Electron-hole pair condensation and Coulomb drag effect in a graphene double layer.

The spatially separated electron-hole pair condensation and Coulomb drag effect are studied theoretically in a graphene double layer. The main research results are presented as follows: we derive a critical average spacing of particles for the pair condensation to start at zero temperature, which is determined by the permittivity and thickness of the dielectric film between the two sheets of the graphene double layer; we obtain the phase diagram of condensates by introducing the ground-state fidelity, which accurately differentiates the Bose-Einstein condensate, Bardeen-Cooper-Schrieffer state, and their crossover; we confirm that the superfluid portion decreases the drag conductivity for a given gate voltage at finite temperatures; we find there exists a minimum drag conductivity by increasing the gate voltage, which results from the combined effect of the longitudinal conductivity in each graphene layer and superfluid density. The last result is especially useful for detecting the pair condensation experimentally.