Microwave impedance of epitaxial high-temperature superconducting films

The changes in the surface resistance of HTS epitaxial films are measured as functions of constant and varying magnetic fields in the X-band (9.4 GHz). The YBa2Cu3O7–δ films on a sapphire substrate have been investigated. It is found that the varying magnetic field produces a much stronger effect on the surface resistance than the constant field. The experimental results are explained by assuming that the HTS film is not an ideal superconductor and is formed by series-connected sections of different types: perfectly superconducting sections, sections with low- and high-resistivity Josephson junctions shunted by an ideal superconductor, and finally a small number (for epitaxial films) of sections with intergranular Josephson junctions. Under these conditions, the dependence of the surface resistance on the constant magnetic field is determined by Abrikosov vortices moving in the sections of ideal superconductor, while the dependence on the varying magnetic field amplitude is due to switching of high-resistance contacts to the low-resistance state.