Why do twisted bilayers behave differently from their untwisted counterparts?

Title: Why do twisted bilayers behave differently from their untwisted counterparts?
Speaker: Prof. Priya Mahadevan (S. N. Bose National Centre for Basic Sciences)
Time: Aug. 15, 2023 (Tue.) 14:00
Venue: NCTS Physics Lecture Hall, 4F, Chee-Chun Leung Cosmology Hall, NTU
Online: https://nationaltaiwanuniversity-zbh.my.webex.com/nationaltaiwanuniversity-zbh.my/j.php?MTID=me5fb23359bcef8d2277a292913af1e17

The Mo and W based transition metal dichalcogenides have been known for several decades as examples of semiconductors whose electronic structure is well described by band theory. It is only recently that one finds unusual phenomena arising in them on doping holes via gating into twisted bilayers [1], an aspect that we would associate with correlated materials. We have recently examined the electronic structure of twisted bilayers of Mo [2] and W [3] based transition metal dichalcogenides. In contrast to graphene, we find the emergence of flat bands for several angles of rotation. The origin of this can be linked to patches of various types of stackings which include an atom-on-atom as well as a staggered stacking. The former lead to larger inter layer separations because of the larger repulsion between the electrons in the two layers in contrast to the latter. This leads to larger perturbations in some regions of the moire cell. Building on the fact that these materials represent van der Waals structures, and so the perturbation induced by one layer on the other should be small, we explore different twist angles and quantify the perturbation in each instance from the untwisted limit. Surprisingly, at large twist angles we find that we recover the low energy electronic structure of the untwisted limit, while at small angles we find flat band formation as well as other unusual aspects of the electronic structure.

[1]. L. Wang et al., Nat. Mat. 19, 861 (2020).
[2]. Sumanti Patra, Poonam Kumari and Priya Mahadevan, Phys. Rev B 102, 205415 (2020).
[3]. Sumanti Patra, Prasun Boyal and Priya Mahadevan, Phys. Rev. B 107, L041104 (2023).