COMPLEX CONDUCTIVITY OF PROXIMITY-SUPERCONDUCTING NB CU BILAYERS/

The effective complex conductivity sigma(eff)=sigma(1)(eff)-i sigma(2) (eff) of proximity-coupled Nb/Cu bilayer films at 11.7 GHz is examined . The peak in the real part sigma(1)(T) just below T-c in bare Nb film s gives way to a broader, shallower peak in sigma(1)(eff)(T) as the Cu layer thickness increases, consistent with the existence of coherence effects in proximity-superconducting Cu. The imaginary part sigma(2)( eff)(T) changes curvature from concave down to concave up as the Cu th ickness d(N) increases, We extract proximity-induced penetration depth in Cu in the range 320 Angstrom less than or equal to lambda(N)(0,0)l ess than or equal to 580 Angstrom, with lambda(N)(0,0) increasing slig htly with increasing d(N), and an order-parameter decay length in Cu o f K-1(4.6 K)=225 Angstrom+/-60 Angstrom. The temperature dependence of K-1(T) is consistent with k(-1)(T)similar to T-2. Our results suggest that the single-frequency approximation of Werthamer and de Gennes do es not adequately describe the behavior of very thin proximity-coupled Cu layers, in which the exact near-interface profile of the induced o rder parameter in Cu plays an important rule.