Microwave studies of granular superconductivity

Microwave absorption in low magnetic fields is a sensitive contactless indicator of surface granular superconductivity, a significant source of surface resistance. Granularity is also responsible for microwave absorption in the ceramic high-temperature superconductors and in ceramic Chevrel-phase compounds. Modulated microwave adsorption signals from granular ceramics show critical behavior with pinning and depinning of intergranular flux over a modulation cycle. In single crystal YBa₂Cu₃O_7-δ the adsorption arises from the motion of flux within twin-plane domain boundaries. Modulated adsorption signals from these domain boundaries show additional adsorption from fluxon nucleation above critical levels of microwave current. Such losses may contribute significantly to microwave adsorption in zero magnetic field