Composite fermions with a valley degree of freedom

This thesis explores the Physics of the fractional quantum Hall (FQH) effect in two-dimensional electron systems confined to AlAs quantum wells. In addition to spin, these electrons also posses a valley degree of freedom which raises the exciting possibility of a transfer of this characteristic to the composite fermions (CFs). We experimentally tune the valley occupation in our system via the application of strain.;We find that the CFs formed around the Landau level filling factor nu = 3/2 are fully spin polarized but possess a valley degree of freedom of their own. The response of the various FQH states around nu = 3/2 to strain is captured by a CF Landau level fan diagram drawn for a single-spin, two-valley system. We measure the energy needed to completely valley polarize these CFs. We repeat this measurement for CFs around nu = 1/2 which are the particle-hole conjugates of the CFs formed around nu = 3/2. While we observe that the CFs around nu = 1/2 also have a valley degree of freedom, their valley polarization energies are smaller by up to a factor of four than those of their counterparts around nu = 3/2. This discrepancy shows a particle-hole symmetry breaking in our system. Interestingly, the theory developed for single-valley, two-spin systems agrees better with the results around nu = 3/2.;We also report the observation of persistent FQH states at nu = 1/3 and 5/3 at zero strain when the two valleys are degenerate. In the CF picture these FQH states are analogous to the integer quantum Hall state at nu = 1, which is known to form a quantum Hall ferromagnet in the limit of zero valley splitting. We compare the energy gaps measured at nu = 1/3 and 5/3 to the theory developed for single-valley, two-spin systems and find that the gaps and their rates of rise with strain are much smaller than predicted.