Dissecting intervalley coupling mechanisms in monolayer transition metal dichalcogenides

TL;DR

This study investigates the mechanisms behind intervalley coupling in monolayer transition metal dichalcogenides, revealing the roles of phonon-assisted scattering and Coulomb interactions in exciton valley polarization dynamics.

Contribution

It provides a comprehensive analysis combining experiments and simulations to dissect intervalley coupling processes in monolayer TMDs, highlighting phonon-assisted scattering and spin-flip mechanisms.

Findings

Phonon-assisted scattering is crucial for exciton valley depolarization.

Coulomb scattering and spin-flip processes significantly influence valley dynamics.

Experimental and microscopic simulations offer a unified understanding of intervalley coupling.

Abstract

Monolayer (1L) transition metal dichalcogenides (TMDs) provide a unique opportunity to control the valley degree of freedom of optically excited charge carriers due to the spin-valley locking effect. Despite extensive studies of the valley-contrasting physics, stimulated by perspective valleytronic applications, a unified picture of competing intervalley coupling processes in 1L-TMDs is lacking. Here, we apply broadband helicity-resolved transient absorption to explore exciton valley polarization dynamics in 1L-WSe${}_2$. By combining experimental results with microscopic simulations, we dissect individual intervalley coupling mechanisms and reveal the crucial role of phonon-assisted scattering in the fast decay of the A exciton circular dichroism and the formation of the dichroism of opposite polarity for the B exciton. We further provide a consistent description of the valley depolarization driven by the combined action of Coulomb scattering processes and indicate the presence of efficient single spin-flip mechanisms. Our study brings us closer to a complete understanding of exciton dynamics in TMDs.