  Transition metal dichalcogenides have been the primary materials of interest
in the field of valleytronics for their potential in information storage, yet
the limiting factor has been achieving long valley decoherence times. We
explore the dynamics of four monolayer TMDCs (MoS$_2$, MoSe$_2$, WS$_2$,
WSe$_2$) using ab initio calculations to describe electron-electron and
electron-phonon interactions. By comparing calculations which both omit and
include relativistic effects, we isolate the impact of spin-resolved spin-orbit
coupling on transport properties. In our work, we find that spin-orbit coupling
increases carrier lifetimes at the valence band edge by an order of magnitude
due to spin-valley locking, with a proportional increase in the hole mobility
at room temperature. At temperatures of 50~K, we find intervalley scattering
times on the order of 100 ps, with a maximum value ~140 ps in WSe$_2$. Finally,
we calculate excited-carrier generation profiles which indicate that direct
transitions dominate across optical energies, even for WSe$_2$ which has an
indirect band gap. Our results highlight the intriguing interplay between spin
and valley degrees of freedom critical for valleytronic applications. Further,
our work points towards interesting quantum properties on-demand in transition
metal dichalcogenides that could be leveraged via driving spin, valley and
phonon degrees of freedom.
