Impact of atomic reconstruction on optical spectra of twisted TMD homobilayers

TL;DR

This paper investigates how atomic reconstruction in twisted TMD homobilayers significantly alters the exciton energy landscape and optical spectra, revealing multiple flat bands and multi-peak absorption features.

Contribution

It provides a microscopic, material-specific prediction of the impact of atomic reconstruction on moiré excitons and optical properties in twisted TMD homobilayers, highlighting new spectral signatures.

Findings

Significant change in potential depth for moiré excitons due to reconstruction

Emergence of multiple flat bands and shifted trapping sites

Multi-peak structure in optical absorption spectra of WSe₂ homobilayers

Abstract

Twisted bilayers of transition metal dichalcogenides (TMDs) have revealed a rich exciton landscape including hybrid excitons and spatially trapped moir\'e excitons that dominate the optical response of the material. Recent studies have shown that in the low-twist-angle regime, the lattice undergoes a significant relaxation in order to minimize local stacking energies. Here, large domains of low energy stacking configurations emerge, deforming the crystal lattices via strain and consequently impacting the electronic band structure. However, so far the direct impact of atomic reconstruction on the exciton energy landscape and the optical properties has not been well understood. Here, we apply a microscopic and material-specific approach and predict a significant change in the potential depth for moir\'e excitons in a reconstructed lattice, with the most drastic change occurring in naturally stacked TMD homobilayers. We show the appearance of multiple flat bands and a significant change in the position of trapping sites compared to the rigid lattice. Most importantly, we predict a multi-peak structure emerging in optical absorption of WSe$_2$ homobilayers - in contrast to the single peak that dominates the rigid lattice. This finding can be exploited as an unambiguous signature of atomic reconstruction in optical spectra of moir\'e excitons in naturally stacked twisted homobilayers.