Investigation of the vortex phase diagram and dynamics in single crystalline samples of the high temperature superconductor YBa2Cu3O7-8

Abstract

A method based on partial magnetization loops is used to study memory effects in pure YBA₂Cu₃O_7-δ 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.

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 in 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 on the current at intermediate temperatures. Finally, from the oscillations we extract the anisotropy of this compound and study its dependence on the oxygen content.

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