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.

No description available.

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.

Magnetic hysteresis modelling plays a crucial role in determining the electromagnetic properties of soft magnetic materials. A new approach to modelling is the use of recurrent neural networks, capable of time dependent, dynamic, sequential mapping. This research investigates the implementation and accuracy of this approach among other available hysteresis models. / Thesis (MEng(ElectronicsEngineering))--University of South Australia, 2004.

Measurements were made of the rate of magnetization, and the magnetization of the series of Dy (Co, Ni)2 compounds to investigate the intrinsic magnetic after effect. This effect is demonstrated by the existence of an intrinsic coercivity such that a critical value of applied field, Hc, must be reached before the domain walls can move freely through the material. However there is some movement of domain walls of fields below Hc, and this gives rise to time dependent magnetization. The mechanisms by which the domain walls overcome the energy barrier to their motion at fields below Hc was investigated in pulsed field, and in steady fields. After measuring the critical field and magnetic moments of each sample, the pulsed field magnetometer results were analysed to show that there is a certain temperature above which the mechanism for domain wall motion appears to be that described by Taylor???s thermal activation model. Below that particular temperature, the magnetization rate depends on the reciprocal of the applied field, and an empirical model is suggested. Estimates are made of the number of spines contained in the average wall kink; the wall jump frequency and the energy barrier. A vibrating sample magnetometer was used to study the magnetization of the specimens in steady fields. The results indicate that both the mechanisms described above are in operation, with thermal activation being a possibility at temperatures as low as 4.2K.

2 v. : ill., maps ; 30 cm. + 5 fold. maps in end pocket of v.2 / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / Thesis (Ph.D.)--University of Adelaide, Dept. of Economic Geology, 1976

Magnetic hysteresis modelling plays a crucial role in determining the electromagnetic properties of soft magnetic materials. A new approach to modelling is the use of recurrent neural networks, capable of time dependent, dynamic, sequential mapping. This research investigates the implementation and accuracy of this approach among other available hysteresis models. / thesis (MEng(ElectronicsEngineering))--University of South Australia, 2004.

Thesis (Ph. D.)--University of New South Wales, 1981. / Also available online.