Evidence for a Superfluid-to-solid Transition of Bilayer Excitons

Double layer graphene, which consists of two graphene layers separated by a few-layer-thick insulating barrier, provides a highly tunable platform to investigate the rich phase diagram of strongly correlated bosons in the form of interlayer exciton. For instance, it has long been established that a superfluid phase of interlayer excitons emerges In the quantum Hall regime, when each graphene layer is tuned to half filling of the lowest landau level. In this work, we report an excitonic insulating phase stabilized in the dilute limit of the exciton phase diagram, achieved in the regime of large layer imbalance. Two observations point towards the exotic nature of this insulator. First, with increasing temperature, the insulator undergoes a melting transition into a high temperature phase that behaves like a superfluid. Secondly, we show that the phase boundary between the insulator and superfluid to be hysteretic, the location of which depends on the direction of temperature sweep. The temperature hysteresis offers a strong indication for a first order transition, which arises from a non-zero latent heat associated with solid melting. Together, our observations point towards a superfluid-to-solid phase transition in interlayer excitons.