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

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We report measurements of the spin susceptibility, χ∝ gᵥ g*m*, in an AlAs two-dimensional electron system where, via the application of in-plane stress, we transfer electrons from one conduction-band valley to another (gᵥ 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 (gᵥ=2), the measured g*m* is smaller than when only one valley is occupied (gᵥ=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.