Multiple topological transitions in twisted bilayer graphene near the first magic angle

Abstract

The noninteracting physics of electrons in twisted bilayer graphene is considered by varying the twist angle close to the magic angle using a continuum model. The magic angle is known to occur when the dispersion at the two Dirac cones of the two bands close to neutrality become quadratic. These are topological transitions, in which three moving Dirac points pass through each of the two immobile ones as the angle varies. Indeed, the motion of Dirac points is not limited to this and their participation in several topological transitions is very rich, which is the main subject of this study.