The dependence of the effective surface impedanceZeff=Reff+iXeffof superconducting thin films on the film thicknessd, on the magnetic field penetration depth lgr;, and on the dielectric properties of the substrate material is investigated theoretically by means of impedance transformations. It was found that the effective surface resistanceReffcan be expressed byRSf(d/lgr;)+RtranswhereRSis the intrinsic surface resistance of the superconductor. The functionf(d/lgr;) describes the altered current density distribution in the film.Rtransarises from power transmission through the film. It depends ondand lgr; 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 reactanceXeffof a superconducting thin film can be expressed byXS cosh(d/lgr;) whereXS=ohgr;mgr;0lgr; is the intrinsic surface reactance. Measurements ofZeffat 87 GHz have been performed for YBa2Cu3O7−dgr;thin films grown epitaxially by laser ablation on SrTiO3, MgO, and LaAlO3. With the best films,Reff(77 K) values of 21 mOHgr; andRS(77 K) values of 8 mOHgr; were achieved. The temperature dependence of lgr; 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 lgr; (0 K) values ranging from 140 to 170 nm and 205 to 250 nm, respectively.
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