Strongly Interacting Phases in Twisted Bilayer Graphene at the Magic Angle

TL;DR

This paper uses large-scale Hartree-Fock calculations to reveal that Coulomb interactions induce ferromagnetic and spin-valley polarized insulating phases, including quantum anomalous Hall states, in twisted bilayer graphene at the magic angle.

Contribution

It provides the first comprehensive lattice-based analysis showing Coulomb interactions lead to ferromagnetic and topologically nontrivial insulating states in TBG at various fillings.

Findings

Coulomb interactions produce ferromagnetic insulating states at integer fillings.

The $ u=0$ state is a pure ferromagnet, others are spin-valley polarized.

Odd fillings exhibit quantum anomalous Hall effect with Chern number 1.

Abstract

Twisted bilayer graphene near the magic angle is known to have a cascade of insulating phases at integer filling factors of the low-energy bands. In this Letter we address the nature of these phases through an unrestricted, large-scale Hartree-Fock calculation on the lattice that self-consistently accounts for all electronic bands. Using numerically unbiased methods, we show that Coulomb interactions produce ferromagnetic insulating states at integer fillings $\nu\in[-3,3]$ with maximal spin polarization $M_{\text{FM}}=4-|\nu|$. We find that the $\nu=0$ state is a pure ferromagnet, whereas all other insulating states are spin-valley polarized. At odd filling factors $|\nu|=1,3$ those states have a quantum anomalous Hall effect with Chern number $\mathcal{C}=1$. Except for the $\nu=0,-2$ states, all other integer fillings have insulating phases with additional sublattice symmetry breaking and antiferromagnetism in the remote bands. We map the metal-insulator transitions of these phases as a function of the effective dielectric constant. Our results establish the importance of large-scale lattice calculations to faithfully determine the ground states of TBG at integer fillings.