Valley spin lifetimes reaching 100 ns in monolayer MoSe$_2$ at room temperature
We present time-resolved Kerr-rotation measurements on a monolayer of MoSe$_2$ revealing spin lifetimes up to 100 ns at room temperature. This extraordinary long-lived spin signal only weakly depends on temperature between 60 K and 300 K. At lower temperatures, it gets masked by an additional spin signal with significantly larger amplitude but shorter spin lifetimes reaching 8 ns. The latter spin signal exhibits a Kerr resonance which coincides with the photoluminescence spectrum from neutral and charged excitons showing that the spin dynamics at low temperatures are dominated by excitonic effects. In contrast, the long-lived spin signal at higher temperatures shows no resonance in the energy regime of the excitons. The absence of such resonance combined with the long spin lifetimes at room temperature is expected if the spin dynamics at elevated temperatures are not dominated by excitonic effects but by a polarization of resident holes, which is protected even at room temperature due to the large spin splitting in the valence bands of transition metal dichalcogenides.