  We demonstrate that, in a quasi-two-dimensional electron system confined to
an AlAs quantum well and occupying two conduction-band minima (valleys), a
parallel magnetic field can couple to the electrons' orbital motion and tune
the energies of the two valleys by different amounts. The measured density
imbalance between the two valleys, which is a measure of the valley
susceptibility with respect to parallel magnetic field, is enhanced compared to
the predictions of non-interacting calculations, reflecting the role of
electron-electron interaction.
