  The recently-discovered monolayer transition metal dichalcogenides (TMDCs)
provide a fertile playground to explore new coupled spin-valley physics.
Although robust spin and valley degrees of freedom are inferred from polarized
photoluminescence (PL) experiments, PL timescales are necessarily constrained
by short-lived (3-100ps) electron-hole recombination. Direct probes of
spin/valley polarization dynamics of resident carriers in electron (or hole)
doped TMDCs, which may persist long after recombination ceases, are at an early
stage. Here we directly measure the coupled spin-valley dynamics in
electron-doped MoS_2 and WS_2 monolayers using optical Kerr spectroscopy, and
unambiguously reveal very long electron spin lifetimes exceeding 3ns at 5K (2-3
orders of magnitude longer than typical exciton recombination times). In
contrast with conventional III-V or II-VI semiconductors, spin relaxation
accelerates rapidly in small transverse magnetic fields. Supported by a model
of coupled spin-valley dynamics, these results indicate a novel mechanism of
itinerant electron spin dephasing in the rapidly-fluctuating internal
spin-orbit field in TMDCs, driven by fast intervalley scattering. Additionally,
a long-lived spin coherence is observed at lower energies, commensurate with
localized states. These studies provide crucial insight into the physics
underpinning spin and valley dynamics of resident electrons in atomically-thin
TMDCs.
