Flat bands in twisted bilayers of polar two-dimensional semiconductors

Abstract

We investigate the Bloch flat bands in twisted bilayers from nonpolar to polar two-dimensional semiconductors using first-principles calculations and density functional based tight-binding simulations. First, to delineate the underlying mechanism of the formation of the flat bands, we rely on a tight-binding model of modified graphene where a bias between the A-B sublattice of the hexagonal lattice is introduced. By analyzing the evolution of the valence and conduction band edges of the bilayer of the modified graphene with different stacking patterns, a mechanism attributed to the splitting of the defect-like band edge states induced by different stacking patterns is revealed. The magic angle mechanism is no longer needed. Next, guided by the revealed mechanism, we predict the formation of flat bands in twisted bilayers of a series of two-dimensional systems from nonpolar to polar semiconductors. Our finding has important implications for exploring the flat band physics in low dimensions.