Microstructural Effects in Oxide Superconductors
Mechanical deformation (ball milling) has been found to degrade the superconducting properties of YBa2Cu3O7−x. The degraded samples have been found to contain a large density of non-intersecting (110) twins and at the same time exhibit a large EPR signal. A clear correlation betwen the degree of degradation (as measured by the relative susceptibility), the twin density and the EPR intensity was established. A detailed analysis of the EPR parameters shows conslusively that the copper(II) complexes involved cannot lie in perfect defect-free YBa2Cu3O7−x. Rather, they have very different environments most probably associated with twin boundaries. The twin structure and associated EPR spectra are very sensitive to modest thermal and mechanical stress, and a new “ribbon-like” defect is subsequently observed. On the other hand, annealing at 500 °C, produces the “tweed-like” twin structure, with no associated EPR spectra and with restored superconducting properties.
KeywordsBall Milling Twin Boundary Milling Time Superconducting Property Twin Structure
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