Giant Paramagnetism-Induced Valley Polarization of Electrons in Charge-Tunable Monolayer MoSe_{2}.

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 T leads to a near-complete valley polarization of electrons in a MoSe_{2} monolayer with a density 1.6×10^{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 of 38. 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 MoSe_{2}, which paves the way for valleytronics applications.