  We investigate with polarized microphotoluminescence the optical pumping of
the valley degree of freedom in charge-tunable MoS2 monolayers encapsulated
with hexagonal boron nitride at cryogenic temperatures. We report a large
steady state valley polarization of the different excitonic complexes following
circularly-polarized laser excitation 25 meV above the neutral exciton
transition. For the first time in this material we reveal efficient valley
pumping of positively-charged trions, which were so far elusive in
non-encapsulated monolayers due to defect and laser-induced large electron
doping. We find that negatively-charged trions present a polarization of 70 %
which is unusually large for non-resonant excitation. We attribute this large
valley polarization to the particular band structure of MoS2, where an
optically dark exciton ground state coexists with a bright conduction band
ordering in the single-particle picture, leading to a supression of the valley
relaxation for negatively-charged trions. In addition, we demonstrate that
circular excitation induces a dynamical polarization of resident electrons and
holes, as recently shown in tungsten-based monolayers. This manifest itself as
a variation in the intensity of different excitonic complexes under circular
and linear excitation.
