Magnetic phases from competing Hubbard and extended Coulomb interactions in twisted bilayer graphene

TL;DR

This paper investigates how competing Hubbard and extended Coulomb interactions influence magnetic phases in twisted bilayer graphene, revealing conditions for antiferromagnetic and spin-polarized states at different fillings.

Contribution

It introduces a self-consistent Hartree-Fock approach to analyze the interplay of interactions in twisted bilayer graphene, highlighting the conditions for various magnetic symmetry breakings.

Findings

Antiferromagnetic ground state at charge neutrality with large Hubbard repulsion.

Full spin polarization at half-filling of the valence band.

Screening of Coulomb interaction is crucial for observing magnetic states.

Abstract

We implement a self-consistent Hartree-Fock approximation based on a microscopic model in real space, which allows us to consider the interplay between the Hubbard and the extended Coulomb interaction in twisted bilayer graphene at the magic angle. These two interactions tend to favor different symmetry breaking patterns, having therefore complementary roles in the regimes where one or the other dominates. We show that, for sufficiently large values of the on-site Hubbard repulsion, magic angle graphene has an antiferromagnetic ground state at the charge neutrality point, while at half-filling of the lowest valence band the state becomes fully spin-polarized. In general, a suitable screening of the extended Coulomb interaction is required to observe the magnetic state in either case, as otherwise the instabilities take place in the charge sector, preferentially in the form of time-reversal, chiral or valley symmetry breaking.