This thesis describes an experimental study of the electronic properties of semiconductor heterostructure tunnel devices. InAs self-assembled quantum dots (QDs) are incorporated into the barrier layer of a GaAs/AlAs/GaAs tunnel diode. When a voltage, V, is applied across the device, we observe resonant features in the tunnel current, I, whenever an electron state in one of the qds comes into resonance with an occupied electron state in the emitter. We employ an electron state of a single qd as a spectroscopic probe of a two-dimensional electron system (2DES), from the Fermi energy to the subband edge [1]. For magnetic field B applied parallel to the current, we observe peaks in the I(V) characteristics corresponding to the formation of Landau levels in the 2DES. We obtain quantitative information about the energy dependence of the quasiparticle lifetime, Tqp, of the 2DES. We find that Tqp ~ 2.5 hbar=(Ef - E), in contrast with the expectation for a normal Fermi liquid, but in agreement with predictions for a Fermi liquid state of a disordered 2DES. Close to filling factor nu = 1 we observe directly the exchange enhancement of the g factor. This thesis also describes the design, realisation and measurement of a tunnel diode incorporating InAs QDs and a series of 4 planar electrostatic gates. By applying a bias to the gates, it is possible to selectively inject current into a particular QD. We use magneto-tunnelling spectroscopy to determine the energy levels of the ground and excited state of a single QD, and to map the spatial form of the wave functions of these states [2]. The effect of pressure on the resonant tunnelling of the QDs is also described. [1] P. C. Main et al., Phys. Rev. Lett. 84, 729 (2000) [2] R. J. A. Hill et al., Appl. Phys. Lett. 79, 3275 (2001)

This thesis describes an experimental investigation and analysis of two topical problems in condensed matter physics: 1.) the effect of a magnetic field on quantum states of an electron bound to a shallow donor impurity in a quantum well heterostructure and 2.) the breakdown of the quasi-dissipationless state of the integer quantum Hall effect. Two introductory chapters describe important material parameters and the experimental equipment and techniques used. Magneto-tunneling spectroscopy (MTS) is used to probe the spatial form of the eigenfunction of electrons bound in the ground state of a shallow Si-donor impurities in a GaAs/(A1Ga)As quantum well. An in-plane magnetic field, B[subscript] |, acts to tune the k-vector of the tunnelling electron through the effect of the Lorentz force. The variation with B [subscript] | of the tunnel current through the donor ground state provides a map of the Fourier transform, |ψ(k)| [superscript]2, of the probability density of the ground state donor wavefunction in real space. By applying a strong magnetic field component, B [subscript] ||, parallel to the direction of tunnel current, it is possible to magneto-compress the donor function in real space. The magneto-compression is investigated using MTS and the data are analysed in terms of a simple model, which is critically discussed. The breakdown of the integer quantum Hall effect is investigated by measuring the variation of the voltage drop Vxx along the direction of current flow for a range of currents and magnetic fields and for a number of sample geometries including Hall bars with narrow channels. The data are discussed in terms of two complementary models of breakdown: the bootstrap electron heating model and magneto-exciton formation at a charged impurity. Evidence is found for both types of breakdown depending on the type of sample used and on experimental parameters. For samples with constrictions, it is found that in the breakdown region the value of Vxx measured across a pair of contacts on one side of the Hall bar can differ substantially from that measured on the other side. A model based on magneto-exciton formation at impurities is proposed to explain this unexpected effect. The thesis concludes with a brief summary and suggestions for future work.

BURST is a fast single-shot imaging technique used in magnetic resonance imaging. Most previous implementations of BURST on whole body systems have been carried out at fields of 1.5T and lower. In this work BURST has been implemented on a 3T whole body system. The signal and attenuation characteristics are discussed, leading to an approximate expression for the optimum echo time. A novel method for controlling the pulse amplitude envelope is described. It is shown that this can lead to a modest gain in signal with little loss of resolution, or to a much greater gain in signal with more severe blurring of the image. Frequency modulated RF pulses have been introduced in order to reduce the peak RF power required, which was found to be beyond the range of a 2kW amplifier. The resulting images show a quadratic phase roll in the readout direction, with no reduction in magnitude or image artefacts. A reduction in peak power of more than ten-fold is demonstrated. The maximum reduction factor possible is shown to be approximately equal to the number of pulses applied in the RF train. The total RF power has been reduced by superimposing a sinusoidal oscillation onto the constant excitation gradient. Each pulse is applied at a gradient minimum, and hence need be of lower bandwidth, while the overall excitation is unchanged. The RF pulses are transformed using VERSE. A reduction in SAR of 32% is demonstrated. Greater reductions can be achieved by increasing the amplitude of the oscillation. However, this technique introduces a moderate amount of acoustic noise into the sequence.

