| In this thesis we study the fractional quantum Hall (FQH) effect in very high quality two dimensional electrons confined to GaAs single wide quantum wells (WQWs). In these systems typically two electric subbands are occupied at zero magnetic field and the electron charge distribution in the quantum well is bilayer-like. The additional layer (or equivalently, subband) degree of freedom leads to a new variety of quantum Hall states. In the lowest Landau level, we study the intralayer and interlayer interaction as we engineer the charge distribution. For a symmetric charge distribution and appropriate electron density, a unique even-denominator FQH state emerges at filling factor nu = 1/2 which has no counter-part in standard, single-layer systems. We revisit this problem by studying WQW samples with narrow well widths. We find that, based on the large tunneling and the dependence of the energy gap on density, it is not clear whether this state has a Pfaffian or a two-component Halperin origin.;We also find a developing FQH state near nu = 1/2 which is only stabilized in samples with a highly asymmetric charge distribution. In addition to the nu = 1/2 FQH state, we also observe a clear but weak FQH state at nu = 1/4. We find that the evolution of this state with charge asymmetry and density correlates with the FQH state at nu = 1/2.;Our magneto-transport measurements further reveal that the sequence of FQH states observed in two-subband WQWs at high fillings (nu > 2) are very different from those of a single-subband system. When the Fermi level lies in the lowest Landau level of either of the subbands, no quantum Hall states are seen at the even-denominator nu = 5/2 and 7/2 fillings, instead odd-denominator FQH state following the usual, composite fermion filling sequences are observed. The evolution of these states with changing the Zeeman and subband energies is consistent with coincidences of composite fermion Landau levels. |
