# Dark excitons and the elusive valley polarization in transition metal dichalcogenides

## TL;DR

This paper introduces a rate equation model to explain the variability in valley polarization in transition metal dichalcogenides, highlighting the significant role of dark excitons in PL emission polarization.

## Contribution

It proposes a novel model emphasizing dark excitons' influence on valley polarization and clarifies the mechanisms behind inter-valley scattering in TMD monolayers.

## Key findings

- Dark excitons serve as a reservoir for valley polarization.
- Electron-hole exchange interaction dominates inter-valley scattering.
- Model explains variation in circular polarization across different TMDs.

## Abstract

A rate equation model for the dark and bright excitons kinetics is proposed which explains the wide variation in the observed degree of circular polarization of the PL emission in different TMDs monolayers. Our work suggests that the dark exciton states play an important, and previously unsuspected role in determining the degree of polarization of the PL emission. A dark exciton ground state provides a robust reservoir for valley polarization, which tries to maintain a Boltzmann distribution of the bright exciton states in the same valley via the intra valley bright dark exciton scattering mechanism. The dependence of the degree of circular polarization on the detuning energy of the excitation in MoSe$_2$ suggests that the electron-hole exchange interaction dominates over two LA phonon emission mechanism for inter valley scattering in TMDs.