Microwave surface impedance of proximity-coupled Nb/Al bilayer films

The surface resistance and magnetic penetration depth in proximity-coupled superconductor/normal-metal (Nb/Al) bilayers are examined to study the electrodynamics of layered structures with spatially inhomogeneous superconducting order parameter. The effective penetration depth λ_eff(T) obeys Δλ_eff(T)∼Tⁿ, where n≤1, at low temperatures, distinctly different from the exponential behavior of Nb. An accompanying drop in the surface resistance R_s occurs in the same temperature range. A model of the bilayer electrodynamics explicitly including the effects of proximity coupling is developed, and the behavior of both λ_eff and R_s is consistently described. We find that a treatment of coherence effects in the proximity-coupled normal-metal layer is necessary to properly describe the behavior of R_s(T).