  Coulomb interaction between electrons lies at the heart of magnetism in
solids. In contrast to conventional two-dimensional (2D) systems, electrons in
monolayer transition metal dichalcogenides (TMDs) possess coupled spin and
valley degrees of freedom by the spin-orbit interaction. The electrons are also
strongly interacting even in the high-density regime because of the weak
dielectric screening in two dimensions and a large band mass. The combination
of these properties presents a unique platform for exploring spin and valley
magnetism in 2D electron liquids. Here we report an observation by
magneto-photoluminescence spectroscopy of a nonlinear valley Zeeman effect,
correlated with an over fourfold enhancement in the exciton g-factor in
monolayer WSe2. The effect occurs when the Fermi level crosses the spin-split
upper conduction band, corresponding to a change of the spin-valley degeneracy
from 2 to 4. The enhancement increases, shows no sign of saturation as the
sample temperature decreases. Our result suggests the possibility of rich
many-body ground states in monolayer TMDs with multiple internal degrees of
freedom.
