Trion valley relaxation dynamics in monolayer WSe₂

Valley degrees of freedom of atomically thin materials have received much attention, with potential in future applications of electronics and optoelectronics as emerging valley-tronics. The exploration of novel valley-dependent phenomena has been boosted by strongly Coulomb-bounded electrons and holes acting as neutral excitons and charged excitons (trions) with finite Berry curvature in monolayers of transition metal dichalcogenides. However, neutral excitons have been reported to undergo fast intervalley scattering on the order of 10 ps, which blurs observations of valley-dependent phenomena. Here we report the valley relaxation dynamics of the bright state of positively charged excitons (bright positive trions) in monolayer WSe₂ through polarization- and time-resolved photoluminescence and femtosecond transient reflection measurements. A long valley relaxation time, exceeding 100 ps, is observed for positive trions at low temperature, which is notably prolonged when compared with the characteristic excitonic valley relaxation time of 10 ps. With increasing temperature, the relaxation time decreases to a few ps. This finding implies that phonon-mediated intervalley scattering is important for the relaxation process of the valley-polarized bright state of positive trions.