Abstract
We report measurements of the superconducting to normal transition of ultra-thin granular Al films in parallel magnetic fields, H ‖. The parallel critical field, H c, 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, ΔH c‖ ∼ 2.5 kG. In the hysteretic region we find that the films are far out of thermodynamic equilibrium and exhibit very slow, τ ∼ 10 4s, glass-like, stretched-exponential relaxation. In addition, we observe significant avalanches. These observations are discussed in terms of a random array of Josephson junctions.
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