We investigate the effective microwave surface resistance of superconductor/dielectric/normal metal structures through numerical simulations. In particular, the setup with a thin superconducting film on a dielectric substrate, backed by a metallic endplate, is analysed. The results demonstrate that the effects of the substrate on the measured surface resistance of superconducting thin films must be taken into account. We show that in this setup several spurious effects are predicted. At characteristic frequencies, dependant on the substrate material, the commonly used thin-film approximation fails even for films thinner than the London penetration depth. In general, when the film is thicker than, but of the same order as, λ, the measured surface resistance can exceed both the thin-film and the bulk values. We investigate how the effective surface resistance of a superconducting film can be approximated by the bulk or the thin-film value, as a function of frequency, film and substrate thickness, and temperature. We exploit a thin-film model to take into account the effects of the backing materials. Measurements of surface resistance are performed on a BSCCO/SrTiO 3 /metal structure to stress the nonintrinsic behaviours and to test the proposed model.

We describe a system for the measurement of the surface resistance of high-T c superconductors (HTCSs) at 48 GHz as a function of the temperature, the external magnetic field and its orientation with respect to the crystallographic axes of the sample. The set-up is based upon a resonant cavity. Thin films, bulk ceramics and single crystals can be accommodated in the cavity. A geometry in which there is no c axis microwave current flow, or c axis microwave field penetration, is chosen, so that unwanted effects depending on the anisotropy of HTCSs are ruled out. Geometrical factors for samples smaller than the cavity endplate are calculated. Some measurements on HTCS thin films and single crystals are presented, as well as some of the items of physical information obtainable from the measurements.

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