Long-lived nanosecond spin relaxation and spin coherence of electrons in monolayer MoS_2 and WS_2

摘要

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 polarizedphotoluminescence (PL) experiments, PL timescales are necessarily constrainedby short-lived (3-100ps) electron-hole recombination. Direct probes ofspin/valley polarization dynamics of resident carriers in electron (or hole)doped TMDCs, which may persist long after recombination ceases, are at an earlystage. Here we directly measure the coupled spin-valley dynamics inelectron-doped MoS_2 and WS_2 monolayers using optical Kerr spectroscopy, andunambiguously reveal very long electron spin lifetimes exceeding 3ns at 5K (2-3orders of magnitude longer than typical exciton recombination times). Incontrast with conventional III-V or II-VI semiconductors, spin relaxationaccelerates rapidly in small transverse magnetic fields. Supported by a modelof coupled spin-valley dynamics, these results indicate a novel mechanism ofitinerant electron spin dephasing in the rapidly-fluctuating internalspin-orbit field in TMDCs, driven by fast intervalley scattering. Additionally,a long-lived spin coherence is observed at lower energies, commensurate withlocalized states. These studies provide crucial insight into the physicsunderpinning spin and valley dynamics of resident electrons in atomically-thinTMDCs.