Brightening of spin- and momentum-dark excitons in transition metal dichalcogenides

TL;DR

This paper investigates how dark excitons in monolayer transition metal dichalcogenides become optically active through phonon interactions and magnetic fields, enhancing understanding of their excitonic properties for future applications.

Contribution

It reveals the mechanisms behind brightening of spin- and momentum-dark excitons in TMDs, providing new insights into their exciton landscape and dynamics.

Findings

Detection of momentum- and spin-dark excitons in WS2, WSe2, and MoS2

Identification of brightening mechanisms via phonons and magnetic fields

Agreement with recent experimental observations

Abstract

Monolayer transition metal dichalcogenides (TMDs) have been in focus of current research, among others due to their remarkable exciton landscape consisting of bright and dark excitonic states. Although dark excitons are not directly visible in optical spectra, they have a large impact on exciton dynamics and hence their understanding is crucial for potential TMD-based applications. Here, we study brightening mechanisms of dark excitons via interaction with phonons and in-plane magnetic fields. We show clear signatures of momentum- and spin-dark excitons in WS$_2$, WSe$_2$ and MoS$_2$, while the photoluminescence of MoSe$_2$ is only determined by the bright exciton. In particular, we reveal the mechanism behind the brightening of states that are both spin- \textit{and} momentum-dark in MoS$_2$. Our results are in good agreement with recent experiments and contribute to a better microscopic understanding of the exciton landscape in TMDs.