MICROWAVE SURFACE IMPEDANCE OF PROXIMITY-COUPLED NB AL BILAYER FILMS/

The surface resistance and magnetic penetration depth in proximity-cou pled superconductor/normal-metal (Nb/A1) bilayers are examined to stud y the electrodynamics of layered structures with spatially inhomogeneo us superconducting order parameter. The effective penetration depth la mbda(eff)(T) obeys Delta lambda(eff)(T) similar to T-eta, where n less than or equal to 1, at low temperatures, distinctly different from th e exponential behavior of Nb. An accompanying drop in the surface resi stance R(s) occurs in the same temperature range. A model of the bilay er electrodynamics explicitly including the effects of proximity coupl ing is developed, and the behavior of both lambda(eff) and R(s) is con sistently described. We find that a treatment of coherence effects in the proximity-coupled normal-metal layer is necessary to properly desc ribe the behavior of R(s)(T).