Surface acoustic wave investigations of spin and pseudospin systems

Surface acoustic waves (SAW) have previously provided valuable information in the study of the quantum Hall effect (QHE). Particularly, the SAW attenuation and dispersion both show features at ν = 1/2 that have been attributed to the enhanced conductivity of a composite fermion in zero effective magnetic field. In this thesis the properties of two-dimensional carrier system (2DCS) with an extra degree of freedom are investigated: a bilayer 2DCS where the extra layer degree of freedom is described by a pseudospin, and a narrow quantum well where the g-factor → 0 and spin effects become important. This work extends previous investigations into 2D hole systems (2DHS) by measuring the SAW dispersion and attenuation of single and bilayer 2DHS at frequencies of up to 1GHz at a temperature of 350mK. These measurements were compared to a prediction calculated from simultaneously measured magnetotransport. The single layer 2DHS (a GaAs/AIGaAs heterojunction) was found to exhibit large deviation from the conductivity relaxation model when the Fermi level lies in the localized states of the 2DCS. Several mechanisms for this effect have been considered. Similar SAW measurements of a GaAs/AlGaAs p-type double quantum well showed a greater deviation from the simple relaxation model, attributed to interlayer effects. Evidence for parallel conduction and a correlated state at total ν = 1 has been found in SAW dispersion and attenuation. The longitudinal acoustoelectric field was found to exhibit anomalous behaviour reminiscent of bilayer Coulomb drag experiments. The huge longitudinal resistance (HLR) measured in n-type narrow quantum wells has been investigated. Near ν 2/3 a peak in longitudinal resistance forms over a typical time scale of several minutes. This has been explained in previous work as due to an Ising type ferromagnet formed at ν = 2/3 when different Landau levels become coincident. As the magnetic field is moved away from ν = 2/3 spin-polarization domains form. It is the scattering from domain walls (and a dynamic nuclear polarization) that results in the HLR. Surface acoustic wave measurements were found to be unaffected by the formation of a HLR. This is clear evidence for domain formation, as carriers do not need to move across domain walls to screen the SAW electric field. (author)