  Transition metal dichalcogenide (TMD) monolayers present a singular coupling
in their spin and valley degrees of freedom. Moreover, by applying an external
magnetic field it is possible to break the energy degeneracy between their K
and $-$K valleys. Thus, this analogous valley Zeeman effect opens the
possibility of controlling and distinguishing the spin and valley of charge
carriers in TMDs by their optical transition energies, making these materials
promising for the next generation of spintronic and photonic devices. However,
the free excitons of pristine TMD monolayer samples present a moderate valley
Zeeman splitting, which is measured by their g-factor values that are
approximately $-4$. Therefore, for application purposes it is mandatory
alternative excitonic states with higher magnetic responses. Here we
investigate the valley Zeeman effect in aged WS$_{2}$ and WSe$_{2}$ grown
monolayers by magneto-photoluminescence measurements at cryogenic temperatures.
These samples present a lower energy defect-bound exciton emission related to
defects adsorbed during the aging process. While the free excitons of these
samples exhibit g-factors between $-3$ and $-4$, their defect-bound excitons
present giant effective g-factor values of $-(25.0 \pm 0.2)$ and $-(19.1 \pm
0.2)$ for WS$_{2}$ and WSe$_{2}$ aged monolayers, respectively. In addition, we
observe a significant spin polarization of charge carriers in the defective mid
gap states induced by the external magnetic fields. We explain this spin
polarized population in terms of a spin-flip transition mechanism, which is
also responsible for the magnetic dependent light emission of the defect-bound
exciton states. Our work sheds light in the potential of aged TMDs as
candidates for spintronic based devices.
