Flatbands in transition-metal dichalocogenides – when and why do we have them?

We have examined the evolution of the electronic structure in twisted bilayers of MoSe₂ within ab-initio electronic structure calculations, assuming the moire potential to be a small perturbation to the untwisted limit. Its role in modifying the electronic structure is probed by mapping the calculated band structure for the moire cell onto the primitive cell direction which represents the untwisted limit. At large twist angles such as 19.03^o, we find that the moire cell band structure is identical to the primitive cell one in the low energy window. This allows certain simplifications of the electronic structure of the twisted bilayers at large angles [1]. There are however significant deviations for small twist angles such as 3.48^o which have large patches of high symmetry regions of AA and AB' stackings. These lead to enhanced interlayer hopping interaction strengths in some regions, and hence stronger perturbations leading to sub-band formation of the highest occupied band which has a bandwidth of 19 meV, and is found to be localized both in real space as well as momentum space.

References
[1] Poonam Kumari, Joydeep Chatterjee and Priya Mahadevan, Phys. Rev. B 101, 045432 (2020).