Exciton condensation in biased bilayer graphene

Abstract

We consider suspened bilayer graphene under applied perpendicular electric bias field that is known to generate a single particle gap $2\Delta$ and a related electric polarization ${\cal P}$. 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 $2\Delta$. We predict the critical bias (converted to band gap), $\Delta_c\approx 60$ meV, below which the excitons condense. The critical temperature, $T_c(\Delta)$, is maximum at $\Delta \approx 25$ meV, $T_c^\text{max}\approx 115$ K, decreasing significantly at smaller $\Delta$ due to thermal screening. Entering the condensate phase, the superfluid transition is accompanied by a cusp in the electric polarization ${\cal P}(\Delta)$ at $\Delta\to\Delta_c$, which provides a striking testable signature. Additionally, we find that the condensate prefers to form a pair density wave.