Anomalous Exciton-Condensation in Graphene Bilayers

TL;DR

This paper predicts the existence of anomalous exciton condensates in Bernal stacked graphene bilayers at various filling factors, highlighting unique phase transitions caused by Landau level degeneracy.

Contribution

It introduces the prediction of exciton condensates in graphene bilayers at multiple filling factors, revealing novel phase transition behaviors due to Landau level degeneracy.

Findings

Exciton condensates predicted at non-zero integer filling factors in graphene bilayers.

Vanishing superfluid density and first-order phase transition at specific filling factors.

Degeneracy of Landau levels with different orbital characters causes anomalous properties.

Abstract

In ordinary semiconductor bilayers, exciton condensates appear at total Landau level filling factor $\nu_{T}=1$. We predict that similar states will occur in Bernal stacked graphene bilayers at many non-zero integer filling factors. For $\nu_{T} = -3,1$ we find that the superfluid density of the exciton condensate vanishes and that a finite-temperature fluctuation induced first order isotropic-smectic phase transition occurs when the layer densities are not balanced. These anomalous properties of bilayer graphene exciton condensates are due to the degeneracy of Landau levels with $n=0$ and $n=1$ orbital character.