Evidence for excitonic solid states in double layer graphene

Spatially indirect excitons can form when electron-doped and hole-doped 2D quantum wells are brought into close proximity. In the condition that interlayer separation is sufficiently large to suppress recombination and small enough that interlayer coulomb interactions are strong, the system can spontaneously condense into a superfluid-like macroscopically coherent state, termed an exciton condensate. In a double layer graphene system in the presence of magnetic field, upon decreasing the exciton density, we observe a sharp transition from an exciton superfluid to an exciton insulator, which shows insulating behavior in both charge transport and exciton transport measurement. At elevated temperature, the excitonic insulating state exhibits perfect coulomb drag, which is the signature of an exciton superfluid. Our data suggests a phase diagram consisting of a pinned excitonic Wigner crystal at lower exciton density, which transitions to a superfluid-like state at elevated temperature.