Bulk properties, structure and pinning of magnetic flux-line arrays in high-temperature superconductors

A serious impediment to many potential applications of the high-{dollar}Tsb{lcub}c{rcub}{dollar} copper oxide superconductors is the relative ease with which magnetic flux-lines move within these materials, thereby producing finite electrical resistance. To devise methods for rigidly fixing flux-lines in these materials, which is necessary to achieve a truly superconducting state, requires an understanding of their fundamental structural properties.; The most direct measurement of the microscopic structure of the flux-line lattices has come from the Bitter magnetic decoration technique, which uses fine ferromagnetic particles to reveal the positions of individual flux-lines emerging from the sample surface. We have built upon the traditional decoration technique and extended it to give information out of the plane of the surface by decorating opposite sides of a sample and probing the correlations between the flux-line patterns on the top and bottom surfaces. By analyzing the propagation of density and orientational correlations of the flux-lines as they pass through {dollar}rm Bisb2Srsb2CaCusb2Osb8{dollar} (BSCCO) superconductors, we have been able to extract the compressional {dollar}(csb{lcub}11{rcub}),{dollar} tilt {dollar}(csb{lcub}44{rcub}){dollar} and shear {dollar}(csb{lcub}66{rcub}){dollar} moduli for flux-line arrays as a function of magnetic field for the first time. In addition, the excitation spectrum of the flux-line arrays is remarkably similar to the phonon-roton spectrum in superfluid helium-4.; We have studied the effect of both point disorder and correlated disorder on the flux-line lattice structure. Dislocation-free images containing order of magnitude greater number of vortices than previous work have been obtained in high quality BSCCO samples, allowing us to characterize the nature of translational order in the presence of weak point disorder to a length-scale on the order of hundred lattice constants. We have also studied the samples containing columnar defects introduced by high-energy heavy-ion irradiation. The positions of individual columnar defects and vortices on opposite sample sides were simultaneously determined by combining two-sided chemical etching and decoration. Correlations between vortices and columnar defects were studied for different orientations of the applied field with respect to the columnar defects.; Lastly, we have used the double-sided decoration to study the vortex chain state in BSCCO superconductors formed when the applied field is tilted away from the c-axis. Our results support the theoretical proposal of a combined vortex lattice (one set of vortices parallel and the other perpendicular to the c-axis) for a tilted magnetic field due to the large anisotropy in this material.