Zero resistance at integer values of frustration in out-of-equilibrium classical Josephson-junction arrays

The magnetic field dependence of the resistance of classical Josephson-junction arrays, with 500000 or 1000000 junctions has been studied with significant transport currents through the array. The resistance vanishes into the noise only at integer values of the flux quantum per plaquette (integer frustration f) and is up to seven orders of magnitude larger than the noise floor at other fields. These arrays are apparently superconducting only at integer values of f. The only other surviving features of the complex modulation of the array resistance with field at low currents are small dips in the resistance vs field when f=1/2. The possible role of vortex pinning as being the source of these observations is discussed.