Moiré Exciton Condensates in Twisted Transition Metal Dichalcogenide Bilayers

Exciton gases have long been considered an attractive platform for exploring Bose-Einstein condensates (BECs) in the solid state, due to their strong interactions and small mass. Transition metal dichalcogenide (TMD) heterobilayers are promising systems for realizing exciton BECs, thanks to the long-lived, tightly bound interlayer excitons which they can host. Introducing a twist between the layers produces a moiré superlattice, in which the modulated bandgap acts as an energy landscape that localizes excitons to the superlattice sites. We study a Bose-Hubbard model which could describe these systems, and derive the properties of a novel exciton BEC phase that is governed by the interplay of underlying TMD optoelectronic properties: A coexistence of two exciton species corresponding to each of the TMD electronic valleys, distinct optical selection rules, and a moiré momentum mismatch. While the latter renders the condensed excitons optically inactive, we find that interactions generally enable emission to leak from the condensate. Furthermore, excitonic coherence results in a charge-density wave which can lead to interesting interplay of the two components.