Flat band in twisted bilayer Bravais lattices

This paper derives symmetry-based constraints on the effective potential in twisted bilayers of generic but high-symmetric 2D lattices, i.e., Bravais lattices, which help us predict how flat bands are formed. The generic theory also reveals a possibility of anisotropic band flattening, where a band is flattened in one direction but not in the orthogonal direction.