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After nearly 15 years of research effort, High Temperature Superconductors (HTS) arefinding a wide range of practical applications. A clear understanding of the factors controllingthe current carrying capacity of these materials is a prerequisite to their successfultechnological development. The critical current density (Jc) in HTS is directly dependent onthe structure and pinning of the Flux Line Lattice (FLL) in these materials.This thesis presents an investigation of the Jc anisotropy in HTS. The use of thin films grownon off c-axis (vicinal) substrates allowed the effect of current directions outside the cuprateplanes to be studied. With this experimental geometry Berghuis, et al. (Phys. Rev. Lett. 79,12, pg. 2332) observed a striking flux channelling effect in vicinal YBa2Cu3O7-δ (YBCO)films. By confirming, and extending, this observation, it is demonstrated that this is anintrinsic effect. The results obtained, appear to fit well with the predictions of a field angledependent cross-over from a three dimensional rectilinear FLL to a kinked lattice of stringsand pancakes. The pinning force density for movement of strings inside the cuprate planes isconsiderably less than that on vortex pancake elements. When the FLL is entirely string-likethis reduced pinning leads to the observed channelling minima. It is observed that anti-phaseboundaries enhance the Jc in vicinal YBCO films by strongly pinning vortex strings.The effect on the FLL structure cross-over of increasing anisotropy has been elucidated usingde-oxygenated vicinal YBCO films. Intriguingly, the counter intuitive prediction that therange of applied field angle for which the kinked lattice is fully developed reduces withincreasing anisotropy, appears to be confirmed.Although vortex channelling cannot be observed in c-axis YBCO films, the pinning forcedensity for vortex string channelling has been extracted by observing string dragging. Bystudying the effect of rotating the applied field at a constant angle to the cuprate planes, it ispossible to observe the cross-over into the string pancake regime in c-axis films. In the 3Dregion, the observed behaviour is well explained by the anisotropic Ginzburg-Landau model.Measurements were also made on thin films of the much more anisotropic Bi 2Sr2CaCu2O8+xmaterial, grown on vicinal substrates. The absence of any flux channelling effect and clearadherence to the expected Kes-Law behaviour in the observed Jc characteristics does notprovide evidence for the existence of the predicted ‘crossing lattice’ in Bi 2Sr2CaCu2O8+x .
Materials Science and Metallurgy, University of Cambridge 2001
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