Fabrication and characterization of electrically tunable high-T_c superconducting resonators incorporating barium strontium titanate as a tuning material

We have made the electrically tunable microstrip resonators by using both high-T_c superconducting and dielectric films. The two-pole resonators employ a dielectric barium strontium titanate film on their centre in the form of flip chip. The superconducting YBa₂Cu₃O_y (YBCO) and dielectric Ba_0.1Sr_0.9TiO₃ were deposited on the CeO₂-buffered sapphire substrate and LaAlO₃ substrate, respectively, by a pulsed laser deposition technique. Variations of the relative permittivity, ε_r, and dielectric loss tangent, tan δ, of the Ba_0.1Sr_0.9TiO₃ were studied as a function of the applied dc bias at liquid-nitrogen temperature. The tunability, defined as C_{(0 V)}/C_{(100 V)}, and loss tangent of the resonators were measured to be approx. 1.9 and 1.5 × 10^-2 (at 100 V), respectively.