Novel exciton transport in double-layer graphene structures*

A spatially indirect exciton is created when an electron and a hole, confined to separate layers of a double quantum well system, bind to form a composite boson. A system of such excitons is demonstrated to host a Bose–Einstein condensate phase at low temperature, featuring dissipationless exciton flow and perfect Coulomb drag. In this talk, I will discuss novel exciton phases in various double-layer structures characterized by exotic transport properties. In double monolayer graphene, an insulating exciton phase is stabilized in the presence of large density imbalance. With increasing temperature, the insulating phase “melts” into an exciton superfluid exhibiting perfect Coulomb drag. In double bilayer graphene, Coulomb drag measurement reveals strong anisotropy in exciton flow when n = 0 and n = 1 orbitals states are degenerate in one of the layers. Along the easy direction, drag measurement is consistent with an exciton condensate, whereas an anomalous drag response is observed in the orthogonal direction. These exotic exciton phases are studied in a multidimensional phase space defined by a variety of experimental parameters available to double-layer structures, which provides insight into the nature of electron correlation and the resulting physical structures.