SURFACE IMPEDANCE OF YBA2CU3O7 Y0.6PR0.4BA2CU3O7 BILAYERS - POSSIBLE EVIDENCE FOR THE PROXIMITY EFFECT/

Measurements of the surface impedance of superconductor/normal-metal b ilayers provide information on the proximity coupling between supercon ductors with different transition temperatures. Surface impedance data on c-axis YBa2Cu3O7-delta/Y0.6Pr0.4Ba2Cu3O7 bilayers is consistent wi th either proximity coupling in the c direction between the two materi als with a very weak proximity-induced condensate and/or the behavior of two isolated superconductors in a bilayer geometry. At least one of the characteristics of proximity coupled superconductors, namely, a s trong downturn of the effective penetration depth above the lower of t he two transition temperatures, was not observed in the c-axis high-T- c bilayer films studied. However, the increase of the surface resistan ce above the T-c of the Pr-doped layer is a sign that the conductivity of the Y0.6Pr0.4Ba2Cu3O7 layer is strongly enhanced above its T-c, in agreement with THz spectroscopy measurements.