Flat bands and topological properties of twisted bilayer WSe₂ under external stimuli

Abstract We present systematic calculations on the electronic band structures and topological properties of twisted bilayer WSe2 with twist angle near 1° under different strains, pressures and vertical electric fields. We find that extremely narrow bandwidth less than 1 meV can be obtained for the top two valence bands for a wide range of strain smaller than 2%. Remarkably, the minimum bandwidth with strain around 1.8% is very robust under different pressures and electric fields, so the strain-induced flat band provides a robust 2D system for studying of electron strong correlation. The band gaps between the top valance bands increase significantly under pressure and vertical electric field, offering effective approaches for band engineering. Furthermore, we show that vertical electric field can lead to topological transitions for the flat bands, which are analyzed in details from a close examination of the band touching process. Our results not only provide new insights into the flat bands and their topological properties under various external stimuli in this versatile moiré superlattice system, but also reveal the important parameter spaces for exploring its novel physics in future studies.