SLOW RELAXATION AND AVALANCHES IN THE SPIN-PARAMAGNETIC TRANSITION OF A THIN-FILM GRANULAR SUPERCONDUCTOR

We report measurements of the superconducting to normal transition of ultra-thin granular Al films in parallel magnetic fields, H-parallel to. The parallel critical field, H-c parallel to, in our samples is Pauli spin-paramagnetically limited. We find that the critical field transition goes from second-order to strongly first-order at a tricritical point, T-tr, near 250 mK. The first-order transition is characterized by a giant hysteresis in the critical field, Delta H-c parallel to similar to -2.5 kG. In the hysteretic region we find that the films are far out of thermodynamic equilibrium and exhibit very slow, tau similar to 10(4) s, glass-like, stretched-exponential relaxation. In addition, we observe significant avalanches. These observations are discussed in terms of a random array of Josephson junctions.