Interlayer excitonic superfluidity in graphene

TL;DR

This paper investigates the potential for high-temperature interlayer excitonic superfluidity in double layer graphene, considering realistic disorder effects and screening, and compares monolayer and bilayer systems.

Contribution

It provides a detailed analysis of how disorder and screening influence the critical temperature for excitonic condensation in double layer graphene.

Findings

Disorder from impurities significantly suppresses the superfluid transition temperature.

Screening effects, especially dynamic screening, alter the estimated critical temperature.

Quadratic bilayer graphene exhibits similar pairing temperatures to monolayer graphene.

Abstract

We discuss the conditions under which the predicted (but not yet observed) zero-field interlayer excitonic condensation in double layer graphene has a critical temperature high enough to allow detection. Crucially, disorder arising from charged impurities and corrugation in the lattice structure --- invariably present in all real samples --- affects the formation of the condensate via the induced charge inhomogeneity. In the former case, we use a numerical Thomas-Fermi-Dirac theory to describe the local fluctuations in the electronic density in double layer graphene devices and estimate the effect these realistic fluctuations have on the formation of the condensate. To make this estimate, we calculate the critical temperature for the interlayer excitonic superfluid transition within the mean-field BCS theory for both optimistic (unscreened) and conservative (statically screened) approximations for the screening of the interlayer Coulomb interaction. We also estimate the effect of allowing dynamic contributions to the interlayer screening. We then conduct similar calculations for double quadratic bilayer graphene, showing that the quadratic nature of the low-energy bands produces pairing with critical temperature of the same order of magnitude as the linear bands of double monolayer graphene.