Collective optical emission by dark moire excitons

In two-dimensional Van der Waals heterostructures, tightly bound and long-lived interlayer excitons allow us to explore rich many-body physics. Particularly compelling are twisted bilayers, where a trapping moire potential promotes strong interactions and correlated phases like exciton condensates. Yet the twist-induced indirect band gap renders these excitons momentum-dark, leading to an optically inaccessible dark condensate. We show that the strong interactions drive an emergent emission process where the recombination recoil is absorbed by longitudinal collective modes – a kind of excitonic Mossbauer effect. This many-body "leaky emission" dominates the condensate optical signature at low temperatures. Strong interactions and a valley degree of freedom may lead to additional collective phenomena, such as excitonic density ordering.