  For applications exploiting the valley pseudospin degree of freedom in
transition metal dichalcogenide monolayers, efficient preparation of electrons
or holes in a single valley is essential. Here, we show that a magnetic field
of 7 Tesla leads to a near-complete valley polarization of electrons in MoSe2
monolayer with a density 1.6x10^{12} cm^{-2}; in the absence of exchange
interactions favoring single-valley occupancy, a similar degree of valley
polarization would have required a pseudospin g-factor exceeding 40. To
investigate the magnetic response, we use polarization resolved
photoluminescence as well as resonant reflection measurements. In the latter,
we observe gate voltage dependent transfer of oscillator strength from the
exciton to the attractive-Fermi-polaron: stark differences in the spectrum of
the two light helicities provide a confirmation of valley polarization. Our
findings suggest an interaction induced giant paramagnetic response of MoSe2,
which paves the way for valleytronics applications.
