Valley susceptibility of interacting electrons and composite fermions

We report magnetotransport measurements of integer and fractional quantum Hall states in an AlAs quantum well where two conduction band valleys are occupied. By monitoring the valley level crossings for these states as a function of applied symmetry-breaking strain, we determine the “valley susceptibility” (the change of valley population with strain) for both electrons and composite Fermions. The data reveal that these valley susceptibilities are significantly enhanced over the band values, reflecting the role of strong interaction. Moreover, the measured valley susceptibilities are quite similar to the values of spin susceptibility, establishing the analogy between the spin and valley degrees of freedom.