Model for the angular dependence of critical currents in technical superconductors

Abstract

We propose a model for the effect of anisotropic pinning on the criticalcurrents in superconductor wires as a function of the direction of an externalmagnetic field. The model emphasizes the statistical distribution of the pinningencountered by a vortex as determining the functional forms observed. Ifwe assume anisotropic defect populations with normal distributions of pinningthen we can derive exact functions for the critical current as a function of angle:Jc(θ). The model predicts that the combination of extended defects in thec-axisand ab-plane directions can lead to peaks at intermediate angles. The predicted forms agree wellwith experiments on superconductors with layered structures including cases wherepeaks occur at intermediate angles. The results suggest that common features ofJc(θ) can be identified with combinations of defects. We discuss some implications of thestatistical model including a possible sample thickness dependence of criticalcurrents.