published_or_final_version / Electrical and Electronic Engineering / Master / Master of Philosophy

published_or_final_version / Physics / Doctoral / Doctor of Philosophy

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Current melt texturing processes can only produce high-quality High Temperature Superconducting (HTSC) domains of a few centimetres in size. Increasing the size of the domain by a joining technique is investigated. The parent HTSC melt textured domains of YBa2CU307-li (Y 123), were used in this research to investigate the joining technique. The solder powders used to form the seam were HTSC YbBa2Cu307-li (YbI23), which has a peritectic temperature of approximately 925 cC, Yb2BatCu\07_o (Yb211), Silver Oxide (Ag20) and Y123. A total of 8 different superconducting 'solders' were manufactured using these powders. Microstructural analyses using a scanning electron microscope (SEM) and an optical microscope were conducted on each sample. It was found that mechanisms occurring during the growth of the seam affect the performance of the join. During the growth, three interfaces are formed. The interface between the parent and the growth front produced an excellent join whereas most of the microstructural defects were present at the intersection of the two growth fronts also known as the impingement boundary. The effect of the addition of Yb211 and silver particles to the join were investigated. It was found that although Yb211 particles improve the flux pinning and viscosity of the solder and the silver improves the intergranular critical current density (Jc) [1], they also reduce the effective cross-sectional area of the joined sample. Thus, the amount of non-superconducting particles added needs to be optimised. Transport measurements were taken to evaluate the current carrying capacity of each sample. It was found that Sample 2-A (Yb123 + 5%Ag20) has the highest Jc of approximately 142 A/cm 2 at 80 K while Sample 3-A (Y123 + 5%Ag20) has the poorest Jc of about 37 A/cm2 at 80 K. Resistivity vs. Temperature graphs show that all joined samples were superconducting implying that the joining process was successful in establishing a superconducting join. Magnetic field maps of the parent and joined samples were captured using a Hall Probe. It was found that the joined sample could trap almost 95 % of the field that could be trapped by the parent. From these results, it was concluded that joining HTSC bulk pieces is possible using an external 'soldering' agent. / Thesis (M.Sc.Eng.)-University of KwaZulu-Natal, Durban, 2005.

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In this thesis, we study the effects of doping in two-dimensional quantum antiferromagnets. We consider cases where the undoped parent compound is a Mott insulator with long-range antiferromagnetic order and focus on the low-doping situations. The limit of localized impurities is studied in a system consisting of a host magnet and two additional weakly coupled spins. We derive the effective Hamiltonian describing the interaction between these impurities as a function of their distance and show that it exhibits xyz anisotropy, leading to NMR and EPR line broadening. We calculate the magnetization disturbance in the host magnet induced by a single impurity and find that it always enhances Neel order. Relaxing the localization constraint, we investigate the single-hole dynamics of the t-J model on the honeycomb lattice. Using exact diagonalizations, series expansion and the self-consistent Born approximation, we calculate the quasi-particle dispersion, bandwidth and residues and compare our findings with the well-established results for the square lattice. Similar to the latter case, we find an almost flat band along the edges of the magnetic Brillouin zone and well-defined hole pockets around the corners. The most important part of this thesis is devoted to the magnetic properties of lightly doped La2-xSrxCuO4, the simplest and by far most studied cuprate superconductor. Starting from the undoped parent compound, we calculate the spin-wave spectrum and the spin-flop transitions in a uniform magnetic field at zero temperature. We then consider the low-doping regime and derive the effective field theory describing the spin dynamics in insulating La2-xSrxCuO4, x ≤ 0.055, at low temperature. The spin structure resulting from the spiral solution of the extended t-J model, obtained by taking into account the Coulomb trapping of holes by Sr ions, is confined in the copper-oxide planes. Our solution explains why the incommensurate structure is directed along the orthorhombic b axis and allows us to calculate the positions and shapes of the neutron scattering peaks numerically. These results are in perfect agreement with experimental data. We also show that topological defects (spin vortex-antivortex pairs) are an intrinsic property of the spin-glass ground state.

Thesis (Ph. D.)--Virginia Polytechnic Institute and State University, 1990. / Vita. Abstract. Includes bibliographical references (leaves 138-141). Also available via the Internet.