  Valleytronics targets the exploitation of the additional degrees of freedom
in materials where the energy of the carriers may assume several equal minimum
values (valleys) at non-equivalent points of the reciprocal space. In single
layers of transition metal dichalcogenides (TMDs) the lack of inversion
symmetry, combined with a large spin-orbit interaction, leads to a conduction
(valence) band with different spin-polarized minima (maxima) having equal
energies. This offers the opportunity to manipulate information at the level of
the charge (electrons or holes), spin (up or down) and crystal momentum
(valley). Any implementation of these concepts, however, needs to consider the
robustness of such degrees of freedom, which are deeply intertwined. Here we
address the spin and valley relaxation dynamics of both electrons and holes
with a combination of ultrafast optical spectroscopy techniques, and determine
the individual characteristic relaxation times of charge, spin and valley in a
MoS$_{2}$ monolayer. These results lay the foundations for understanding the
mechanisms of spin and valley polarization loss in two-dimensional TMDs:
spin/valley polarizations survive almost two-orders of magnitude longer for
holes, where spin and valley dynamics are interlocked, than for electrons,
where these degrees of freedom are decoupled. This may lead to novel approaches
for the integration of materials with large spin-orbit in robust
spintronic/valleytronic platforms.
