Microwave surface resistance of epitaxial YBa₂Cu ₃O₇ thin films on sapphire

Microwave surface resistance data were measured on pairs of YBa ₂Cu₃O₇ thin films on Al₂O ₃ {1102} substrates by a parallel plate resonator technique. The surface resistance R_s at 10 GHz was 65 μΩ at 10 K and 850 μΩ at 77 K. These epitaxial YBa₂Cu₃O₇ films were grown on 500-Å-thick buffer layers of SrTiO₃. X-ray diffraction data showed that the YBCO thin films with the SrTiO₃ buffer layers have better in-plane epitaxy than those without such buffer layers. Critical current density of 2×10⁶ A/cm² at 74 K was measured by the ac mutual inductance response of the films. The improved microwave surface resistance and the higher critical current density are believed to be the results of better in-plane epitaxy.