Influence of columnar defects on surface resistance of YBa2Cu3Ox superconducting thin films; nonlinear effects

We report on measurements of surface resistance (Rs) of YBa2Cu3Ox thin films irradiated with 0.9 GeV Pb ions. Measurements of Rs versus microwave-frequency (rf) magnetic field and temperature were carried out using a dielectric resonator technique. At low temperatures, no significant difference was observed between surface resistance of irradiated and nonirradiated films. However, at higher temperatures the irradiated films exhibited not only higher Rs but also a nonlinear dependence of Rs on rf magnetic field. A simple model for microwave losses in thin superconducting films induced by columnar defects is proposed. Using this model, the observed changes in Rs have been attributed to additional losses and/or heating effects in the insulator-superconductor boundary between an amorphous core of the columnar defects (several nanometers in diameter) and the surrounding superconducting material.