Time-reversal versus chiral symmetry breaking in twisted bilayer graphene

J. Gonzalez, T. Stauber

Published 2020-02-27Version 1

By applying a self-consistent Hartree-Fock approximation, we show that the mechanism of dynamical symmetry breaking can account for the insulating phase that develops around the charge neutrality point of twisted bilayer graphene at the magic angle. At strong and intermediate coupling of the Coulomb interaction, the opening of a gap between the lowest-energy valence and conduction bands proceeds preferentially through the breakdown of chiral symmetry. This effect is mainly driven by the extended component of the interaction. When this is under very strong dielectric screening, however, we enter a regime where the on-site Coulomb repulsion becomes dominant, leading to the prevalence of time-reversal and parity symmetry breaking in a weak-coupling window of the phase diagram. Moreover, when the long-range tail of the Coulomb interaction is not screened, we see the appearance of another dominant pattern, which is characterized by breaking the time-reversal invariance but preserving parity. This gives rise to the superposition of two different effects, namely the opening of a gap at the Dirac cones and the splitting of the degeneracy of the low-energy bands at the K points of the Moir\'e Brillouin zone