  Using a spatially-resolved optical pump-probe experiment, we measure the
lateral transport of spin/valley polarized electrons over very long distances
(tens of micrometers) in a single WSe2 monolayer. By locally pumping the Fermi
sea of 2D electrons to a high degree of spin/valley polarization (up to 75%)
using circularly-polarized light, the lateral diffusion of the electron
polarization can be mapped out via the photoluminescence induced by a
spatially-separated and linearly-polarized probe laser. Up to 25% spin-valley
polarization is observed at pump-probe separations up to 20 microns.
Characteristic spin/valley diffusion lengths of 18 +/- 3 um are revealed at low
temperatures. The dependence on temperature, pump helicity, pump intensity, and
electron density highlight the key roles played by spin relaxation time and
pumping efficiency on polarized electron transport in monolayer semiconductors
possessing spin-valley locking.
