Instabilities of the AA-stacked graphene bilayer

TL;DR

This paper investigates the electronic instabilities in AA-stacked graphene bilayers, revealing that degeneracy leads to spontaneous symmetry breaking, with antiferromagnetism being the most stable order, potentially persisting at room temperature.

Contribution

It demonstrates that AA-stacked graphene bilayers are inherently unstable due to degeneracy, and identifies antiferromagnetism as the dominant symmetry-breaking order.

Findings

Degeneracy causes instability in AA-stacked graphene bilayers.

Antiferromagnetism is the most stable order parameter.

Antiferromagnetic gap can exist up to room temperature.

Abstract

Tight-binding calculations predict that the AA-stacked graphene bilayer has one electron and one hole conducting bands, and that the Fermi surfaces of these bands coincide. We demonstrate that as a result of this degeneracy, the bilayer becomes unstable with respect to a set of spontaneous symmetry violations. Which of the symmetries is broken depends on the microscopic details of the system. We find that antiferromagnetism is the more stable order parameter. This order is stabilized by the strong on-site Coulomb repulsion. For an on-site repulsion energy typical for graphene systems, the antiferromagnetic gap can exist up to room temperatures.