Vortex pinning by disordered columnar defects in large Josephson junctions

To study the pinning of Josephson vortices by well-characterized correlated disorder, we have fabricated and made measurements on a series of large Josephson junctions with lithographically defined parallel columnar defects arranged with varying degrees of positional disorder. Commensurability between vortices and a periodic array of defects gives rise to sharp peaks in I-c(H). Randomly shifting the defects from initially periodic positions causes the decay of these commensurate peaks as well as the emergence of a randomly fluctuating background with an oscillatory envelope. Completely random defect placement induces similar I-c fluctuations with weaker envelope oscillations. Using a model based on the blocking of Josephson currents by the defects, we have performed numerical calculations which accurately reproduce I-c(H) for any given defect configuration and provide a physical understanding of its essential features.