Dependence of spin susceptibility of a two-dimensional electron system on the valley degree of freedom

We report measurements of the spin susceptibility, chiproportional tog(v)g(*)m(*), in an AlAs two-dimensional electron system where, via the application of in-plane stress, we transfer electrons from one ellipsoidal conduction-band valley to another (g(v) is the valley degeneracy, and m(*) and g(*) are the electron effective mass and g factor). At a given density, when the two valleys are equally populated (g(v)=2), the measured g(*)m(*) is smaller than when only one valley is occupied (g(v)=1). This observation counters the common assumption that a two-valley two-dimensional system is effectively more dilute than a single-valley system because of its smaller Fermi energy.