Coupled spin-valley-dynamics in singe-layer transition metal dichalcogenides

Single layers of transition metal dichalcogenides (TMDCs) like MoS$_2$ and WS$_2$ can be produced by simple mechanical exfoliation. Offering a direct bandgap at the K-points in the Brillouin zone, they re\-present a promising semiconductor material for flexible and transparent optoelectronic applications. Due to inversion symmetry breaking together with strong spin-orbit-interaction, the valley and spin degrees of freedom are coupled in monolayer TMDCs. Via circularly polarized optical excitation, an efficient polarization of the $K^+$ or the $K^-$ valley can be generated. Here, we investigate the dynamics of these coupled spin-valley polarizations in monolayer MoS$_2$ and WS$_2$ by means of photoluminescence spectroscopy and time-resolved Kerr rotation (TRKR). The results indicate a maximum achievable spin-valley-lifetime in these materials exceeding one nanosecond at low temperatures. Furthermore, we extract the dependence of the spin-valley lifetime on temperature. By varying the excitation energy, we reveal the excitonic resonances as well as the spin-polarized bandstructure around the K valleys common to monolayer TMDCs.