MICROWAVE SURFACE-RESISTANCE PEAKS IN BSCCO THIN-FILMS

We measure the microwave surface resistance R-S at 24 GHz in BSCCO fil ms of different thickness on two different substrates: LaAlO3 and SrTi O3. One of the films deposited on LaAlO3 presents an anomalous peak in R, at a temperature close to the transition temperature. The film dep osited on SrTiO3 presents multiple peaks as a function of the temperat ure. We show that both effects can be ascribed to substrate effects, i nstead of physical phenomena in the superconducting films. To reflect the actual experimental setup, we develop a simplified model for the m icrowave surface resistance of thin superconducting film deposited ont o a dielectric substrate backed by a metal. We show that at characteri stic frequencies, dependent on the substrate material, the commonly us ed thin film approximation fails even for films thinner than the Londo n penetration depth. With this model, supplemented with a two-fluid ex pression for the conductivity, the single peak in the film deposited o nto 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 b e taken into account.