Grain boundaries form the basis of an important Josephson junction technology in the cuprates and also limit the superconducting critical currents attainable in practical, polycrystalline materials. An improved understanding of these defects is therefore important for applications. The status of the current understanding of cuprate grain boundaries is summarised and experimental investigations are presented, focusing on the less well understood high angle boundaries. Measurements of the capacitance of grain boundaries in the overdoped superconductor Y1-xCaxBa2Cu3O7-8, were performed as a function of the calcium content, using the Josephson coupling across the boundaries. Particular care was taken to eliminate the effects of heating and stray capacitance due to the substrate. The effect of thermal noise was also assessed. These measurements provide important information about the area and the width of the grain boundaries, that highlights their inhomogeneous nature. A new technique was applied to measure the normal state properties of YBa2Cu3O7-8 grain boundaries above the critical temperature. Since the resistance of the adjacent material at high temperatures is comparable to, or greater than, that of the grain boundary a compensating Wheatstonebridge structure was used. The errors involved in this technique are carefully assessed and quantified. The normal state resistance of a number of different grain boundary orientations was measured from room temperature to the critical temperature. Detailed characterisation of the grain boundaries, including measurements of the critical current and the current voltage characteristics at low temperatures, was performed. The results obtained are used to assess the validity of the various theories for the grain boundary electrical structure. A tunneling model that accounts for the band structure of the material is developed and applied to potential barriers consistent with a band bending model. This theory is shown to provide a convincing account of the experimental results presented in this thesis.

Critical currents in the latest biaxially textured 'coated conductors' are now limited by 2D networks of low angle grain boundaries (LAGBs) with misorientation θm = 1 - 10°. In order to understand and optimise current transfer in these materials it is essential to elucidate the electromagnetic behaviour of the LAGB. This work presents an investigation into the transport properties of [001]-tilt LAGBs formed by the thin film deposition of YBa2Cu3O7 onto bicrystalline substrates. Through the use of a precision two-axis goniometer, measurements of the V-I characteristic and critical current density were made as magnetic field was rotated in angles θ and φ relative to the LAGB defect. It is found that for fields applied parallel to the LAGB defect plane, dissipation is dominated by the viscous flux flow of vortices along the boundary. Clear evidence for this is found in the V-I characteristic, which displays an increased linearity indicative of the viscous regime. It is shown that the number of vortex rows involved in the flow process can discontinuously switch, leading to a V-I characteristic made up of straight segments of different gradient. For fields applied away from the LAGB defect plane a kinked vortex structure develops and the boundary critical current density, JcGB, is determined by the channelling of vortex segments still lying within the LAGB. The channelling regime is seen in angular measurements as a marked decrease in JcGB(θ,φ) as field becomes aligned to within critical angles φK or θK of the LAGB. The aligned vortices experience a reduction in dimensionality that is manifest in a reduced temperature dependence of JcGB(T). For fields applied at a sufficiently large angle from the defect plane the deleterious effects of the LAGB on current transport are mitigated considerably. In this regime a collinear vortex structure is regained and transport is controlled by the intragranular (IG) sections of the LAGB track; JcGB = δJcIG where δ ~ 0.8, a result that is independent of field, temperature and angle for a 4.9° LAGB. For rotation of the applied field within the LAGB defect plane, the presence of a pinning peak in JcGB for field aligned to the dislocation array is confirmed and modelled. The peak is found to be absent in IG track measurements and increases relative to the intrinsic peak with both increasing field and temperature. In addition, a pronounced angular hysteresis is presented, which is directly linked to a corresponding 'static' hysteresis in JcGB(B) with field. Magneto-optic measurements confirm that this effect is controlled by the flux density profile in the IG regions of the LAGB track. Finally, above a temperature, angle and sample dependent merging field, B*, the LAGB is found to be effectively transparent, as δ ~ 1. This is due to the irreversibility line, above which dissipation occurs across the whole LAGB track.

Large electrical current transport in the absence of energy losses is thekey factor in commercial applications of high temperature superconductors. This thesis demonstrates an easy and inexpensive bottom-uptechnique to produce self assembled nanorods, segmented nanorodsas well as nanoparticles in YBa2Cu3O7-δ thin films grown by pulsedlaser deposition. The structural and morphological characteristic ofthe pinning landscapes produced are investigated and correlated totheir effects on the superconducting properties of the thin films. In particular two pinning landscapes are investigated: Ba2YNbO6nanorods are grown in YBa2Cu3O7-δ thin films using a Ba2YNbO6doped YBa2Cu3O7-δ pulsed laser deposition targets andBa2(Y/Gd)(Nb/Ta)O6 segmented nanorods together with (Y/Gd)2O3nanoparticles are grown in (Y/Gd)Ba2Cu3O7-δ thin films using aBa2YNbO6 + Gd3TaO7 doped YBa2Cu3O7-δ pulsed laser depositiontargets. The Ba2YNbO6 + YBa2Cu3O7-δ is deeply characterised and the effectsof the deposition parameters are analysed. Ba2YNbO6 is demonstratedto be an interesting novel pinning addition capable to increasethe critical current and to reduce the YBa2Cu3O7-δ critical currentsangular dependencies anisotropy. The Ba2YNbO6 + Gd3TaO7 + YBa2Cu3O7-δ is found to produce anew complex pinning landscape extremely effective. At high fieldsthe synergetic combination of the different defects typology is shownto generate an interesting new feature in the critical current angulardependencies. Chapter 1 is an introduction to superconductivity, the fundamentals ofthe field are briefly presented. In chapter 2 the discussion in focused onpinning in high temperature superconductors. Cuprates and in particularYBa2Cu3O7-δ are presented. The pinning phenomenon andthe practical pinning engineering in thin films is also discussed in thischapter. Chapter 3 describes the thin films preparation methods andthe characterisation techniques used in the research work. Chapter 4and 5 are focused on the Ba2YNbO6 doped YBa2Cu3O7-δ thin films. Chapter 4 is an introduction to Ba2YNbO6 doped YBa2Cu3O7-δ, thepreliminary results obtained on Ba2YNbO6 doped YBa2Cu3O7-δ thin films are shown in this chapter. The crystalline structure, the morphologyand the superconducting properties of thin films depositedadopting different deposition parameters are analysed and discussedin chapter 5. In chapter 6 the new complex pinning landscape ofBa2(Y/Gd)(Nb/Ta)O6 and (Y/Gd)2O3 in (Y/Gd)Ba2Cu3O7-δ is presented. Concluding remarks on the research described in the workends the thesis in a brief final chapter 7.

