Thesis (MScEng (Electrical and Electronic Engineering))--University of Stellenbosch, 2005. / High-temperature superconductive (HTS) logic has become a large research field worldwide. The HTS advantages compared to the low-temperature circuits are the reduced cooling requirements, increased or comparable operation speeds and the "ease" of manufacturing. The potential of HTS logic circuits is vast and surpassed by very few other technologies. However, before these circuits can reach the market, suitable simulation packages and fabrication processes have to be designed. The simulation of HTS Rapid Single Flux Quantum (RSFQ) circuits are performed at temperatures between 30 and 77 Kelvin. The noise sources in Josephson junctions (JJ’s) are investigated and incorporated into these simulations. The results show that it is possible to operate these circuits in this temperature range. An objective of this thesis is the fabrication of HTS step-edge JJ’s. A process to fabricate these junctions was investigated and improved. This includes the etching of substrate steps with the use of the argon ion mill, deposition of the YBCO thin films with the Pulsed Laser Deposition (PLD) system and the etching of the final circuit by the use of dry or wet etching.

We investigated the electronic structure of the two nematic materials Sr₃Ru₂O₇ and Ca(Fe₀.₉₇Co₀.₀₃As)₂ using spectroscopic – imaging scanning tunneling microscopy (SI-STM) and angle resolved photoemission spectroscopy (ARPES). – – – Sr₃Ru₂O₇ is an itinerant metamagnet that shows a putative quantum critical endpoint at 8 Tesla, submersed by the formation of a nematic electronic phase. Using ARPES, we identified at least 5 Fermi pockets in agreement with quantum oscillation measurements. Surprisingly, we found Fermi velocities up to an order of magnitude lower than in single layer Sr₂RuO₄ and up to 35 times lower than predicted by ab initio calculations. Many bands are confined in an energy range of only ∼10 meV below the Fermi level. This, as well as distinct peak-dip-hump shapes of the spectra with a characteristic energy of around ∼5 meV indicate strong correlations and a possible nontrivial mechanism that is absent in single layer Sr₂RuO₄ and connected to the nematicity. The quasiparticle interference of one of the bands was detected by SI-STM, which was also used to measure subatomic features with the symmetries of the relevant Ru d orbitals. – – – In the second mate- rial, the iron-based high-temperature superconductor Ca(Fe[subscript(1-x)]Co[subscript(x)]As)₂, we discovered electronic nematic nano-pattern in its under-doped ‘parent’ state. We spectroscopically imaged this state in real space over large areas and across domain boundaries that change the directionality of the nano-pattern by 90°. We propose that oriented, dimer-shaped electronic nematogens are responsible for this pattern, in striking contrast to what has been expected and observed in electronic nematic materials. The dimers consist of two Gaussian conductance peaks separated by about 8 a[subscript(FeFe)]. Unidirectionality also shows in the quasiparticle interference pattern of the delocalized electrons. The dispersion is in agreement with scattering from the α₂ band discovered by ARPES but has distinct C₂ symmetry, not inconsistent with a C₄-symmetric band scattered by the proposed dimers.

Strongly correlated metals are known to give rise to a variety of exotic states. In particular, if a system is tuned towards a quantum critical point, new ordered phases may arise. Sr₃Ru₂O₇ is a quasi-two dimensional metal in which field-tuned quantum criticality has been observed. In very pure single crystals of this material, a phase with unusual transport properties forms in the vicinity of its quantum critical point. Upon the application of a small in-plane field, electrical resistivity becomes anisotropic, a phenomenon which has led to the naming of this phase as an `electron nematic'. The subject of this thesis is a study of the electrical transport in high purity crystals of Sr₃Ru₂O₇. We modified an adiabatic demagnetisation refrigerator to create the conditions by which the entire temperature-field phase diagram can be explored. In particular, this allowed us to access the crossover between the low-temperature Fermi liquid and the quantum critical region. We also installed a triple axis `vector magnet' with which the applied magnetic field vector can be continuously rotated within the anisotropic phase. We conclude that the low- and high-field Fermi liquid properties have a complex dependence on magnetic field and temperature, but that a simple multiple band model can account for some of these effects, and reconcile the measured specific heat, dHvA quasiparticle masses and transport co-efficients. At high temperatures, we observe similarities between the apparent resistive scattering rate at critical tuning and those observed in other quantum critical systems and in elemental metals. Finally, the anisotropic phase measurements confirm previous reports and demonstrate behaviour consistent with an Ising-nematic, with the anisotropy aligned along either of the principal crystal axes. Our observations are consistent with the presence of a large number of domains within the anisotropic phase, and conclude that scattering from domain walls is likely to contribute strongly to the large measured anisotropy.

