Quenching of Intervalley Exchange Coupling in the Presence of Momentum-Dark States in TMDCs

TL;DR

This paper investigates how low-lying momentum-dark exciton states affect the intervalley exchange coupling and valley exciton lifetime in monolayer transition metal dichalcogenides, revealing significant quenching effects.

Contribution

It provides a microscopic theoretical analysis of valley exciton dynamics considering the presence or absence of momentum-dark states in TMDCs.

Findings

Valley exciton lifetime is hundreds of femtoseconds without dark states.

Presence of momentum-dark states strongly quenches valley lifetime.

Differences between W-based and Mo-based TMDCs in exciton dynamics.

Abstract

Monolayers of transition metal dichalcogenides are promising materials for valleytronic applications, since they possess two individually addressable excitonic transitions at the non-equivalent $K$ and $K'$ points with different spins, selectively excitable with light of opposite circular polarization. Here, it is of crucial importance to understand the elementary processes determining the lifetime of these optically injected valley excitons. In this study, we perform microscopic calculations based on a Heisenberg equation of motion formalism to investigate the efficiency of the intervalley coupling in the presence (W based TMDCs) and absence (Mo based TMDCs) of energetically low lying momentum-dark exciton states. While we predict a valley exciton lifetime on the order of some hundreds of fs in the absence of low lying momentum-dark states we demonstrate a strong quenching of the valley lifetime in the presence of such states.