Gate-Controlled Spin-Valley Locking of Resident Carriers in WSe2 Monolayers

Abstract

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...

Optical initialization of a single spin-valley in charged WSe2 quantum dots

“…Even after the recombination of exciton, which takes place after a few nanoseconds [19,20,22], one expects the spin-valley of single, localized hole to be preserved for a much longer time. In fact from the valley lifetime of free holes [14,16,17], we expect a single spin-valley lifetime on the order of microseconds, if not longer. On longer timescales, valley relaxation could be mediated by hyperfine interaction with nuclear spins, which is expected to be quite small in TMDs [24,36].…”

“…However, as recombination lifetimes of photogenerated excitations in TMDs is on the order of a few picoseconds, optically generated valley lifetime is limited to a similar timescale. On the other hand, valley polarization of free charge carriers, as opposed to photogenerated excitations, shows promising prospect with lifetimes on the order of microseconds reported for holes [14][15][16][17][18]. A natural question for quantum information science and quantum metrology applications is whether a single spin-valley can be optically addressed and manipulated.…”

Hyperfine interaction in atomically thin transition metal dichalcogenides

“…For resident charge carriers this limitation is released. The polarization can be preserved for a few nanoseconds in MoS 2 [45][46][47], and even longer in WSe 2 [48][49][50]. Particularly long polarization relaxation times can be obtained for the localized * smirnov@mail.ioffe.ru charge carriers, where the dominant role in the spin and valley dynamics is played by the hyperfine interaction with the host lattice nuclear spins [28,51].…”

Engineering Valley Polarization of Monolayer WS2: A Physical Doping Approach

“…The introduced resident carriers in TMDs not only tailor the exciton species, but are expected to considerably tune the valley polarization dynamics which dominates P c values as well. In previous reports, the changes of valley polarization/relaxation dynamics due to carrier doping have been noticed but have not been investigated systematically and the mechanism remains elusive . In addition, the unintentional doping and localized states in TMD samples play important roles to influence their optical properties, which may lead to discrepancies in experiments using layered TMD samples with different initial electronic states .…”