Antiferromagnetic states and phase separation in doped AA-stacked graphene bilayers

TL;DR

This paper investigates the electronic and magnetic phase behavior of doped AA-stacked graphene bilayers, revealing phase separation, antiferromagnetic order, and complex phase transitions influenced by doping, temperature, and disorder.

Contribution

It provides a detailed phase diagram of doped AA-stacked graphene bilayers, highlighting the conditions for antiferromagnetic order and phase separation, including re-entrant transitions.

Findings

Phase separation into antiferromagnetic insulator and metal at low doping.

Metallic phase can be antiferromagnetic or paramagnetic depending on parameters.

Re-entrant transition from paramagnetic to antiferromagnetic phase under certain conditions.

Abstract

We study electronic properties of AA-stacked graphene bilayers. In the single-particle approximation such a system has one electron band and one hole band crossing the Fermi level. If the bilayer is undoped, the Fermi surfaces of these bands coincide. Such a band structure is unstable with respect to a set of spontaneous symmetry violations. Specifically, strong on-site Coulomb repulsion stabilizes antiferromagnetic order. At small doping and low temperatures, the homogeneous phase is unstable, and experiences phase separation into an undoped antiferromagnetic insulator and a metal. The metallic phase can be either antiferromagnetic (commensurate or incommensurate) or paramagnetic depending on the system parameters. We derive the phase diagram of the system on the doping-temperature plane and find that, under certain conditions, the transition from paramagnetic to antiferromagnetic phase may demonstrate re-entrance. When disorder is present, phase separation could manifest itself as a percolative insulator-metal transition driven by doping.