Coulomb Interactions and Ferromagnetism in Pure and Doped Graphene

TL;DR

This paper investigates how Coulomb interactions and doping influence ferromagnetism in graphene, revealing conditions under which ferromagnetic phases are stabilized, especially considering disorder effects.

Contribution

It demonstrates that exchange interactions can induce ferromagnetism in graphene near half-filling, with phase diagrams showing multiple transitions and phases, including effects of disorder.

Findings

Ferromagnetism is stabilized at low doping with strong Coulomb interactions.

Two distinct ferromagnetic phases are identified: one with single carrier type, another with both electrons and holes.

Disorder further enhances the stability of ferromagnetic phases.

Abstract

We study the presence of ferromagnetism in the phase diagram of the two-dimensional honeycomb lattice close to half-filling (graphene) as a function of the strength of the Coulomb interaction and doping. We show that exchange interactions between Dirac fermions can stabilize a ferromagnetic phase at low doping when the coupling is sufficiently large. In clean systems, the zero temperature phase diagram shows both first order and second order transition lines and two distinct ferromagnetic phases: one phase with only one type of carriers (either electrons or holes) and another with two types of carriers (electrons and holes). Using the coherent phase approximation (CPA) we argue that disorder further stabilizes the ferromagnetic phase.