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.

Epitaxial thin films of YBa2C!u307 have been prepared on SrTiOs and LaAlOs substrates by a high-pressure planar dc-sputtering technique. By covering the substrate heater with a frame of polycrystalline YBazCu30~ -substantial improvements of the YBa2Cu307 film properties were achieved. These are characterized by dc-resistivity values p(T) of less than 50 @I cm at 100 K and ~(300 K)/p( 100 K) values of up to 3.9. Significant deviations from the usual linear p(T) behavior were found. Critical temperatures above 90 K, resistive transition widths down to 0.3 K, and critical current densities of about 5 x lo6 A/cm2 at 77 K confirm the high quality of the films. As indicated by Rutherford backscattering and high-resolution transmission electron microscopy the films exhibit a microstructure characterized by a reduced density of lattice defects. However, lattice-coherent precipitates with a diameter of about 5-10 nm were observed. As an outstanding feature the films exhibit, besides the initial steep falloff at T, a further gradual decrease of the microwave surface resistance at 87 GHz below 50 K by at least one order of magnitude. These results are very promising for millimeter-wave applications of epitaxial YBaz.Cu307 thin films.

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