The purpose of this thesis is to explore some of the consequences of the valley degree of freedom for the properties of AlAs two-dimensional electron systems.; We start by demonstrating our ability to tune the relative valley occupancy in wide AlAs quantum wells by applying a shear, in-plane strain. We continue by characterizing the piezoresistance of AlAs 2DES at both zero and finite magnetic fields. We then proceed to show that, unlike the single-particle predictions, the perpendicular (but not parallel) magnetic field couples to the valley degree of freedom. This suggests that electron-electron interaction is the origin of the observed field-induced valley splitting. We then focus on the role of electron-electron interaction by measuring the energy gap of the quantum Hall state at filling factor nu = 1 as a function of valley splitting. We find that at nu = 1 valleys are strongly coupled through the electronelectron interaction, and that the lowest energy charged excitations of the nu = 1 quantum Hall state are valley Skyrmions. Finally, we explore the interplay between the valley and the spin degrees of freedom by measuring the spin-susceptibility of the AlAs two-dimensional electrons as a function of valley occupancy. At a given density, when two AlAs valleys are equally populated, the measured spin-susceptibility is smaller than when only one valley is occupied. This observation, which is counter to the single-particle prediction, suggests that the valley and spin degrees of freedom are also coupled.
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