Héctor Ochoa

Two-dimensional materials are formed by mechanically stiff atomic layers weakly coupled by van der Waals forces. Thanks to this separation of energy scales we can exfoliate them down to the atomic limit or combine them to form superstructures such as moiré superlattices. In this talk I will discuss a new degree of freedom associated with incommensurate stacking arrangements and their interplay with electrons. First I will describe how to engineer stacking textures by applying heterostrain in a multilayer MoSe$_2$ sample, which happens to behave as a very effective trap for electrons [1]. The collective vibrational modes of this texture (which can be envisioned as a network of domain walls connecting almost commensurate stackings) are called phasons [2]. These modes are akin to the acoustic phonons of the moiré superlattice in twisted devices. Contrary to acoustic phonons, however, these modes are not protected by a local conservation law and become overdamped at low frequencies [3]. For the same reasons they are very effective in propagating disorder in the twist angle. I will connect these observations with some pieces of the phenomenology in twisted bilayer graphene. 

[1] D. Edelberg, H. Kumar, V. Shenoy, H. Ochoa, and A. N. Pasupathy, Nat. Phys. 16, 1097 (2020).
[2] H. Ochoa, Phys. Rev. B 100, 155426 (2019).
[3] H. Ochoa and R. M. Fernandes, Phys. Rev. Lett. 128, 065901 (2022).