Microwave surface resistance peaks in BSCCO thin films.

We measure the microwave surface resistance R-S at 24 GHz in BSCCO films of different thickness on two different substrates: LaAlO3 and SrTiO3. One of the films deposited on LaAlO3 presents an anomalous peak in R, at a temperature close to the transition temperature. The film deposited on SrTiO3 presents multiple peaks as a function of the temperature. We show that both effects can be ascribed to substrate effects, instead of physical phenomena in the superconducting films. To reflect the actual experimental setup, we develop a simplified model for the microwave surface resistance of thin superconducting film deposited onto a dielectric substrate backed by a metal. We show that at characteristic frequencies, dependent on the substrate material, the commonly used thin film approximation fails even for films thinner than the London penetration depth. With this model, supplemented with a two-fluid expression for the conductivity, the single peak in the film deposited onto LaAlO3 and the multiple peaks of the SrTiO3-deposited film find a quantitative explanation. The results demonstrate that the effects of the substrate on the measured R-S of superconducting thin films must be taken into account.