Granular superconductors and their intrinsic and extrinsic surface impedance

Abstract

High-frequency experiments depend sensitively on homogeneous and inhomogeneous “defects” in the normal and superconducting state. As homogeneous effects, the intrinsic scattering time is of great theoretical importance above 0.1 THz in the surface impedanceZ. Of practical importance are the planar defects, “the weak links (WL),” which interrupt the rf shielding currents and thus enhanceZeff. In the superconducting state, the Josephson currentjcJ crosses the WL in parallel with the normal, leakage currentjbl. The latter explains the observed, finite rf residual lossesRres(T∼0) quantitatively and as a function of material parameters, temperatureT, fieldH, and frequencyΩ for Nb, NbN, and cuprate superconductors. With increasing field,Z deteriorates likeH2 up toHcIJ, where Josephson fluxons (JF) penetrate into the WL yieldingδZ∫ αH. Above HcIJ≈0.1–10 mT, JF dynamics dominatesZ with hysteresis losses and reactive components. The nonlinear JF effects are enforced by thin-film edge enhancements limiting the performance of various devices by enhanced dissipation, reactance, and flux noise. A method is presented which is able to separate electron dynamics at the WL from their strength and distribution.