This thesis discusses experiments that have been performed to investigate ultrafast acoustoelectric effects in semiconductor devices. Current commonly employed techniques to generate ultrafast acoustic pulses and detect them with spectral resolution require a powerful pulsed laser system that is bulky, expensive and complicated. If the acoustic pulses could instead be generated and detected by electrical methods, picosecond acoustic techniques could become more readily available as a tool for other users. This thesis focusses on the electrical detection of acoustic pulses with spectral resolution. In many of the key experiments described in this thesis a picosecond strain pulse was generated optically on the opposite face of the sample to the semiconductor device of interest. The strain was generated either in a thin Al film thermally deposited on the sample surface, or directly in the GaAs substrate. Acoustic phonons generated by this method propagated across the substrate to the device. Transient voltages across the semiconductor device caused by the incident phonons were detected using a high frequency real-time oscilloscope. The first evidence of heterodyne mixing of coherent acoustic phonons with microwaves was obtained, for frequencies up to about 100 GHz. First, it was confirmed that Schottky diodes can produce a fast transient voltage in response to an incident acoustic wavepacket. The detection process occurs at the semiconductor-metal interface, and is due to the deformation potential. Bow-tie antenna fabricated directly onto the GaAs substrate proved to be ineffective at coupling microwaves from free space to the Schottky diode. A waveguide-coupled beam-lead Schottky diode provided by e2v had a sufficient response to the incident microwaves to proceed with the mixing experiments. The microwave local oscillator signal was mixed with a tunable narrow frequency band acoustic signal that was produced using a Fabry-Perot etalon external to the laser cavity. The intermediate frequency components were in the range of 1-12 GHz, which could be detected on the oscilloscope. Mixing was performed using both the fundamental frequency acoustic wave and the second harmonic generated in the sample. Semiconductor superlattices were also investigated as electrical detectors for ultrafast acoustic pulses. In this case, the transient voltage measured across the device contained an unexpected contribution in the form of a peak with a width of approximately 2 ns. This signal is too slow to be caused by a strain pulse and too fast for a heat pulse. It is proposed that this peak is caused by long-lived phonon modes from the centre of the mini-Brillouin zone being confined in the superlattice due to Bragg reflections. The peak caused by confined phonons and the two peaks caused by heat pulses also present in the detected signal were investigated for a range of experimental conditions. This allowed comparisons to be made to previous works. A similar superlattice structure had a very different response to the incident acoustic wavepacket. The polarity of the transient voltage detected was inverted and there was no evidence of an electronic response to the confined phonon modes, which would have been present in both samples. It is proposed that the barriers of the NU1727 superlattice sample are thicker than expected, and this strongly affects the electron transport through the structure. This thesis shows that semiconductor devices can be suitable for the electrical detection of ultrafast strain pulses. For this technique to reach its full potential, it is also necessary to be able to generate these strain pulses electrically. A step recovery diode has been considered for this purpose as part of the suggested future work.

The aim of this project is the conception, implementation, and application of a simulation tool for the accurate modeling of electromagnetic fields within inhomogeneous materials with complex shapes and the propagation of the resulting fields in the surrounding environment. There are many methods that can be used to model the scattering of an electromagnetic field, however one of the most promising for hybridisation is the Boundary Element Method (BEM), which is a surface technique, and the Unstructured Transmission Line Modeling (UTLM) method, which is a volume technique. The former allows accurate description of the scatterer's boundary and the field's radiation characteristics, but cannot model scattering by materials characterized by a non-uniform refraction index. The latter, on the contrary, can model a very broad range of materials, but is less accurate, since it has to rely on approximate absorbing boundary conditions. A method resulting in the hybridisation of BEM and UTLM can be used to construct a tool that takes into account both the interaction with non-uniform tissue and propagation in its environment. The project aims to describe in detail the implementation of the novel method, and deploy it in a heterogeneous distributed computing environment.

No description available.

