| In this thesis, we explore the role of spin and valley degrees of freedom on physical properties of interacting two-dimensional electron systems (2DESs) confined to AlAs quantum wells. In addition to controlling the spin polarization via the application of magnetic field, in this system, we also change the conduction-band valley occupation via engineering the quantum well width and the application of strain.;Our measurements reveal that both spin and valley degrees of freedom play crucial roles in renormalizing the fundamental properties of the 2DES including the effective mass (m*), the effective Lande g-factor (g*) and the conduction-band deformation potential ( E*2 ). For a partially spin/valley polarized system, m* is larger than its band value, consistent with previous results on various 2DESs. However, for a fully spin and valley polarized system, m* is unexpectedly suppressed and falls even below the band mass. Moreover, m* measurements in the partially polarized regime reveal a marked contrast between the spin and valley degrees of freedom: When electrons occupy two spin subbands, m* depends on the valley occupation, but not vice versa. Combining m* data with the measured spin and valley susceptibilities, we find that the effective Lande g-factor strongly depends on valley occupation, but the renormalized conduction-band deformation potential is independent of the spin occupation.;We also study the role of valley degree of freedom and the band mass anisotropy on the properties of composite fermions, the emergent particles at high magnetic fields. Our measurements of the piezoresistance at zero magnetic field and at Landau level filling factors nu = 1/2 and 3/2 demonstrate that, qualitatively similar to their electron counterparts, the composite fermions in this system also possess a valley degree of freedom and an anisotropic Fermi surface. |
