FLUX-PINNING AND PHASE-TRANSITIONS IN MODEL HIGH-TEMPERATURE SUPERCONDUCTORS WITH COLUMNAR DEFECTS

We calculate the degree of flux pinning by defects in model high-temperature superconductors (HTSC's). The HTSC is modeled as a three-dimensional network of resistively shunted Josephson junctions in an external magnetic field, corresponding to a HTSC in the extreme type-II limit. Disorder is introduced either by randomizing the coupling between grains (model-A disorder) or by removing grains (model-B disorder). Three types of defects are considered: point disorder, random line disorder, and periodic line disorder; but the emphasis is on random line disorder. Static and dynamic properties of the models are determined by Monte Carlo simulations and by solution of the analogous coupled over-damped Josephson equations in the presence of thermal noise. Random line defects considerably raise the superconducting transition temperature T(c)(B), and increase the apparent critical current density J(c)(B, T), in comparison to the defect-free crystal. They are more effective in these respects than a comparable volume density of point defects, in agreement with the experiments of Civale et al. Periodic line defects commensurate with the flux lattice are found to raise T(c)(B) even more than do random line defects. Random line defects are most effective when their density approximately equals the flux density. Near T(c)(B), our static and dynamic results appear consistent with the anisotropic Bose-glass-scaling hypotheses of Nelson and Vinokur, but with possibly different critical indices.