Microwave characterization of high-temperature superconductors

Thick (10-15 µm) Tl-Ba-Ca-Cu-O films have been deposited onto yttria-stabilized zirconia and Ag substrates by d.c. magnetron sputtering techniques. Direct deposition onto 1" diameter yttria-stabilized zirconia yields films with typical 22-GHz surface resistance (Rs) values of 5.2 ± 2 mS2 and 52 ± 2 mK2 at 10 K and 77 K, respectively. For comparison, Rs of Cu at this same frequency is 10 mK2 at 4 K and 22 mg2 at 77 K. Tl-Ba-Ca-Cu-O films have also been deposited onto 1" diameter Ag substrates using Au/Cu, Cu, and BaF2 buffer layers. The lowest Rs values were obtained on films with a BaF2 buffer layer, typical values being 7.8 ± 2 mC2 and 30.6 ± 2 mC/ (measured at 22 GHz) at 10 K and 77 K, respectively. Larger films (1.5" diameter) with similar Rs values were prepared using this same technique, demonstrating that the fabrication process can be scaled to larger surface areas. These films are promising for radiofrequency cavity applications because they are thick (50 -75 times the London penetration depth), have relatively large surface areas, are fabricated on metallic substrates, and have Rs values that are competitive with Cu at 77 K and are lower than Cu at 4 K. Because they are polycrystalline and unoriented, it is anticipated that their Rs values can be lowered by improving the processing technique. High-quality films of YBa2Cu3O7 have been electron-beam deposited onto 1" LaGa03 and 1.5" LaA103 substrates. The 1" sample is characterized by Rs values of 0.2 ± 0.1 mS2 at 4 K and 18.6 ± 2 mΩ at 77 K. The 4-K value is only 2 - 4 times higher than Nb. The 1.5" sample has Rs values (measured at 18 GHz) of 0.93 ± 2 Ω and 71 ± 3 mΩ at 10 K and 77 K, respectively.