Robustly protected carrier spin relaxation in electrostatically doped transition-metal dichalcogenides

Transition-metal dichalcogenides are unique semiconductors because of their exclusive coupling between the spin and the valley degrees of freedom. The spin flip simultaneously requires a large amount of the crystal momentum variation; hence most of the carrier scattering is expected to be the spin-conserving intravalley scattering. Analysis of the quantum interference effects on the magnetoconductivity in WSe2,MoSe2, and MoS2 reveals that the spin-relaxation time is orders of magnitude longer than the carrier momentum scattering time, indicating that the valley-spin coupling robustly protects the spin polarization from carrier scatterings. In addition, the electron-spin-relaxation time of MoSe2 is found to be anomalously short compared to other members, which is likely the origin of the ultrafast valley scattering of excitons in MoSe2