Electric-field-induced metal-semiconductor transitions in twisted bilayers of WSe2

We have examined the electronic structure of twisted bilayers of WSe2 within ab initio electronic structure calculations. Flat bands are found to emerge above a set of dispersing bands at a twist angle of 3.48 degrees, whereas they are absent at 19.03 degrees which has a similar-sized moire cell. The presence of the flat bands at the valence -band maximum are traced to the strong perturbations from zone-boundary scattering found for small angles. This follows from the fact that the strongest Fourier components of the lattice potential are associated with the primitive cell vectors, justifying a description of the electronic structure as a perturbation to the unrotated limit. The layer polarization of the charge density associated with the valence-band maximum also allows for tuning the electronic structure with an electric field, thereby driving a semiconductor-metal-semiconductor transition.