Strong coupling phases of partially filled twisted bilayer graphene narrow bands

TL;DR

This paper investigates the effects of Coulomb interactions on the narrow bands of twisted bilayer graphene at the magic angle, revealing various magnetic phases and their dependence on electron filling and magnetic field.

Contribution

It provides a detailed analysis of interaction-driven phases in twisted bilayer graphene, highlighting the emergence of SU(4) ferromagnetism and stripe phases at specific fillings.

Findings

At two electrons/holes per unit cell, an SU(4) ferromagnetic insulator is favored.

Ground state involves equal mixing of valleys with opposite spins, with no net magnetic moment.

Extended excited states have a gap that decreases under magnetic field.

Abstract

We identify states favored by Coulomb interactions projected onto the Wannier basis of the four narrow bands of the "magic angle" twisted bilayer graphene. At the filling of two electrons/holes per moire unit cell, such interactions favor an insulating SU(4) ferromagnet. The kinetic terms select the ground state in which the two valleys with opposite spins are equally mixed, with vanishing magnetic moment per particle. We also find extended excited states, the gap to which decreases in magnetic field. An insulating stripe ferromagnetic phase is favored at one electron/hole per unit cell.