Dirac points for twisted bilayer graphene with in-plane magnetic field

Abstract

We study Dirac points of the chiral model of twisted bilayer graphene (TBG) with constant in-plane magnetic field. For a fixed small magnetic field, we show that as the angle of twisting varies between magic angles, the Dirac points move between $ K, K' $ points and the $ \Gamma $ point. The Dirac points for zero magnetic field and non magic angles lie at $ K $ and $ K'$, while in the presence of a non-zero magnetic field and near magic angles, they lie near the $ \Gamma $ point. For special directions of the magnetic field, we show that the Dirac points move, as the twisting angle varies, along straight lines and bifurcate orthogonally at distinguished points. At the bifurcation points, the linear dispersion relation of the merging Dirac points disappears and exhibit a quadratic band crossing point (QBCP). The results are illustrated by links to animations suggesting interesting additional structure.