Flux Pinning and Phase Transitions in Model High-Temperature Superconductors with Columnar Defects

TL;DR

This study models high-temperature superconductors with various defects to analyze flux pinning effects, showing that line defects notably enhance critical temperature and current density, aligning with experimental observations and theoretical scaling hypotheses.

Contribution

It introduces a detailed simulation framework for flux pinning in HTSCs with different defect types, emphasizing the effectiveness of line defects over point defects.

Findings

Random line defects significantly increase T_c(B) and J_c(B,T).

Periodic line defects further enhance T_c(B) compared to random defects.

Results are consistent with Bose glass scaling hypotheses near T_c(B).

Abstract

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 overdamped 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 {\it 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: