Exciton valley depolarization in monolayer transition-metal dichalcogenides

TL;DR

This paper investigates the mechanisms behind exciton valley depolarization in monolayer transition-metal dichalcogenides, revealing how Rashba-induced coupling causes faster depolarization in certain materials.

Contribution

It uncovers the role of Rashba-induced coupling between dark and bright excitons in explaining anomalously fast valley depolarization in specific monolayer TMDs.

Findings

Valley depolarization occurs within a few ps at low temperatures.

Rashba-induced coupling explains the faster depolarization in MoSe₂ and MoTe₂.

Depolarization is slower in WSe₂, WS₂, and likely MoS₂ due to negligible Rashba effects.

Abstract

The valley degree of freedom is a sought-after quantum number in monolayer transition-metal dichalcogenides. Similar to optical spin orientation in semiconductors, the helicity of absorbed photons can be relayed to the valley (pseudospin) quantum number of photoexcited electrons and holes. Also similar to the quantum-mechanical spin, the valley quantum number is not a conserved quantity. Valley depolarization of excitons in monolayer transition-metal dichalcogenides due to long-range electron-hole exchange typically takes a few ps at low temperatures. Exceptions to this behavior are monolayers MoSe$_2$ and MoTe$_2$ wherein the depolarization is much faster. We elucidate the enigmatic anomaly of these materials, finding that it originates from Rashba-induced coupling of the dark and bright exciton branches next to their degeneracy point. When photoexcited excitons scatter during their energy relaxation between states next to the degeneracy region, they reach the light cone after losing the initial helicity. The valley depolarization is not as fast in monolayers WSe$_2$, WS$_2$ and likely MoS$_2$ wherein the Rashba-induced coupling is negligible.