  Controlling the momentum of carriers in semiconductors, known as valley
polarization, is a new resource for optoelectronics and information
technologies. Materials exhibiting high polarization are needed for
valley-based devices. Few-layer WS$_2$ shows a remarkable spin-valley
polarization above 90%, even at room temperature. In stark contrast,
polarization is absent for few-layer WSe$_2$ despite the expected material
similarities. Here, we explain the origin of valley polarization in both
materials based on the interplay between two indirect optical transitions. We
show that the relative energy minima at the $\Lambda$- and K-valleys in the
conduction band determine the spin-valley polarization of the direct K-K
transition. Polarization appears as the energy of the K-valley rises above the
$\Lambda$-valley as a function of temperature and number of layers. Our results
advance the understanding of the high spin-valley polarization in WS$_2$. This
insight will impact the design of both passive and tunable valleytronic devices
operating at room temperature.
