Exciton condensation in biased bilayer graphene

TL;DR

This paper predicts a quantum phase transition in biased bilayer graphene from a band insulator to an exciton superfluid condensate, with specific critical bias, temperature, and observable polarization signatures.

Contribution

It introduces the concept of exciton condensation driven by electric bias in bilayer graphene and predicts critical parameters and experimental signatures.

Findings

Critical bias for exciton condensation is approximately 60 meV.

Maximum critical temperature is around 115 K at 25 meV bias.

A cusp in electric polarization signals the phase transition.

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.