BEC-BCS crossover of exciton condensation in graphene double-layer

In an electronic double-layer system under strong magnetic fields, electron-like and hole-like carriers of different layers can bind by Coulomb interaction to form excitons, which condense into superfluid phases at low temperatures. When the densities of the excitons are low (electron separation l_B >> interlayer distance d), the electrons and holes are spatially paired (BEC condensate), while at high densities (l_B ∼ d), the pairing is in momentum space and caused by the Fermi surface instability (BCS condensate). In our study, we observed exciton condensation in dual-graphite-gated graphene double-layer devices for a large range of l_B parameter. For d/l_B << 1, the counter-flow resistance shows an activation behavior. On the contrary, at high densities (d/l_B ≈ 0.75), the counter-flow resistance exhibits sharp transitions in temperature. In this BCS regime, we observed characters of Berezinskii–Kosterlitz–Thouless (BKT) transition from current (I)-voltage (V) relation: V ∝ I^α, where α grows rapidly as temperature decreases. In the low-density regime (d/l_B << 1), I-V curves exhibit much smoother transitions (α < 3), deviating from the BKT expectation. Our experimental observations thus suggest the exciton condensate in graphene undergoes a BCS-BEC crossover around d/l_B ≈ 0.5.