SIMULATION STUDY OF THE CRITICAL CURRENT-DENSITY OF YBA2CU3O7 CERAMICS

A two-dimensional Josephson-junction-array model is used to study the effect of grain boundaries on the critical current density of YBa2Cu3O7 superconducting ceramics. The model represents a network of superconducting grains with a distribution of tilt angles-theta. Each grain boundary has a critical current density J(c)(theta) and normal-state resistance R(theta). The current-voltage characteristics are calculated numerically for different tilt-angle distributions. The scaling law and statistics of extremes, introduced by Duxbury, Beale, and Leath for general breakdown behavior, based on the most critical defect (normal region) in the network, are tested and found to be accurate for the predicted critical-current distribution of random samples. When the applied current is larger than but close to its critical value, there is a periodic V(t) with discrete power spectra. When the applied current gets larger, chaotic behavior appears with nearly continuous power spectra.