Atomistic theory of twist-angle dependent intralayer and interlayer exciton properties in twisted bilayer materials

TL;DR

This paper develops an atomistic method to analyze how twist angles in bilayer 2D materials affect exciton properties, enabling detailed predictions of optical spectra and exciton energies.

Contribution

The authors introduce an efficient atomistic approach using Wannier functions to solve the Bethe-Salpeter equation for twisted bilayer materials, addressing large moiré superlattice challenges.

Findings

Optical spectrum shows three low-energy peaks for twist angles below 2°.

Energy splitting between peaks matches experimental data.

Identifies two low-energy interlayer excitons with weak oscillator strengths.

Abstract

Twisted bilayers of two-dimensional (2D) materials have emerged as a highly tunable platform to study and engineer properties of excitons. However, the atomistic description of these properties has remained a significant challenge as a consequence of the large unit cells of the emergent moir\'e superlattices. To address this problem, we introduce an efficient approach to solve the Bethe-Salpeter equation that exploits the localization of atomic Wannier functions. We then use this approach to study intra- and interlayer excitons in twisted WS$_{2}$/WSe$_{2}$ at a range of twist angles. In agreement with experiment, we find that the optical spectrum exhibits three low-energy peaks for twist angles small than $2^\circ$. The energy splitting between the peaks is described accurately. We also find two low-energy interlayer excitons with weak oscillator strengths. Our approach opens up new opportunities for the design of light-matter interactions in ultrathin materials.