AC MAGNETIC-SUSCEPTIBILITY AND FLUX NOISE IN SIMPLE JOSEPHSON-COUPLED SUPERCONDUCTING CLUSTERS

We have studied a simple model of granular superconductors which includes an ensemble of simple loops formed by superconducting grains, with weak intergrain junctions. Thermally activated phase slippage transitions between stable states of the N-contact loop are shown to determine the hysteretic behavior in an applied AC field. The imaginary part of the AC magnetic susceptibility, chi'', and the power spectral density of magnetic flux noise, S(phi), have been calculated. We have shown the importance of two-level potential configurations of the system. The 1/f-like flux noise has been found to arise from the superposition of random flux transitions in two-level systems with statistically distributed parameters.