Gate-Controlled Spin-Valley Locking of Resident Carriers in ${\mathrm{WSe}}_2$ Monolayers

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 ${\mathrm{WSe}}_2$. In the $n$-type regime, we observe long ($\sim 130\mathrm{ns}$) polarization relaxation of electrons that is sensitive to in-plane magnetic fields $B_y$, indicating spin relaxation. In marked contrast, extraordinarily long ($\sim 2\mu \mathrm{s}$) polarization relaxation of holes is revealed in the $p$-type regime, which is unaffected by $B_y$, directly confirming long-standing expectations of strong spin-valley 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.

Figure 1

(a) TRKR experiment on gated monolayer ${\mathrm{WSe}}_2$. $V_G$ tunes the resident carrier density between electron- and hole-doped regimes. External coils provide in-plane magnetic fields $B_y$. RCP or LCP pump pulses from a 645 nm diode laser (DL) imprint a spin-valley polarization on the resident carriers, which is detected via the Kerr rotation ${\theta }_K$ imparted on probe pulses from a Ti:S laser. A pulse picker (PP) and electronic delay generator (EDG) allow access to $\mu \mathrm{s}$ pump-probe delays $\mathrm{\Delta }t$. (b) Normalized reflection spectra $R/R_0$ versus $V_G$ at 5 K (from 80 nm ${\mathrm{SiO}}_2$ sample); the oscillator strength evolves from neutral exciton $X^0$ to negatively charged exciton $X^{-}$ with increasing electron density. (c) Similar evolution from $X^0$ to positively charged exciton $X^{+}$ with increasing hole density. (d) TRKR in the heavily electron-doped regime at 5 K (from 300 nm ${\mathrm{SiO}}_2$ sample), with the probe tuned to 727 nm. For large $\mathrm{\Delta }t$ ($>50\mathrm{ns}$), the signal decays exponentially with $\sim 130\mathrm{ns}$ time constant. The decays are strongly suppressed in small $B_y$. (For technical reasons, $\mathrm{\Delta }t$ between 150 and 260 ns are not accessible [24].) (e) TRKR at 5 K in the hole-doped regime at the $X^{+}$ transition: Extraordinarily long polarization decays are revealed ($\sim 2\mu \mathrm{s}$) that are independent of $B_y$, confirming strong spin-valley locking in the valence band. (f),(g) The diagrams depict the simplest ${\mathrm{WSe}}_2$ band structure in the electron and hole-doped regimes, along with available scattering pathways.

Figure 2

(a)–(h) CWKR spectra at 5 K (from ${\mathrm{WSe}}_2$ on 80 nm ${\mathrm{SiO}}_2$) as the resident carrier density is tuned from electron doped to hole doped. Within each panel, $B_y$ is varied from 0 mT (red trace) to 360 mT (violet trace). The ${\mathrm{WSe}}_2$ is weakly pumped by a circularly polarized 632.8 nm continuous-wave laser, and the induced Kerr rotation is detected by a narrow band continuous-wave Ti:sapphire ring laser that is scanned from 700–750 nm. Positions of the neutral and charged exciton transitions from reflectivity studies of this sample [see Figs. 1 and 1] are indicated.

Figure 3

(a) Hanle-Kerr effect as the ${\mathrm{WSe}}_2$ monolayer is tuned from the $n$-type to $p$-type regime (in the 80 nm ${\mathrm{SiO}}_2$ structure). The ${\mathrm{WSe}}_2$ is weakly pumped by a circularly polarized 632.8 nm continuous-wave laser. $B_y$ is continuously varied while the induced Kerr rotation is measured by a tunable continuous-wave probe laser. Here the probe is fixed at 727 nm, which is sensitive to both $X^{-}$ and $X^{+}$ transitions. The width of the Hanle curves increases with increasing electron density in the $n$-type regime. In the $p$-type regime the Hanle curves show no dependence on $B_y$. (b) Same, but the probe laser is fixed at 706 nm, which is sensitive to the neutral $X^0$ exciton transition. No dependence on $B_y$ is observed.