Using time-resolved Kerr rotation, we measure the spin/valley dynamics of resident electrons and holes in single charge-tunable monolayers of the archetypal transition-metal dichalcogenide (TMD) semiconductor WSe2. In the n-type regime, we observe long (∼70 ns) polarization relaxation of electrons that is sensitive to in-plane magnetic fields By, indicating spin relaxation. In marked contrast, extraordinarily long (∼2 µs) polarization relaxation of holes is revealed in the p-type regime, that is unaffected by By, directly confirming long-standing expectations of strong spinvalley locking of holes in the valence band of monolayer TMDs. Supported by continuous-wave Kerr spectroscopy and Hanle measurements, these studies provide a unified picture of carrier polarization dynamics in monolayer TMDs, which can guide design principles for future valleytronic devices.Besides their obvious promise for 2D optoelectronics [1][2][3], monolayer transition-metal dichalcogenide (TMD) semiconductors such as MoS 2 and WSe 2 have also revitalized interests in exploiting both the spin and valley pseudospin of electrons and holes for potential applications in (quantum) information processing [4][5][6][7][8][9]. This notion of "valleytronics" arises due to their crystalline asymmetry and strong spin-orbit coupling, which leads to spin-valley locking and valley-specific optical selection rules [7,8]. These rules mandate that the K or K ′ valleys in momentum space can be selectively populated and probed using polarized light, in contrast with most conventional III-V, II-VI, and group-IV semiconductors. Therefore, information may be readily encoded not only by whether an electron (or hole) has spin "up" or "down", but also by whether it resides in the K or K ′ valley -or, indeed, in some quantum-mechanical superposition thereof.The intrinsic timescales of carrier spin and valley dynamics in monolayer TMDs are therefore of considerable interest. However, most studies to date [10][11][12][13][14][15][16] have focused on photogenerated neutral and charged excitons, whose dynamics at low temperatures are inherently limited by their short (3-30 ps) recombination lifetimes [13,17]. An essential but altogether different question, however, concerns the intrinsic spin/valley lifetimes of the resident electrons and holes that exist in n-type and p-type TMD monolayers. In future valleytronic devices, it is likely the properties of these resident carriers that will determine performance -analogous to how the scattering timecales and mobility of resident carriers (not excitons) determines the performance of modernday transistors and interconnects.Several recent time-resolved studies point to encouragingly long polarization dynamics of resident carriers in monolayer TMDs. 3-5 ns polarization decays were observed in CVD-grown MoS 2 and WS 2 monolayers that were unintentionally electron-doped [18,19], while somewhat longer timescales were observed in unintentionally hole-doped CVD-grown WSe 2 [20,21]. However, a significant shortcoming in all these st...
