We report direct measurements of the valley susceptibility, the change of
valley population in response to applied symmetry-breaking strain, in an AlAs
two-dimensional electron system. As the two-dimensional density is reduced, the
valley susceptibility dramatically increases relative to its band value,
reflecting the system's strong electron-electron interaction. The increase has
a remarkable resemblance to the enhancement of the spin susceptibility and
establishes the analogy between the spin and valley degrees of freedom.Comment: 5 pages, 2 figure