Absorption at high microwave power by large-area Tl-based superconducting films on metallic substrates

Microwave surface resistance measurements have been made on large-area Ti-Ba-Ca-Cu-O thick films magnetron-sputtered onto oriented Ag alloy substrates by replacing the end wall of an 18 GHz TE/sub 011/ mode Cu cavity with the superconducting film. The best surface resistance values obtained are 4 and 14 m Omega at 10 K and 77 K, respectively; corresponding Cu values are 8 and 21 m Omega . The dependence of the surface resistance on microwave power was measured in a similar way except that a Nb cavity was used instead of a Cu cavity. Typically, the surface resistance of the film begins to rise in 1-10 Oe of microwave field and saturates in 20-60 Oe. A model is presented relating the observed saturation to critical penetration of Josephson junctions. Films exhibiting the highest degree of c-axis texturing show the weakest dependence of surface resistance on power and also exhibit the sharpest transition to the superconducting state as measured at high frequency.>