Disorder, anisotropy, and the loss of coherence in superconducting wire networks

We have investigated the effects of disorder and anisotropy on the superconducting transitions of square aluminum wire networks. We have found that disorder in the network tile areas induces a crossover to a glassy incoherent state in which T_c is independent of the applied field, and we can accurately model this behavior by considering only the energy cost of the frustration-induced supercurrents. On networks made anisotropic by widening the horizontal wires of the square network, we find even more striking results. Numerous measurements (including transport, T_c(H) phase boundary, and AC susceptibility) indicate that the narrow wires exhibit the same behavior as the wide wires only at strongly commensurate fields; away from these fields, the narrow wires remain resistive far below the temperature at which the wide wires lose resistive sensitivity. Furthermore, this weakening of superconductivity along the narrow wires at incommensurate fields becomes more extreme as the anisotropy is increased, even though the anisotropy is introduced by adding superconducting material to make the other set of wires wider. We describe these results in terms of anisotropic localization of the superconducting order parameter.