Quantum critical point in graphene approached in the limit of infinitely strong Coulomb interaction

TL;DR

This paper investigates a model of graphene with multiple Dirac fermions interacting via Coulomb forces, revealing a quantum critical point at infinite interaction strength and scale invariance in a large momentum range.

Contribution

It demonstrates that in the limit of strong Coulomb interaction, the system approaches a quantum critical point with a variable dynamic critical exponent depending on fermion degeneracy.

Findings

At infinite Coulomb strength, the system exhibits scale invariance.

The dynamic critical exponent z approaches 1-4/( ext{N}) for large N.

Graphene in vacuum remains near the scale-invariant regime over a broad momentum range.

Abstract

Motivated by the physics of graphene, we consider a model of N species of 2+1 dimensional four-component massless Dirac fermions interacting through a 3D instantaneous Coulomb interaction. We show that in the limit of infinitely strong Coulomb interaction the system approaches a quantum critical point, at least for sufficiently large fermion degeneracy. In this regime the system exhibits invariance under scale transformations in which time and space scale by different factors. The elementary excitations are fermions with dispersion relation omega ~ p^z, where the dynamic critical exponent z depends on N. In the limit of large N we find z=1-4/(\pi^2 N). We argue that due to the numerically large Coulomb coupling, graphene (freely suspended) in vacuum stays near the scale-invariant regime in a large momentum window, before eventually flowing to the trivial fixed point at very low momentum scales.