Ing-Jye Hwang, Seungoh Ryu, and D. Stroud, Department of Physics,
The Ohio State University, Columbus, Ohio 43210-1106
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 {\em not} screened out.
When current is injected perpendicular to the ladder edges,
the critical current is unchanged from its $f=0$ value up to a
penetration field of
f_{c1\perp} ~ 0.12 flux quanta per plaquette.
Similarly, there are only integer Shapiro steps below a critical
f_{c1\perp}^*, where vortices first penetrate. A narrower critical 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 which flow along the edges of the ladder.