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Vertical Bell Lab Layered Space-Time (VBLAST) is an emerging spatial modulation technique, that provides high spectral efficiency due to its use of multiple transmit and receiver antennas. We present novel schemes; consisting of combinations of Transmit Antenna Selection (TAS) with Power Allocation (PA), to improve the error rate performance of VBLAST systems. Our schemes provide significant performance gain over VLAST because of enhanced transmit diversity introduced by transmit antenna selection. Our scheme may be used for uplink wireless system, where the total power is limited by the mobile terminal battery. The effect of limited rate feedback link is investigated by sending quantized power weights from receiver to transmitter. The simulation results show that even one-bit, codebook for power weights can achieve performance close to that of unlimited rate feedback link in low and medium SNR regions. We also present a novel PA technique alloting power in the joint space-frequency domain to reduce the error rate, and apply our scheme to frequency selective MIMO-OFDM channels. In addition, an analytical analysis of transmit correlated Successive interference Cancelling Zero Forcing (SIC-ZF) VBLAST in flat fading channels is presented. Over frequency selective channels, the performance of SIC-ZF VBLAST is assessed by computer simulation.

The free fatty acids of milk fat are believed to be involved in imparting flavor properties to milk and other dairy products. In the past the free fatty acids have largely been related to quality deterioration and hence the methods for measurement have been devised to determine the changes in the free fatty acid content and to relate these data with quality. No method has been reported to determine the quantities of individual free fatty acids. The purpose of this investigation was to evaluate procedures and adapt methods for isolation and measurement of the free fatty acids of milk fat and then to evaluate adapted methods by quantitative measurement of the individual acids in fresh cream fat samples and in butter made therefrom. The ion exchange method of Hornstein et al. (50) was modified to isolate and esterify the free fatty acids from milk fat. The free fatty acids were adsorbed on Amberlite IRA-400 resin, the resin was made fat free and the bound acids were simultaneously esterified and eluted with anhydrous methanol-HCl. The methyl esters were extracted from the reaction mixture with ethyl chloride (b.p. 12.3°C.). The ethyl chloride was evaporated and the esters weighed. The methyl esters were then separated by gas-liquid chromatography using the thermal conductivity detector. The recovery of each saturated even numbered fatty acid from 4:0 to 18:0 was checked. The percent recoveries obtained were: 4:0, 71.4; 6:0, 86.5; 8:0, 66.6; 10:0, 75.1; 12:0, 94.3; 14:0, 100.2; 16:0, 99.5 and 18:0, 92.5. The ion exchange resin was checked for its fat hydrolysing capacity, for retention of fatty acids when used in successive analyses and for leachings of brown polymers during each analysis. The resin did not show detectable hydrolysis of triglycerides nor did it retain or exchange fatty acids from previous use. It was necessary to pretreat the resin with stearic acid and anhydrous methanol-HCl to avoid interference of a leached polymer with quantitative results. An average of 5.0 mg of residue leached from the resin during every analysis, but this did not interfere with the quantitative determination of free fatty acids. Twenty samples of milk fat; ten from pasteurized sweet cream, nine from cultured butter and one from sweet cream butter were analyzed for free fatty acids. The results obtained were compared with the esterified fatty acid content of milk fat. The percent composition of free fatty acids was similar to that of the esterified fatty acids in milk fat. Also the manufacturing process of butter had little or no effect upon the free fatty acid composition of the fat. The values obtained for volatile fatty acids, especially 4:0, were not consistent. One reason for this probably was that evaporation of the ethyl chloride from the solution of the methyl esters was carried out at room temperature and the evaporation rate was not controlled. It is believed that the results would be more consistent if the evaporation of ethyl chloride were carried out under controlled and standardized conditions and if internal standards are employed for quantitative references rather than weighing the ester mixture. Samples of autoxidized milk fat, sweet cream fat and rancid cream fat were analyzed for further evaluation of the method. / Graduation date: 1963

Thesis (PhD)--University of South Australia, 2002.