High-T_c films by thermal co-evaporation: First phonon experiments

We have prepared 800 nm YBa₂Cu₃O_x films on Al₂O₃, SrTiO₃ and Si substrates by simultaneously evaporating the three metals in an oxygen atmosphere. Resistance measurements indicate the onset of superconductivity in the films on Al₂O₃ and SrTiO₃ at about 96K. The transtion widths depend on the substrate and the details of the preparation method. The best films have onset temperatures of 96 K and transition widths (90 % to 10 %) 0f 2 K. The films were characterised by RBS, X-ray, electron microscope and Raman analyses. We have investigated the absorption of phonons (f 〉 100 GHz) and found that their mean free path is larger than 1 μm below about 300 GHz and decreases rapidly with increasing frequency.