AC-SUSCEPTIBILITY MEASUREMENTS ON ISOTROPIC AND ANISOTROPIC SUPERCONDUCTING NETWORKS

We report measurements of the complex ac susceptibility chi(T) for bot h isotropic and anisotropic square superconducting wire networks as we ll as direct measurements of the magnetic normal-to-superconducting ph ase boundary T(c)(H)\chi for these systems. The chi(T) transition is s ubstantially broader and exhibits greater depression in magnetic field than the resistive transitions R(T). Commensurability structures foun d in the T(c)(H)\chi measurements at low-order rational fields are gre atly enhanced compared to those found in resistively measured phase bo undaries T(c)(H)\R. For square networks made anisotropic by different wire widths in the two perpendicular directions, the T(c)(H)\chi phase boundaries demonstrate that increasing the anisotropy greatly increas es the depression of the susceptibility transition temperature at inco mmensurate applied magnetic fields. This indicates a weakening of the network's ability to screen magnetic field with larger anisotropy desp ite the fact that anistropy is increased by adding material to one set of parallel wires. This result supports a picture of anisotropic loca lization of the order parameter and/or anisotropic vanishing of the fl ux pinning or helicity modulus in periodic systems in an irrational ho mogeneous field.