The effect of oxygen partial pressure and temperature on the solid state sintering kinetics of the superconducting ceramic YBa₂Cu₃O₇₋₈ has been investigated. The isothermal contraction behavior of YBa₂Cu₃O₇₋₈ in the temperature range 930 - 960°C with oxygen partial pressures varying from 0.001 to 1.0 atmospheres was studied. The change in the unit cell lattice parameters of YBa₂Cu₃O₇₋₈ over this oxygen partial pressure range was also determined in order to evaluate its effect on the sintering behavior. The sintering kinetics of YBa₂Cu₃O₇₋₈ were investigated through the variation of the isothermal steady state contraction rate with respect to oxygen partial pressure at different temperatures. Isothermal contraction rates between 930 and 955°C reached a maximum at some critical oxygen partial pressure, PO₂CRIT, with decreasing contraction rates both above and below this oxygen partial pressure. This behavior was not observed for YBa₂Cu₃O₇₋₈ sintered at 960°C. SEM micrographs of the fracture surface of these samples suggest that a liquid phase was formed at this temperature. The unit cell volume and a-lattice parameter of YBa₂Cu₃O₇₋₈ decreased with decreasing oxygen partial pressure, while the c-lattice parameter increased. The activation energy for sintering at oxygen partial pressures above PO₂CRIT has been estimated to be approximately 191 kJ/mole, which is similar to oxygen ion diffusion in this compound, indicating that for oxygen partial pressures above PO₂CRIT, oxygen diffusion is the rate controlling mechanism for mass transport. The activation energy for sintering below PO₂CRIT was found to be ∽130 kJ/mole, which could also be due to oxygen ion diffusion. Several possible factors have been considered for the sintering behavior of YBa₂Cu₃O₇₋₈ for oxygen partial pressures below PO₂CRIT. These are formation of copper vacancies, positive hole creation, and lattice strain effect. However, although the oxygen partial pressure dependence on the sintering rate can be correlated with either the formation of Cu²⁺ vacancies in the lattice or hole formation, neither of these mechanisms are considered to be probable. Mathematically, the contraction rate was found to depend exponentially on the lattice strain (Δɛlat), ie. έ∞еΔɛlat. The significance of this relationship is not clearly understood. Intuitively, it appears that as the vacancies are created due to the removal of oxygen ions from the lattice, the lattice is not contracting to the extent needed to accommodate the vacancies. This elastic strain hinders the migration of the oxygen vacancies through the lattice, thus affecting the sintering rate of the YBa₂Cu₃Ox compound below PO₂CRIT. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

This thesis is a study of the phenomenon of superconductivity.

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The purpose of this thesis is to determine the radiation patterns from an acute isosceles triangular superconducting mesa modeled by a pie-shaped geometry. The terahertz band lies between the microwave and infrared regions of the electromagnetic spectrum. The terahertz radiation from atomic-scale layered superconducting mesas is caused by the tunneling of electron pairs in the ac-Josephson effect. To determine the terahertz power radiated per unit solid angle of an acute isosceles triangular superconducting mesa, a model was employed in which the shape of the mesa is approximated as a pie-shaped wedge. This model is shown to have an accuracy of about 1%. Using Love’s Equivalency Principle, the current caused by the Josephson effect is then assumed to be on the edges of the mesa. Since in the mesas used for experiments the electric field is in the direction of the current, it is parallel to the boundary of the sample. Hence, we want the TM modes, and that requires the magnetic field to be transverse to the boundary. We thus require that the tangential component of the magnetic field parallel at the boundary vanishes. Love’s equivalency principle provides the easiest and most straight forward way to satisfy this condition. The surface electric current density was modeled by comparing the magnetic vector potential resulting from the modeled edge current with that given by a standard volume average integration technique. The surface current density that provided the best approximation to the bulk average was used and the radiation patterns were plotted using Mathematica software.

<p> The present knowledge of lattice dynamics in particular solids is applied to the theory of strongly coupled superconductors. From existing phonon data, the product function α²(ω)F(ω) is determined in various materials, where α²(ω) is the electron-phonon coupling term, and F(ω) is the phonon density of states of that material. The Eliashberg gap equations are solved for these particular materials using this product function and predictions of the superconducting energy gap ∆(∆₀) and tunneling electron density of states N_T(ω) are made. </p> <p> Tunneling experiments are performed on selected Tℓ-Pb-Bi alloys where this phonon information is available and comparisons are made both of the predicted and obtained ∆(∆₀) and the tunneling density of states N_T(ω). </p> / Thesis / Doctor of Philosophy (PhD)