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 criti cal current is unchanged from its f=0 value up to a penetration field of f(c1 perpendicular to)approximate to 0.12 flux quanta per plaquette . Similarly, there are only integer Shapiro steps below a critical f( c1 perpendicular to), where vortices first penetrate. A narrower criti cal current plateau also appears to form near f=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 whi ch flow along the edges of the ladder.