High-frequency properties of Tl-based superconductors deposited onto large-area metallic substrates

Microwave surface resistance measurements were made on large-area (11.4-cm²) Tl-Ba-Ca-Cu-O thick films/ss (~10 μm) which were magnetron sputtered onto oriented Ag alloy (Consil 995) substrates. Initial films were prepared by depositing a BaF₂ buffer layer between the superconducting film and the substrate to control the film/substrate interaction; however, films exhibiting the best microwave properties were prepared without a buffer layer. Surface resistance measurements were made by replacing the end wall of an 18-GHz Cu cavity (TE₀₁₁ mode) with the superconducting film. The best surface resistance values obtained on the unbuffered film were 4 and 14 mΩ at 10 K and 77 K, respectively; the corresponding Cu values are 8 mΩ and 21 mΩ. The dependence of surface resistance on surface magnetic field was measured in a similar way except that an Nb cavity was used instead of a Cu one. Films exhibiting the highest degree of c-axis texturing show the weakest dependence of surface resistance on magnetic field and also exhibit the sharpest high-frequency transition into the superconducting state. These results are important for potential cavity applications of these materials