Dark excitons and the elusive valley polarization in transition metal dichalcogenides

Abstract

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