Electron-hole pair condensation in graphene bilayer

TL;DR

This paper investigates electron-hole pairing in graphene bilayers, revealing a BCS-like condensate influenced by unique graphene properties, with estimates of energy gaps and effects of disorder, highlighting differences from traditional BCS-BEC crossover.

Contribution

It demonstrates that electron-hole pairing in graphene bilayers resembles conventional systems but is affected by Berry phase and screening, providing gap estimates and disorder effects analysis.

Findings

Existence of BCS-like electron-hole condensate in graphene bilayers.

Estimated energy gaps for various interlayer distances and carrier densities.

Disorder effects and the absence of localized pairs in the system.

Abstract

We consider the pairing of electrons and holes due to their Coulomb attraction in two parallel, independently gated graphene layers, separated by a barrier. At weak coupling, there exist the BCS-like pair-condensed state. Despite the fact that electrons and holes behave like massless Dirac fermions, the problem of BCS-like electron-hole pairing in graphene bilayer turns out to be rather similar to that in usual coupled semiconductor quantum wells. The distinctions are due to Berry phase of electronic wave functions and different screening properties. We estimate values of the gap in one-particle excitation spectrum for different interlayer distances and carrier concentrations. Influence of disorder is discussed. At large enough dielectric susceptibility of surrounding medium, the weak coupling regime holds even at arbitrarily small carrier concentrations. Localized electron-hole pairs are absent in graphene, thus the behavior of the system versus coupling strength is cardinally different from usual BCS-BEC crossover.