  Measurements on a two-dimensional electron system confined to an AlAs quantum
well reveal that for a given electron density the valley susceptibility,
defined as the change in valley population difference per unit strain, is
enhanced as the system makes a transition from partial to full spin
polarization. This observation is reminiscent of earlier studies in which the
spin susceptibility of AlAs electrons was observed to be higher in a
single-valley system than its two-valley counterpart.
