  We report on scattering induced valley polarization enhancement in monolayer
molybdenum disulfide. With thermally activated and charge doping introduced
scattering, our sample exhibits seven? and twelve-folds of improvements
respectively. This counter-intuitive effect is attributed to disruptions to
valley pseudospin precession caused by rapid modulation of exciton momentum and
concomitant local exchange interaction field, at time scales much shorter than
the precession period. In contrast, the valley coherence is improved by
thermally activated scattering, but not by charge doping induced scattering. We
propose that this is due to anisotropic pseudospin scattering and generalize
the Maialle-Silva-Sham model to quantitatively explain our experimental
results. Our work illustrates that cleaner samples with minimal scattering,
such as those carefully suspended or protected by hexagonal boron nitride, do
not necessarily lead to good valley polarization. Well-controlled scattering
can in fact provide an interesting approach for improving valleytronic devices.
