Probing exciton condensates in double layer graphene using the corbino geometry

Double layer graphene system with a thin tunnel barrier was recently reported to host exciton condensates and novel interlayer fractional quantum hall (FQH) states in the presence of a high magnetic field. The corbino geometry, consisting normally of two concentric rings, provides a direct transport probe of the bulk response, without the contribution of sample edges. In such geometry, combing the parallel flow and counter flow experimental setup, we observe the perfect drag phenomenon, namely the current in each of the two layers has exactly the same intensity but opposite direction. It demonstrates that all the current in the system is carried by excitons and provides an important evidence of the existence of exciton condensate. For the first time, we demonstrate with perfect drag that the exciton coupling remains robust at high landau level where the enhanced screening effect between electrons reduces the interlayer coupling. We study the exciton coupling as a function of interlayer bias, temperature, magnetic field and dc bias. Evidence of a fractional exciton condensate, occurring at total filling fraction of 1/3, is discussed.