Atomic reconstruction and flat bands in strain engineered transition metal dichalcogenide bilayer moiré systems

Abstract

Strain-induced lattice mismatch leads to moiré patterns in homobilayer transition metal dichalcogenides (TMDs). We investigate the structural and electronic properties of such strained moiré patterns in TMD homobilayers. The moiré patterns in strained TMDs consist of several stacking domains which are separated by tensile solitons. Relaxation of these systems distributes the strain unevenly in the moiré superlattice, with the maximum strain energy concentrating at the highest energy stackings. The order parameter distribution shows the formation of aster topological defects at the same sites. In contrast, twisted TMDs host shear solitons at the domain walls, and the order parameter distribution in these systems shows the formation of vortex defects. The strained moiré systems also show the emergence of several well-separated flat bands at both the valence and conduction band edges, and we observe a significant reduction in the band gap. The flat bands in these strained moiré superlattices provide platforms for studying the Hubbard model on a triangular lattice as well as the ionic Hubbard model on a honeycomb lattice. Furthermore, we study the localization of the wave functions corresponding to these flat bands. The wave functions localize at different stackings compared to twisted TMDs, and our results are in excellent agreement with recent spectroscopic experiments [1].

The strong modulation limit of excitons and trions in moiré materials

“…In a TMD heterobilayer moiré superlattice, electrons and holes tend to localize at one of three high-symmetry sites in a moiré unit cell [54][55][56][57][58][59][60][61][62] at which the local stacking places the metal atoms of one layer above either metal, chalcogen, or empty sites of the layer below. As summarized in Fig.…”

“…Theories of the excitonic properties of moiré materials have progressed by treating excitons as point particles moving in a periodic moiré potential [38][39][40]. Recent numerical and experimental studies show that moiré modulation strengthens when strain is taking into account [54][55][56][57], and that the conduction band electrons and valence band holes that combine to form excitons in a moiré material can be attracted to different lateral positions within the moiré pattern [58][59][60][61][62], distorting the excitonic modes. In the strong modulation limit, the electron and hole moiré band widths become extremely small and the moir material can be viewed as an accurately periodic array of quantum dots.…”