  Monolayer transition metal dichalcogenides (TMD) have immense potential for
future spintronic and valleytronic applications due to their two-dimensional
nature and long spin/valley lifetimes. We investigate the origin of these
long-lived states in n-type WS2 using time-resolved Kerr rotation microscopy
and photoluminescence microscopy with ~1 micron spatial resolution. Comparing
the spatial dependence of the Kerr rotation signal and the photoluminescence
reveals a correlation with neutral exciton emission, which is likely due to the
transfer of angular momentum to resident conduction electrons with long
spin/valley lifetimes. In addition, we observe an unexpected anticorrelation
between the Kerr rotation and trion emission, which provides evidence for the
presence of long-lived spin/valley-polarized dark trions. We also find that the
spin/valley polarization in WS2 is robust to magnetic fields up to 700 mT,
indicative of spins and valleys that are stabilized with strong spin-orbit
fields.
