Complex conductivity of proximity-superconducting Nb/Cu bilayers

Abstract

The effective complex conductivity σeff=σ1eff-iσ2eff of proximity-coupled Nb/Cu bilayer films at 11.7 GHz is examined. The peak in the real part σ1(T) just below Tc in bare Nb films gives way to a broader, shallower peak in σ1eff(T) as the Cu layer thickness increases, consistent with the existence of coherence effects in proximity-superconducting Cu. The imaginary part σ2eff(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 ≤λN(0,0)≤580 Å, with λN(0,0) increasing slightly with increasing dN, and an order-parameter decay length in Cu of K−1(4.6 K)=225 ű60 Å. The temperature dependence of K−1(T) is consistent with K−1(T)∼T−2. 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 near-interface profile of the induced order parameter in Cu plays an important role. © 1996 The American Physical Society.

Source-reported materials — not catalogue approval

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