Complex conductivity of proximity-superconducting Nb/Cu bilayers.

The effective complex conductivity s5s1 2is2 eff of proximity-coupled Nb/Cu bilayer films at 11.7 GHz is examined. The peak in the real part s1(T) just below Tc in bare Nb films gives way to a broader, shallower peak in s1 (T) as the Cu layer thickness increases, consistent with the existence of coherence effects in proximity-superconducting Cu. The imaginary part s2 (T) changes curvature from concave down to concave up as the Cu thickness dN increases. We extract proximity-induced penetration depth in Cu in the range 320 Å <lN(0,0)<580 Å, with lN(0,0) increasing slightly with increasing dN , and an order-parameter decay length in Cu of K(4.6 K!5225 Å660 Å. The temperature dependence of K(T) is consistent with K(T);T. Our results suggest that the single-frequency approximation of Werthamer and de Gennes does not adequately describe the behavior of very thin proximity-coupled Cu layers, in which the exact nearinterface profile of the induced order parameter in Cu plays an important role. @S0163-1829~96!06529-0#