The magnetic behaviour (magnetization curves, Meissner effect, hysteresis losses, remanent flux and flux line cutting) for high $T\sb{c}$ samples of Nd and $YBa\sb{2}Cu\sb{3}O\sb{7-x}$ at 77 K, semi-reversible PbIn and hysteretic VTi at 4.2 K have been investigated and compared. The magnetic behaviour of the high $T\sb{c}$ samples and of the PbIn are remarkably similar but contrasts dramatically with the phenomena observed in the VTi. A hump structure appears in the low field region of the initial magnetization of the high $T\sb{c}$ samples. This phenomenon occurs because the sintered samples consist of a compact agglomeration of small irregularly shaped grains which are electrically coupled before the hump structure but are isolated after this feature. We show that the appropriate calibration should be based on the diamagnetic response of the uncoupled grains. A large ratio of $j\sb{c\Vert}$ to $j\sb{c\perp}$ (the critical current densities $\Vert$ and $\perp$ to the flux line density) accounts for the observations on the VTi in the flux cutting regime whereas $j\sb{c\Vert} \approx j\sb{c\perp}$ is indicated by the behaviour of the high $T\sb{c}$ and PbIn samples.

We have developed a novel experimental approach for the study of the interaction of sheets of non parallel flux lines in hysteric type II superconductors. We continuously monitor the evolution of the components of the magnetic flux density $\Vert$ and $\perp$ to $H\sb{a}$ (i.e. $\langle B\sb{z}\rangle$ and $\langle B\sb{y}\rangle$) as $H\sb{a}$ is raised to various intensities, and then reduced to zero. In our investigation of a high $T\sb{c}(YBa\sb{2}Cu\sb{3}O\sb{7-x}$) ceramic, $H\sb{a}$ exceeded $H\sb{*}$, the first full penetration field. We applied the phenomenological Clem general critical state model to the analysis of our extensive observations. A computer program was developed to solve the four coupled differential equations of this theory with appropriate physical constraints for the situations prevailing in our experiment. This analysis provides detailed insight into the evolution of the intricate configurations of the magnetic flux density $\vec B$(x), the critical current density, $\vec J$(x), and electric field $\vec E$(x) patterns as the injected and trapped flux lines are made to interact, unpin, migrate and undergo flux cutting processes. The model is seen to generate the variety of complicated measured curves of $\langle B\sb{y}\rangle$ and $\langle B\sb{z}\rangle$ vs $H\sb{a}$ very satisfactorily. (Abstract shortened by UMI.)

The purpose of this study is to investigate the conduction electrons' interaction mechanisms over the wide temperature range in order to find possible deviations from the existing theories. The temperature dependence of the electrical resistivity of two scattering systems, the crystalline and the amorphous, in the range of 1.7--300 K is studied. In the pure crystalline metals, the nearly-free-electron model can qualitatively and quantitatively account for the temperature dependence of the electrical resistivity. In the low-temperature range, the electron-electron scattering is the dominant scattering mechanism. Above the Debye temperature, the phonon-electron scattering is the dominant scattering mechanism. The temperature dependence of the electrical resistivity for the amorphous metallic alloys can be described qualitatively by the Ziman model. The scattering mechanism of the conduction electrons of the samples can be categorized in the weak scattering limit regime. A small, but significant correction must be added to the Ziman model in order to account for the additional scattering contributions that exist in the wide temperature range. In the very-low-temperature range the electron-electron interaction effect is the major correction to the temperature dependence of the electrical resistivity. In the low-temperature range, the weak localization effect is the major correction to the temperature dependence of the electrical resistivity. In the high-temperature range, the temperature dependence of the electrical resistivity is linear with a small but significant correction. For some samples the magnetic contribution is the major correction, for others the multiphonon scattering is the major correction. For the resistivity minima at temperatures below 20 K could be due to the Kondo effect. A more plausible explanation, however, is the electron-electron interaction effect. The Ziman model with the proper corrections is found to be an adequate and successful model in accounting for the temperature dependence of the electrical resistivity of all of the studied amorphous metallic alloys.

