides Be Bright Excitonic Insulators?

In this work, a first-principles GW-BSE approach is employed to predict that Janus monolayers of ClNiBr and its analogues are a rare class of intrinsic candidates for bright excitonic insulators (EIs). The structural asymmetry of Janus monolayers builds in an electric field along the out-of-plane direction, which activates the optical activity of otherwise dark excitons. Although the EI formation is independent of whether the exciton is bright or dark, there is a significant difference between bright and dark EIs during the transition from the EI phase to the band insulator. The bright EI is capable of producing coherent radiation, which will provide a clear collective signal for the identification of spontaneous exciton condensation and open up new possibilities for the construction of EI-based quantum optical devices.