The Role of Dark Excitons in Valley Depolarisation In Monolayer Transition Metal Dichcalcogenides

We present a theoretical study of the valley magneto-exciton relaxation dynamics in monolayer transition metal dichcalcogenides (TMDs) using a kinetic equation approach. The TMDs are direct band gap semiconductors with strong light-matter coupling which produce optical responses dominated by tightly bound excitons. The combination of the lattice symmetry and strong spin-orbit interaction gives rise to a rich selection rules allowing for optical control of the excitons valley polarisation. Experiments have shown that due to spin-orbit interactions there are spin forbidden transitions which lead to dark exciton states, and these states are found to possess long lifetimes due to their non-radiative decay and play a role in the depolarisation dynamics of the TMDs. Using a kinetic equation approach we investigate the interplay of the exchange interaction, a perpendicular magnetic field and the dark state scattering the time evolution of the exciton valley polarisation. We find that the influence of the dark states leads to longer valley relaxation times. We also explore the effect of an in-plane magnetic, which acts to “brighten” the dark states, which leads to an even greater increase in the valley relaxation time.