Valley susceptibility of an interacting two-dimensional electron system.

In a wide AlAs quantum well grown on a (001) GaAs substrate, 2D electrons occupy two elliptical conduction band minima (valleys), whose major axes lie along the two in-plane $<$100$>$ directions. Strain-induced energy splitting between these valleys results in an inter-valley charge transfer and consequently in a change of the system's valley polarization. We parameterize this strain-induced change of the valley polarization using valley susceptibility $\chi _{v}$, in analogy to the spin-susceptibility traditionally used to characterize the magnetic field induced spin-polarization. We find that $\chi _{v}$ dramatically increases relative to its band value as the electron density is reduced, reflecting the dominant role of electron-electron interaction.