Complex Conductivity of YBCO Films in Normal and Superconducting States Probed by Microwave Measurements

We report on microwave frequency characterization of yttrium barium copper oxide (YBCO) thin films in both normal and superconducting states. A microwave single-post dielectric resonator technique using two different resonators was used for the complex conductivity determination of YBCO samples deposited on dielectric substrates. The intrinsic complex conductivity of YBCO films was determined from the measured resonator quality factor Q and resonant frequency values employing rigorous electromagnetic modeling of the resonant structures with the mode-matching and Rayleigh-Ritz techniques. Such approach allowed us to determine the intrinsic properties of the films (conductivity, permittivity, and penetration depth) without any simplifications and errors associated with approximate modeling employing perturbation theory. We describe the superconducting material only through its intrinsic material properties, such as the complex conductivity, which does not depend on the thickness of the sample and other parameters. From both simulations and experimental results, we show that the proposed method of intrinsic complex conductivity determination is particularly useful for the characterization of very thin YBCO layers. To support the novelty of our approach, it is shown that significant differences appear between the rigorous and perturbation computations for thin superconducting films (below 50 nm).