Surface resistance and magnetic penetration depth of structured YBa₂Cu₃O_7-δ-thin films

We have determined the high-frequency properties of epitaxial YBa₂Cu₃O_7-δ-thin films that are crucial for applications of this high-temperature superconducting material for passive microwave devices. From the quality factor and resonance frequency of integrated coplanar halfwave resonators the surface resistance and the magnetic penetration depth as a function of temperature and rf-power are deduced. Notably this quantities can be obtained for individual thin films and so their microwave performance can comprehensively be characterized. This planar resonator based method not only offers high sensitivity within a wide frequency range but also the ease of integrating resonators as a small-sized test structure with loss mechanisms that would also be present in actual passive devices. As the interpretation of the measured data needs full knowledge of the electric and magnetic field distribution in the cross-section of the resonator line a theoretical field wave analysis is performed employing a "partial wave synthesis" (mode-matching method). Any ambiguity in the determination of the temperature dependence of the magnetic penetration depth is removed by the simultanuous evaluation of pairs of resonator structures of different width. This novel approach yields the magnetic penetration depth as a function of temperature without assuming a specific model. Knowledge of the absolute value of the penetrationg depth also allows the determination of the surface resistance with great accuracy as it is shown in comparison with other methods. Power dependent measurements finally give information about weak-links in the epitaxial films possibly associated with grain- and twin boundaries.