Flat bands and chiral optical response of twisted bilayer insulators

H. Ochoa, A. Asenjo-Garcia

Published 2020-02-23Version 1

We present a low-energy model describing the reconstruction of the electronic spectrum in twisted bilayers of honeycomb crystals with broken sublattice symmetry. The resulting moir\'e patterns can be classified into two families with different symmetry. In both cases, flat bands appear at relatively large angles, without any magic angle condition. These flat bands arise from midgap states spatially localized in regions of unnatural stacking, where ions of the same polarity lie on top of each other. Transitions between these bands give rise to sharp resonances in the optical absorption spectrum at frequencies well below the gap of the monolayer. The system displays circular dichroism (different absorption of left- and right-handed circularly-polarized light) due to the chiral symmetry of the twisted bilayer. In hexagonal boron nitride, we find that circular dichroism is maximized in one type of moir\'e structure, which can be linked to approximate symmetries not contained in the space group. Our calculation illustrates how the chiral response of moir\'e-patterned materials probes geometrical properties of the electronic wave function.