Phonon-induced Markovian and non-Markovian effects on absorption spectra of moir\'e excitons in twisted transition metal dichalcogenide bilayers

TL;DR

This study theoretically explores how phonons influence the absorption spectra of moiré excitons in twisted TMDC bilayers, revealing a transition from non-Markovian to Markovian dynamics with increasing twist angle.

Contribution

It uncovers the twist angle-dependent phonon effects on exciton absorption spectra, highlighting the transition between non-Markovian and Markovian regimes and their impact on spectral features.

Findings

Non-Markovian effects dominate at small twist angles for localized excitons.

At larger twist angles, Markovian processes lead to line broadening.

Optical phonons significantly suppress absorption peaks of higher exciton bands.

Abstract

The properties of moir\'e excitons in twisted bilayers of transition metal dichalcogenides (TMDCs) vary significantly with the twist angle, ranging from quasi localized excitons with flat dispersions for small twist angles to delocalized excitons for larger ones. This twist angle dependence directly impacts the exciton-phonon coupling, which plays a significant role for the optical properties of these materials. In this work we theoretically investigate the twist angle dependent influence of phonons on absorption spectra of intralayer moir\'e excitons in a twisted TMDC hetero-bilayer. For the lowest-lying intralayer moir\'e exciton we find that the exciton-phonon coupling interpolates between two physically distinct regimes when tuning the twist angle. At small twist angles non-Markovian polarization dynamics and phonon sidebands dominate the properties of absorption spectra for localized excitons. For larger twist angles Markovian processes become more important leading to additional line broadening. Furthermore, the absorption spectra here show a characteristic asymmetric peak similar to monolayer TMDCs. When taking into account multiple bright moir\'e exciton bands we find that intraband scattering due to optical phonons has a significant impact on absorption spectra, effectively suppressing absorption peaks of higher lying bands when their bandwidth surpasses the optical phonon energy.