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Superconductors present great potential for weight reduction and increased power delivery when compared to traditional copper power delivery systems, but current systems require cryogenic cooling systems. Traditional superconductor cooling systems consist of helium cooled by helical heat exchangers made of Oxygen Free High thermal Conductivity (OFHC) copper tube. The helium is cooled by bulky heat exchangers consisting of OFHC copper coils wrapped around a cryogenic cooler heat sink for heat transfer into the working fluid. Metal foams have recently been studied in a variety of heat transfer applications, and could greatly reduce the weight of heat exchanger modules in superconductor cooling systems while simultaneously providing increased heat transfer effectiveness. Aluminum and Copper foams have been available for several years, but more recently, graphite foams, such as PocoFoam™, have been developed which have particularly good heat transfer characteristics. Using Computational Fluid Dynamics (CFD) to model a cryogenic heat exchanger application, this study examines the effectiveness and pressure drop of several metal foam heat exchangers, and compares their performance with the traditional helical coil design for superconductor cooling applications. The CFD simulation results show that a heat exchanger with the same heat sink contact area as existing helical heat exchangers weighs up to 95 percent less and can be up to 25 percent more effective, depending on system conditions such as pressure, cryogenic cooler temperature and helium inlet temperature. Aluminum and copper foam heat exchangers had comparable weight to the PocoFoam heat exchanger, but were significantly less effective than the helical or PocoFoam heat exchanger models.

L'apparition des supraconducteurs à hautes températures (HTc) a permis un développement continu des moteurs supraconducteurs. Ces dispositifs supraconducteurs permettent d'obtenir des puissances et des couples massiques et volumiques très élevés, et sont des alternatives intéressantes aux machines conventionnelles. Le dimensionnement des ces moteurs nécessite une bonne connaissance des caractéristiques électromagnétiques, thermiques et mécaniques des supraconducteurs HTc. Ainsi, le travail présenté ici traite du dimensionnement de machines HTc avec la prise en compte des contraintes liées aux fils supraconducteurs. Le manuscrit débute par une étude bibliographique approfondie dans laquelle trois points sont abordés : les fils supraconducteurs, description des réalisations remarquables de machines HTc de fortes puissances et une analyse statistique sur ces réalisations. Ensuite, la caractérisation DC des fils supraconducteurs HTc et la détermination de leurs pertes AC sous champ variable sont abordées. Des méthodes numériques sont utilisées pour la modélisation et les résultats obtenus sont comparés à ceux de l'expérience. La dernière partie aborde le dimensionnement électromagnétique de moteurs supraconducteurs HTc par des méthodes analytiques et numériques, avec la prise en compte des contraintes liées aux courants critiques dans les bobinages. Les premiers résultats concernant le dimensionnement électromagnétique et thermique d'une structure de machine synchrone supraconductrice à pôles saillants sont donnés. Les solutions techniques envisagées sont largement commentées / The appearance of high-temperature superconductors (HTS) has led to a continuous development of superconducting motors. These superconducting devices allow high power and torque densities, so they constitute and novel alternative to conventional motors. The design of such motors requires a deep knowledge of the electromagnetic, thermal and mechanical characteristics of the HTS materials. Hence, the work presented here is related to the design of HTS motors which includes the constraints due to the HTS conductors. The manuscript begins with an extensive literature review where three main points are discussed, viz. HTS conductors, already constructed HTS motors and statistical analysis on these achievements. The dc characterization of HTS superconductors as well as their ac losses under time varying magnetic fields are theoretically and experimentally studied. In so doing, numerical techniques are used and the obtained results are compared the measurements. The last part deals with the design of HTS motors by analytical and finite element methods. A particular attention is given to the constraints introduced by the use of HTS materials. The first results of the electromagnetic and thermal design of a salient pole HTS synchronous machine are provided with the emphasis of the adopted technical solutions

