Half-metal and other fractional metal phases in doped AB bilayer graphene

TL;DR

This paper predicts that doping AB bilayer graphene can lead to the formation of fractional metal phases with fully polarized Fermi surfaces in both spin and valley degrees of freedom, explaining recent experimental observations.

Contribution

It introduces a theoretical mechanism for the emergence of fractional metal phases in doped AB bilayer graphene, extending the concept of half-metals to include valley polarization.

Findings

Prediction of fractional metal phases with valley polarization

Identification of phase transitions with increasing doping

Consistency with recent experimental phase transition observations

Abstract

We theoretically argue that, in doped AB bilayer graphene, the electron-electron coupling can give rise to the spontaneous formation of fractional metal phases. These states, being generalizations of a more common half-metal, have a Fermi surface that is perfectly polarized not only in terms of a spin-related quantum number, but also in terms of the valley index. The proposed mechanism assumes that the ground state of undoped bilayer graphene is a spin density wave insulator, with a finite gap in the single-electron spectrum. Upon doping, the insulator is destroyed, and replaced by a fractional metal phase. As doping increases, transitions between various types of fractional metal (half-metal, quarter-metal, etc.) are triggered. Our findings are consistent with recent experiments on doped AB bilayer graphene, in which a cascade of phase transitions between different isospin states was observed.