Observation of exciton-exciton interaction mediated valley depolarization in monolayer MoSe$_2$

TL;DR

This study directly measures the rapid valley depolarization in monolayer MoSe₂, revealing that exciton-exciton Coulomb interactions significantly accelerate valley pseudospin decay, which impacts potential optoelectronic applications.

Contribution

It provides the first direct measurement of valley depolarization lifetime in monolayer MoSe₂ and links it to exciton-exciton interactions.

Findings

Valley depolarization lifetime scales linearly with excitation density.

Strong exciton-exciton Coulomb exchange interactions enhance valley depolarization.

Rapid valley decay limits the use of valley pseudospin in TMD-based devices.

Abstract

The valley pseudospin in monolayer transition metal dichalcogenides (TMDs) has been proposed as a new way to manipulate information in various optoelectronic devices. This relies on a large valley polarization that remains stable over long timescales (hundreds of ns). However, time resolved measurements report valley lifetimes of only a few ps. This has been attributed to mechanisms such as phonon-mediated inter-valley scattering and a precession of the valley psedospin through electron-hole exchange. Here we use transient spin grating to directly measure the valley depolarization lifetime in monolayer MoSe$_{2}$. We find a fast valley decay rate that scales linearly with the excitation density at different temperatures. This establishes the presence of strong exciton-exciton Coulomb exchange interactions enhancing the valley depolarization. Our work highlights the microscopic processes inhibiting the efficient use of the exciton valley pseudospin in monolayer TMDs.