Microwave properties of YBa2Cu3O7−x thin films studied with coplanar transmission line resonators

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Abstract
The microwave properties of epitaxial YBa2Cu3O7−x thin films with critical temperatures Tc up to 91.5 K and critical current densities Jc up to 6×106 A/cm2 at 77 K have been investigated between 5 and 20 GHz using coplanar transmission line resonators. Information about the surface resistance Rs and the magnetic penetration depth λL has been obtained from temperature‐ and power‐dependent measurements of the quality factors and resonance frequencies of the resonators. The lowest surface resistance was 105 μΩ at 77 K and 38 μΩ at 9 K measured at 6.2 GHz, respectively. The dependence of Rs on the surface magnetic field Brf showed that with decreasing film quality the films exhibit a granular behavior. At high critical current densities Jc≳4×106 A/cm2, the intrinsic properties of the material show up. The magnetic penetration depth was in the range between 160 and 270 nm. Both Rs and λL turned out to be sensitive to the film quality represented by the transition width ΔTc and the critical current density Jc at 77 K.