Many-body instability of Coulomb interacting bilayer graphene: RG approach

TL;DR

This paper uses renormalization group analysis to show that Coulomb interactions in bilayer graphene cause a spontaneous splitting of Fermi points, leading to a nematic phase with broken rotational symmetry at finite temperature.

Contribution

It demonstrates that short-range interactions are marginally relevant in bilayer graphene, causing a transition to a nematic state with four Dirac points, a novel insight into its low-energy physics.

Findings

Quadratic Fermi points split into four Dirac points due to interactions.

A nematic phase emerges, breaking six-fold symmetry to two-fold.

Finite transition temperature for the nematic phase.

Abstract

Low-energy electronic structure of (unbiased) bilayer graphene is made of two Fermi points with quadratic dispersions, if trigonal-warping and other high order contributions are ignored. We show that as a result of this qualitative difference from single-layer graphene, short-range (or screened Coulomb) interactions are marginally relevant. We use renormalization group to study their effects on low-energy properties of the system, and show that the two quadratic Fermi points spontaneously split into four Dirac points, at zero temperature. This results in a nematic state that spontaneously breaks the six-fold lattice rotation symmetry (combined with layer permutation) down to a two-fold one, with a finite transition temperature. Critical properties of the transition and effects of trigonal warping are also discussed.