Ac-susceptibility measurements on isotropic and anisotropic superconducting networks

We report measurements of the complex ac susceptibility χ(T) for both isotropic and anisotropic square superconducting wire networks as well as direct measurements of the magnetic normal-to-superconducting phase boundary Tc(H)χ for these systems. The χ(T) transition is substantially broader and exhibits greater depression in magnetic field than the resistive transitions R(T). Commensurability structures found in the Tc(H)χ measurements at low-order rational fields are greatly enhanced compared to those found in resistively measured phase boundaries Tc(H)R. For square networks made anisotropic by different wire widths in the two perpendicular directions, the Tc(H)χ phase boundaries demonstrate that increasing the anisotropy greatly increases the depression of the susceptibility transition temperature at incommensurate applied magnetic fields. This indicates a weakening of the network's ability to screen magnetic field with larger anisotropy despite the fact that anistropy is increased by adding material to one set of parallel wires. This result supports a picture of anisotropic localization of the order parameter and/or anisotropic vanishing of the flux pinning or helicity modulus in periodic systems in an irrational homogeneous field.