  Semiconductor transition metal dichalcogenides (TMDs) have equivalent
dynamics for their two spin/valley species. This arises from their
energy-degenerated spin states, connected via time-reversal symmetry. When an
out-of-plane magnetic field is applied, time-reversal symmetry is broken and
the energies of the spin-polarized bands shift, resulting in different bandgaps
and dynamics in the K$_+$ and K$_-$ valleys. Here, we use time-resolved Kerr
rotation to study the magnetic field dependence of the spin dynamics in
monolayer MoSe$_2$. We show that the magnetic field can control the
light-induced spin accumulation of the two valley states, with a small effect
on the recombination lifetimes. We unveil that the magnetic field-dependent
spin accumulation is in agreement with hole spin dynamics at the longer
timescales, indicating that the electron spins have faster relaxation rates. We
propose a rate equation model that suggests that lifting the energy-degeneracy
of the valleys induces an ultrafast spin-flip toward the stabilization of the
valley with the higher valence band energy. Our results provide an experimental
insight into the ultrafast charge and spin dynamics in TMDs and a way to
control it, which will be useful for the development of new spintronic and
valleytronic applications.

Mesoscale and Nanoscale Physics