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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.

We report on the design of a planar 200MHz superconducting two-resonator array for magnetic resonance imaging (MRI) applications. The array was made out of a double-sided thin YBa2Cu3O7−x film on r-cut sapphire substrate and consists of two 24mm diameter resonators with built-in planar capacitors for coupling to the tuning and matching electronics. Required for the performance of the MRI array, rf isolation of two resonators was accomplished by built-in planar capacitors, and the mechanism of resonator-to-resonator decoupling was analyzed. The signal-to-noise gain as a result of using high-Tc superconductor resonators/arrays was calculated and compared with experimental data.