Flux Line Lattice Disorder Induced by Pinning Centers and Critical Currents in YBa2Cu3O7-δ
Angular dependencies of the critical current density and the irreversible magnetization for single-crystalline and single-domain-melt-textured samples of moderately anisotropic high-Tc superconductor YBa2Cu3O7-δ (YBCO) are measured both with transport current and magnetic techniques in the range of temperature between 77.6 K and T c and applied magnetic field up to 2 T. It is proven that the “cleaner” YBCO single crystals with no dopants and/or fine precipitates show higher critical current densities than melt-textured YBCO materials: J c up to ~ 5.5104 A/cm2 at 77.6 K, µ0H=1.5 T and the angle 6 between H and C-axis «±20°. It is also shown the main contribution to the net J c comes from the oxygen vacancies. The twin boundaries dramatically reduce J c at orientations close to H\\C (within ± 20° deviation) and at intermediate values of applied field. J c (H\\C) at 77.6 K and 1.5 T equals to 1.5•104 A/cm2 against 5.5•104 A/cm2 at H inclined with 0=20° angle. Thus, in contrast to melt-textured samples the J C Θ dependencies for twinned single crystals reveal a remarkable minimum at H|| C-axis orientation (corresponds to Θ=0). This angular dependence of the J c results from a combination of two contributions into the volume pinning force, F p : from pont-like and planar pis. The random point-like defects are responsible for the major part of measured J c anisotropy. The role of the twin planes as extended defects, which provide a vortex locking proves out to be more ambiguous due to interference of the correlated disorder induced by the twins with point-like one, induced by the oxygen vacancies. We also consider influence of the defect structure of YBCO samples on dynamic properties of the FLL. An essential difference in the flux dynamic behavior appears to take place between melt-textured and single-crystalline YBCO. We relate the difference to the specific features of the defect structure of the samples.
KeywordsOxygen Vacancy Critical Current Density Twin Plane Correlation Radius Flux Pinning
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