Chern insulator phases and spontaneous spin and valley order in a moiré lattice model for magic-angle twisted bilayer graphene

Abstract

At a certain ``magic'' relative twist angle of two graphene sheets it remains a challenge to obtain a detailed description of the proliferation of correlated topological electronic phases and their filling dependence. We perform a self-consistent real-space Hartree-Fock study of an effective moir\'e lattice model to map out the preferred ordered phases as a function of Coulomb interaction strength and moir\'e flat-band filling factor. It is found that a quantum valley Hall phase, previously discovered at charge neutrality, is present at all integer fillings for sufficiently large interactions. However, except for charge neutrality, additional spontaneous spin/valley polarization is present in the ground state at nonzero integer fillings, leading to Chern insulator phases and anomalous quantum Hall effects at odd filling factors, thus constituting an example of interaction-driven nontrivial topology. At weaker interactions, all nonzero integer fillings feature metallic inhomogeneous spin/valley ordered phases which may also break additional point group symmetries of the system. We discuss these findings in the light of previous theoretical studies on and recent experimental developments related to magic-angle twisted bilayer graphene.