Yttrium barium copper oxide: the formulation and magnetic properties of a 93 K superconductor

The focus of this thesis is a systematic study of the relationship between the oxygen stoichiometry and the highly anisotropic magnetic behavior of single crystals of YBa2Cu3O7-y(Y123). While these two areas have received much attention individually, the body of knowledge relating them is scant. The principal objective of this work is to evaluate the dependence of such microscopic parameters as lower critical fields and critical current densities on the oxygen deficiency y. Correlations between oxygen stoichiometry and the lower critical fields are expected since the latter is to a good approximation directly proportional to the superconducting carrier density; this, in turn, is a strong function of the oxygen-regulated normal carrier density. The critical current densities are expected to have a strong dependence on oxygen since flux pinning on microstructural defects such as twinning regulates the time rate of decay of the currents; twinning in Y123 is dictated by oxygen stoichiometry and processing conditions. Independent of all other factors, the oxygen stoichiometry can be adjusted to completely eliminate twins or to render the material insulating. By testing single crystals, the anisotropy of these parameters is also studied, from which inferences are drawn on the nature of the pinning mechanisms. In addition to the above, an accurate account and detailed evaluation of the author's ideas and motivation responsible for the original formulation of Y-Ba-Cu-O is given