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We present time-resolved Kerr rotation measurements, showing spin lifetimesof over 100 ns at room temperature in monolayer MoSe$_2$. These long lifetimesare accompanied by an intriguing temperature dependence of the Kerr amplitude,which increases with temperature up to 50 K and then abruptly switches sign.Using ab initio simulations we explain the latter behavior in terms of theintrinsic electron-phonon coupling and the activation of transitions tosecondary valleys. The phonon-assisted scattering of the photo-excitedelectron-hole pairs prepares a valley spin polarization within the first few psafter laser excitation. The sign of the total valley magnetization, and thusthe Kerr amplitude, switches as a function of temperature, as conduction andvalence band states exhibit different phonon-mediated inter-valley scatteringrates. However, the electron-phonon scattering on the ps time scale does notprovide an explanation for the long spin lifetimes. Hence, we deduce that theinitial spin polarization must be transferred into spin states which areprotected from the intrinsic electron-phonon coupling, and are most likelyresident charge carriers which are not part of the itinerant valence orconduction band states.

Spin States Protected from Intrinsic Electron-Phonon-Coupling Reaching 100 ns Lifetime at Room Temperature in MoSe$_2$