Screening in Josephson-junction ladders.

Both critical current and Shapiro steps in Josephson ladders are found to show effects of vortex exclusion below a critical magnetic field. Vortices are excluded even though the flux itself is not screened out. When current is injected perpendicular to the ladder edges, the critical current is unchanged from its f50 value up to a penetration field of f c1''0.12 flux quanta per plaquette. Similarly, there are only integer Shapiro steps below a critical f c1' * , where vortices first penetrate. A narrower critical current plateau also appears to form near f 5 1 2. No such plateaus occur when current is injected in the parallel direction. We attribute the exclusion in the perpendicular geometry to screening currents which flow along the edges of the ladder.