Tunable band gap in twisted bilayer graphene

At large commensurate angles, twisted bilayer graphene which holds even parity under sublattice exchange exhibits a tiny gap. Here, we point out a way to tune this tiny gap into a large gap. We start from comprehensive understanding of the physical origin of gap opening by density functional theory calculations. We reveal that the effective interlayer hopping, intralayer charge-density wave, or interlayer charge imbalance favors a gap. Then, on the basis of tight-binding calculations, we suggest that a periodic transverse inhomogeneous pressure, which can tune interlayer hoppings in specific regions of the moi$\stackrel{\ifmmode \acute{}\else \'{}\fi{}}{\mathrm{r}}$e supercell, may open a gap of over 100 meV, which is further confirmed by first-principles calculations. Our results provide a theoretical guidance for experiments to open a large gap in twisted bilayer graphene.