Vortex pinning in Josephson-junction arrays

Abstract

Josephson-junction arrays provide a uniform lattice of vortex pinning sites. We investigate the effect of pinning on vortex motion in this model system through comparisons of numerical simulations with measurements on 1000×1000 Nb-Cu-Nb proximity-effect Josephson-junction arrays. The resistive transition is broadened upon application of small magnetic fields near f=0, where f is the number of flux quanta per unit cell. We discuss this broadening in terms of thermally activated vortex motion. The resistive transition at f=1/2 also broadens in the presence of small additional magnetic fields; we attribute this broadening to motion of field-induced defects in the ground-state vortex superlattice. Pinning barriers are found to be field independent for small deviations of the field from both f=0 and 1/2. We also discuss measurements suggesting defect motion near f=1/3 and 1/4. The implications of these results for flux-line lattice dissipation in other systems are discussed.