Valley dynamics of different trion species in monolayer WSe_2

摘要

We develop a model, taking into account optically inactive excitonic dark states, of both spin-forbidden and momentum-forbidden types, to explore valley dynamics of different trion species in monolayer WSe2. We observe two distinct regimes, determined by two typical bright-dark scattering processes, i.e., the spin-flip and spin-conserving ones. When the spin-flip scattering is comparable to the spin-conserving one, we find that the valley polarizations (VPs) of all excitonic states including excitons and singlet and triplet trions are copolarized with respect to the excitation laser, consistent with available experimental measurements. Interestingly, when the spin-conserving scattering dominates over the spin-flip one, we observe the reversal of VP of the singlet trion. This fascinating valley control opens the door to an envisaged valley field-effect transistor (FET) (an analog of spin FET). Furthermore, we reveal that the reversed VP strengthens until it becomes stabilized, offering a promising platform for coherent control of valley pseudospin. Our findings should stimulate experiments probing the VP reversal in samples with strong spin-conserving scattering.