The applicability of the weak-localization theory to highly ordered quasicrystals raises the question of whether or not the long-range order in these alloys can be reconciled with the electronic disorder. This study did not detect any unusual structure-induced contribution to the resistivity at low temperatures other than those known for metallic glasses. The temperature dependence of the resistivity in icosahedral quasicrystals of high structural quality showed that the transport behaviour of these alloys at low temperatures can be satisfactorily explained in terms of conventional weak-localization and electron-electron interaction theories. The temperature dependence of the resistivity in weakly disordered (low-resistivity) alloys can be explained qualitatively in terms of the classical theories extended to liquids and disordered alloys. The experimental evidence shows the existence of a close relationship between the icosahedral quasicrystalline structure and the weak localization tendency of the electronic states at the Fermi level. To explain the temperature dependence of the resistivity at high temperatures additional models based on the concept of the band transition and hopping have to be invoked. In the high-resistivity stable icosahedral Al-Cu-Ru alloy the insulating-like (electron hopping) behaviour was found to dominate the electron transport even at low temperatures. This causes the deviations from the weak-localization theory and is due to the enhancement of the density of localized electronic states at the Fermi level. Consequently, the Anderson localization is collapsed in the vicinity of the metal-insulator transition. The values of the correlation gap in the Al-Cu-Fe-Mn icosahedral system suggest a considerably different nature for the pseudogap around the Fermi level in amorphous and icosahedral phases. However, unless an ideal quasicrystal belongs to the metal-insulator transition region (if it exists), the weak-localization theory will be the most appropriate tool to investigate the low temperature electron transport properties in icosahedral phases.

The desirable properties of the paramagnetic substances are discussed and the possibility of using some metallic alloys as well as dielectric crystals doped with proper substances capable of lowering the temperatures is illustrated. For this purpose some low temperature investigations of MgO:Mn and of Mg:Ce alloys is presented. The MgO:Mn doped with 0.0325% Mn gives a final temperature of 0.04°K when demagnetized from a field of 18 Kgauss and approximately 1.3°K. The specific heat measurements resulted in a value of CT 2/R = 6x10-3 for the "high temperature" tail of the specific heat. The entropy determinations performed on the Mg:Ce alloy system on the other hand indicated that the magnetic entropy at about 1°K is approximately of the same value as that of the conduction electrons.Therefore it seems that this particular alloy system is not suitable for appreciable lowering of the temperatures by adiabatic demagnetization. The specific heat of a well behaved pure substance is proposed as a resonably sensitive thermometric standard. Its usefulness is illustrated by calibrating a secondary thermometer using the proposed method. The difficulties encountered in low temperature calorimetry are described and our solution of them is given. A discussion is also included of the difficulties involved in using mechanical heat switches. A double mechanical heat switch is proposed and a design of it is included together with some illustrations of its usefulness.

The ionic and electronic contributions to the total magnetic susceptibility of a non ferromagnetic metal alloy are discussed, and a review of the experimental techniques for measuring susceptibilities is given. The presence of even small amounts of ferromagnetic impurities can have disastrous effects on the measured susceptibilities, and a method is described for correcting for such impurities when the Gouy method is used. This enabled the true susceptibility of the non ferromagnetic part of a substance to be determined with an accuracy of .1% relative to Ge. The room temperature values of magnetic susceptibility of the dilute alloys of Mn and Fe in Mg and Al were measured in view of obtaining information about the electronic configuration of the transition elements in solution. A localized d electron picture rather than a d band picture was found to be adequate. The measured effective Bohr magneton numbers yielded two possible electronic configurations for the solute atoms. The possibility of distinguishing between these is discussed in terms of the corresponding change in population of the conduction band and its influence on the electronic susceptibility.

This thesis reports on the behavior of the n.m.r signal from Al27 in Al and Al Mn alloys. We have obtained values of 0.162 .002% for the Knight shift and of 8.65 .2 oe. for the line width in pure Al, in good agreement with values reported elsewhere. On alloying with Mn, decreases in intensity, line width and Knight shift are observed. This behavior is readily explained by nuclear electric quadrupole interactions. The small effective magnetic moment of the Mn ions seem to have a negligible effect on the line and from that point of view Al Mn behaves essentially like Al Mg and Al Zn which have been extensively studied recently.