Investigation of the vortex phase diagram and dynamics in single crystalline samples of the high temperature superconductor YBa_2Cu_3O_7_-_#delta#

The present thesis contains an extended experimental investigation of the hysteretic behaviour of the vortex system in pure YBa_2Cu_3O_7_-_#delta# single crystals. A method based on partial magnetization loops is used to study memory effects in pure YBa_2Cu_3O_7_-_#delta# single crystals with various types and densities of pinning sites. For detwinned samples with low densities of point defects, our measurements reveal the transition from a dislocation-free Bragg glass to a disordered vortex phase. This boundary is mapped for the first time in the B - T phase diagram and is located in proximity to the onset of the second magnetization peak. The transition line is further found to be in accordance with theoretical predictions. For fields above this line, metastable topological disorder invades the vortex system leading to a pronounced dependence of the critical current on the formation history of the vortex lattice. The effect of decreasing oxygen stoichiometry is the shift of this boundary to lower fields, and its disappearance for high values of the oxygen deficiency in agreement with theory. We also study the influence of extended defects and find that a low density of twin boundaries does not affect the Bragg glass significantly. However, a high concentration of twins as well as even low densities of columnar defects suppress the transition and eliminate the memory effects. Studies of the second magnetization peak in pure YBa_2Cu_3O_7_-_#delta# reveal several new features, such as a non-monotonous temperature dependence of the peak position and the presence of a rather sharp peak near the melting line. These observations contradict conventional interpretations and support more realistic scenaria for the origin of this phenomenon. The effect of oxygen ordering on the magnetization peak is also investigated. Although for temperatures below 70 K the peak does not depend on the distribution of oxygen vacancies, above 70 K ordering effects become essential leading to the suppression of the peak for a more homogeneous distribution of oxygen defects. These results indicate that while at low temperatures pinning by individual oxygen vacancies is important, at elevated temperatures clusters mainly dominate the hysteresis. Pinning of vortices for fields along the superconducting planes is also investigated. We show that the magnetization exhibits an oscillatory behaviour that can be fully accounted by considering the commensurability of the vortex lattice with the layered structure. The effect of varying point disorder is also studied. We demonstrate that the interplay between pinning and thermal fluctuations leads to a remarkable non-monotonous temperature dependence of the current at intermediate temperatures. Finally, from the oscillations we extract the anisotropy of this compound and study its dependence on the oxygen content. (author)Available from British Library Document Supply Centre-DSC:DXN032239 / BLDSC - British Library Document Supply CentreSIGLEGBUnited Kingdo