Comparing the spin/valley dynamics of resident carriers in gated WSe₂ and MoSe₂ monolayers

The ability to populate and probe specific valleys in monolayer transition-metal dichalcogenides (TMDs) using polarized light has revived long-standing interests in valleytronics. An essential question therefore concerns the spin/valley relaxation timescales of the resident electrons and resident holes (not excitons) that exist in n-type or p-type TMDs. Using time-resolved Kerr rotation, we recently demonstrated extraordinarily long (microsecond) valley polarization lifetimes for resident holes in electrostatically-gated WSe₂ monolayers in the p-type regime [1]. Here we compare the spin/valley dynamics of resident electrons and holes in monolayer WSe₂ with its molybdenum-based counterpart, MoSe₂. Importantly, WSe₂ and MoSe₂ monolayers exhibit conduction band spin-orbit splittings (Δ_c) of opposite sign, which can influence the measured dynamics, particularly in the weakly n-type regime. [1] P. Dey et al., PRL 119, 137401 (2017).