Dependence of Spin Susceptibility of a Two-Dimensional Electron System on the Valley Degree of Freedom

We report measurements of the spin susceptibility, χ∝g_vg^*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.