  We have investigated the steady-sate valley polarization and valley coherence
of encapsulated MoS2 monolayer as a function of the temperature and the power
density with a continuous wave laser excitation. Both valley polarization and
coherence exhibit a non-monotonic dependence on sample temperature, attaining a
local maximum at T=40 K. This has been recently attributed to a motional
narrowing effect: an enhancement of the valley relaxation time occurs when the
scattering rate increases. At a fixed temperature of T=6 K, a two-fold increase
of the steady-state valley polarization is achieved by increasing the laser
excitation power, which we attribute to a local heating induced by the energy
relaxation of photoexcited excitons outside the light cone and to an increase
in the exciton-exciton scattering rate. In contrast, in the same power range
only a moderate enhancement of valley coherence is observed. Further increasing
the excitation power leads to a small reduction of valley polarization but a
dramatic loss of valley coherence. Supported by spatial imaging of the
excitonic luminescence and polarization, we attribute this behaviour to the
detrimental role of exciton-exciton interactions on the pure dephasing rate.