A superfície (001) dos compostos III-V têm grande importância para as modernas tecnologias de crescimento de materiais, assim o estudo da formação dessa superfície, de suas propriedades e dos processos de adsorção de moléculas que ocorrem sobre ela é essencial para o desenvolvimento da ciência de materiais. Sabe-se que o InP, a exemplo de outros compostos III-V, apresenta uma variedade de padrões de reconstrução para superfície (001) que dependem das condições iniciais que prevalecem durante o seu crescimento. Neste trabalho estudamos os padrões da superfície InP(001) originados em ambientes com concentração máxima de átomos de índio e de fósforo e os processos envolvidos com a adsorção da molécula H2S sobre estas superfícies. Usamos cálculos de primeiros princípios dentro do formalismo da Teoria do Funcional da Densidade (DFT) associados à pseudopotenciais de norma conservada com aproximação generalizada do gradiente para o termo da energia de troca e correlação (DFT-GGA) e correlação não linear de caroço (NLCC). Fizemos a adsorção da molécula de H2S sobre os padrões mais estáveis da superfície InP(001), considerando os casos com a molécula dissociada e não dissociada. No padrão reconstrução 2x2 com dois dímeros, superfície originada em ambientes ricos em átomos de fósforo, fizemos o cálculo das barreiras de energia entre as configurações energeticamente mais favoráveis e encontramos um mecanismo de adsorção para a molécula de H2S dissociada Na superfície com padrão de reconstrução 2x4, crescida em ambientes ricos em átomos de índio, com formação de um dímero misto, verificamos que a molécula não dissociada adsorve sobre os sítios formados por átomos de fósforo e que adsorve sobre os sítios formados por átomos de índio. O átomo de enxofre da molécula dissociada adsorve sobre os sítios formados por átomos de fósforo e de índio e as moléculas de hidrogênio permanecem desorvidas, porém as estruturas são desfavoráveis energeticamente. / The (001) surface of the III-V compounds have great importance for modern technologies of materials growth thus the study of these surfaces formation, its properties and the absorption process of the molecules are essential for the development of materials science. It is well known that the InP, as well others III-V compounds, present a variety of reconstruction patterns for (001) surface that depend on the initial growth conditions. In this work we studied the patterns of InP(001) surface considering an environment with high concentration ln and P atoms and the interaction of the H2S molecule with these surfaces. Using first-principles calculations within the Density Functional Theory (DFT) formalism, norm-conserving pseudopotentials with the generalized gradient approximation for the exchange and correlation energies (GGA) and non-linear core-correction (NLCC). We have studied the H2S molecule adsorption on different reconstruction of the InP(001) surface, considering the cases in which the H2S molecule was dissociated and non-dissociated. In the 2x2 pattern with two dimmers, the energy barriers were calculated between the more energetically favorable configurations and a possible mechanism of adsorption of dissociated H2S molecule is proposed. The non-dissociated molecule does not bind over phosphorus sites but over indium sites, for all considered reconstructions. The sulfur atom of dissociated molecule adsorbs over phosphorus and indium sites and the hydrogen molecules stands not joint, however the structure are energetically unfavorable.