The dependence of the effective surface impedance Zeff=Reff+iXeff of superconducting thin films on the film thickness d, on the magnetic field penetration depth λ, and on the dielectric properties of the substrate material is investigated theoretically by means of impedance transformations. It was found that the effective surface resistance Reff can be expressed by RSf(d/λ)+Rtrans where RS is the intrinsic surface resistance of the superconductor. The function f(d/λ) describes the altered current density distribution in the film. Rtrans arises from power transmission through the film. It depends on d and λ as well as on the dielectric properties of the substrate material and is significantly altered in the case of a resonant background. The effective surface reactance Xeff of a superconducting thin film can be expressed by XS cosh(d/λ) where XS=ωμ0λ is the intrinsic surface reactance. Measurements of Zeff at 87 GHz have been performed for YBa2Cu3O7−δ thin films grown epitaxially by laser ablation on SrTiO3, MgO, and LaAlO3. With the best films, Reff (77 K) values of 21 mΩ and RS (77 K) values of 8 mΩ were achieved. The temperature dependence of λ was found to be in good agreement to both weak-coupling BCS theory in the clean limit and the empirical two-fluid model relation with λ (0 K) values ranging from 140 to 170 nm and 205 to 250 nm, respectively.

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We have measured the surface resistance of two c-axis oriented YBa2Cu3O7−x thin-film samples in a copper host cavity at 86.7 GHz between 4.2 and 300 K. High quality films of 0.6 and 0.4 μm thickness have been grown epitaxially on SrTiO3 by pulsed excimer laser ablation. Their millimeter wave absorption drops sharply at a transition temperature of 86 and 88 K to a corresponding surface resistance at 77 K of 18 mΩ and less than 8 mΩ, respectively. These values exceed the best results on polycrystalline samples and come close to the expectation from classical superconductors. Therefore, applications of high Tc superconductors up to THz frequencies can be envisaged now.

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We have deposited 10–20-μm-thick YBa2Cu3O7−x films by an electrophoretic process on silver substrates in a magnetic field of 8 T. X-ray diffraction patterns of these 2.4-cm-diameter samples show a high degree of c-axis texturing perpendicular to the surface. Their microwave surface resistance Rs was measured at 21.5 GHz in the temperature range from 4.2 to 300 K. It was found to drop sharply at 92.4 K to reach 18±3 mΩ at 77 K and less than 3 mΩ at 4.2 K. These first results exceed the values obtained from bulk samples and untextured thick films, and are lower than Rs of pure copper at the same frequency and temperature. The electrophoretic deposition technique is applicable to large substrates of nonplanar geometry and is therefore suitable for applications of high-Tc superconductors in the field of microwave technology.