Exciton condensation in biased bilayer graphene

Abstract

We consider suspended bilayer graphene under applied perpendicular electric bias field that is known to generate a single particle gap 2Δ and a related electric polarization P. We argue that the bias also drives a quantum phase transition from band insulator to superfluid exciton condensate. The transition occurs when the exciton binding energy exceeds the band gap 2Δ. We predict the critical bias (converted to band gap), Δc≈60 meV, below which the excitons condense. The critical temperature, Tc(Δ), is maximum at Δ≈25 meV, Tcmax≈115 K, decreasing significantly at smaller Δ due to thermal screening. Entering the condensate phase, the superfluid transition is accompanied by a cusp in the electric polarization P(Δ) at Δ→Δc, which provides a striking testable signature. Additionally, we find that the condensate prefers to form a pair density wave.