Flat bands and optical activity in twisted boron nitride

We present a minimal continuum model describing the electronic reconstruction in twisted bilayers of hexagonal boron nitride. The relative twist creates regions of unnatural stacking within the moiré cell, where ions of the same polarity lie on top of each other. The gap is strongly modulated, giving rise to very narrow quasi-bands at relative large twist angles. The absence of a magic angle condition emphasizes the different nature of these flat bands as compared to the case of twisted graphene systems. In particular, changing the parameters of our model we can describe both situations, which are separated by a topological phase transition controlled by inter-band coherences of the envelope wave functions around the moiré zone center. Transitions between these flat bands are manifested as sharp resonances in the optical absorption spectrum. We also evaluate the Hall counter-flow ascribed to the chiral symmetry of the system, which gives rise to different absorption cross sections for right-handed and left-handed circularly-polarized light (circular dichroism).