Modeling the microwave properties of the YBa2Cu3O7−x superconductors

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Abstract
We have used the two‐fluid model and the Bruggeman effective‐medium approximation to compute the microwave surface resistance Rs of superconductor‐nonsuperconductor composites. The results are relevant to the new high‐Tc superconductors: because of extreme anisotropy, unfavorably oriented crystallites may be considered nonsuperconducting; because of chemical reactions with the atmosphere, the surface can be a superconductor‐nonsuperconductor composite. The model predicts very low values of Rs and very large unloaded Q’s. In most applications reduction of the operating temperature from 77 to 4 K will not result in technologically important improvements. The conditions on which these conclusions are based are discussed, as well as the dependence of microwave surface resistance on superconducting fraction, conductivity of the nonsuperconducting component, temperature, frequency, normal electron‐scattering time, and grain size.