Superconducting proximity effect at microkelvin temperatures

The ac susceptibility of a wire with a Nb core (1.27 mm diameter) and a Cu cladding (0.37 mm thickness) at T = > 50 mu K and B = > 0.1 mG has been measured. Due to its proximity to Nb, the Cu becomes fully superconducting. From the data, a breakdown field H sub b = 1.2 (mG) and a coherence length xi = 2.2 T exp --1/2 ( mu m) for the Cu, as well as a field penetration depth omega = 34 T exp 1/2 ( mu m) at the Cu/Nb interface are found. 6 ref.--AA