Dependence of effective mass on spin and valley degrees of freedom

We measure the effective mass (m*) of interacting two-dimensional electrons confined to an AlAs quantum well while we change the conduction-band valley occupation and the spin polarization via the application of strain and magnetic field, respectively. Compared to its band value, m* is enhanced unless the electrons are fully valley- and spin-polarized. Incidentally, in the fully spin- and valley-polarized regime, the electron system exhibits an insulating behavior.