Experimental studies of the even-denominator fractions in high Landau levels

In this thesis, I present experimental studies of charge transport at even-denominator filling factors in high Landau levels in a two-dimensional electron system (2DES).; In Chapter 2, I report an ultra-low temperature experiment on the even-denominator fractional quantum Hall effect (FQHE) state at Landau level filling nu = 5/2 in a high mobility specimen of mu = 1.7 x 107 cm 2/Vs. An electron temperature as low as ∼4 mK is achieved, at which we observe vanishing Rxx and, for the first time, a quantized Hall plateau, Rxy = h/(5/2e2) to better than 2 ppm. Rxy at the neighboring odd-denominator states nu = 7/3 and 8/3 is also quantized. The temperature dependence of the Rxx-minima at these fractional fillings yield activation energy gaps: Delta5/2 = 0.11K, Delta7/3 = 0.10K, and Delta8/3 = 0.055K.; In Chapter 3, I investigate the influence of an increasing in-plane magnetic field on the states of half fillings of the second Landau level (nu = 9/2 and 11/2) of a 2DES. In the electrically anisotropic phase at nu = 9/2 and 11/2 an in-plane field of ∼1--2T overcomes its initial pinning to the crystal lattice and reorients this phase. For high in-plane fields the high-resistance axis is always parallel to the direction of the in-plane field.; I also study the effect of in-plane magnetic field on the 5/2 and 7/2 states in Chapter 4. In the initially isotropic phase at nu = 5/2 and 7/2 an in-plane magnetic field induces a strong electrical anisotropy and the high-resistance axis is parallel to the direction of the in-plane field, similar to the 9/2 state.; I have examined the even-denominator Landau level fillings around nu = 9/2 in the so-called GaAs/AlGaAs Hetero-junction Insulated Gate Field-Effect Transistors (HIGFET's) in Chapter 5. We do not observe a strong anisotropic state at nu = 9/2. However, well-developed thermally activated satellite minima at nu ∼ 4.3 and 4.7 are seen adjacent to the 9/2 state, resembling the findings in the fixed-density single GaAs/AlGaAs heterostructures. Their activation energy gaps increase with increasing densities.; I summarize the main results and suggest some possible future work in Chapter 6.