Reversible-magnetization data have been studied for YBa2Cu408 in order to determine the thermodynamic properties and the magnetic-field dependence of the superQuid density. By fitting the Hao-Clem variational model to the reversible magnetization data below T, superconducting parameters such as the Ginzburg-I andau parameter e, the thermodynamic critical field H, and the superconducting order parameter are determined. The upper and lower critical fields H 2, 0 i, the penetration depth A, and the coherence length f are derived. Within the model there is a direct connection between the magnetization and the superBuid density, so the vortex structure, the behavior of the average order parameter in (H, T) plane, and the magnetic-field dependence of the super8uid density are obtained. In the high-field region, the evidence of strong vortex Quctuation effects is observed and explained by vortex Huctuation for a three-dimensional superconductor. The mean-field transition temperature T (H) and dH 2, /dT near T, derived from the analysis of vortex Huctuation effects are comparable with fitting results.
The connectivity and flux pinning components of the critical current density J c in a high-J c Pb) 2 Sr 2 Ca 2 Cu 3 O x (Bi-2223) composite throughout important steps in its thermomechanical treatment were investigated by mass density measurement, microstructural observation and extensive superconducting property characterization. The mechanical integrity of the filaments was greatest after the first heat treatment, because damage produced by intermediate rolling was not completely erased by subsequent heat treatment. The final relative filament mass density was less than 75%, even though J c more than tripled to reach ∼50 kA cm −2 (77 K, 0 T) after final heat treatment. Filament resistance and J c data showed that the electrical connectivity improved significantly throughout the process, although ultrasonic fracturing, magneto-optical images and density measurements all showed that the mechanical and physical connectivity degraded. Our study suggests that the attainable J c of Bi-2223 is determined by the electrical connectivity of each filament which is itself an uncertain compromise between minimizing the significant porosity produced by the Bi-2223 formation reaction and the extensive crack network that porosity reduction by intermediate rolling produces.
The elementary pinning force f p has been measured for a single vortex trapped in one of the superconducting layers of a cross-strip Josephson junction. At temperatures close to the transition temperature the vortex can be pushed across the junction by a transport current. The vortex is found to move in a small number of discrete steps before it exits from the junction. The pinning force for each site is found to be asymmetric and to have a value of about 10~6 N/m at the reduced temperature t ^T/Tc =0.95. As a function of temperature, f p is found to vary approximately as (1 -t) l/1 .
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