Measurement of the surface resistance of YBa/sub 2/Cu/sub 3/O/sub 7- delta / by the use of a coaxial resonator

The design and initial testing of a coaxial cavity for microwave surface resistance (R/sub s/) measurements on high-temperature superconductors are described. The cavity design is optimized so as to produce the maximum number of harmonic measurements using TEM (transverse electromagnetic modes), in order for accurate measurements of R/sub s/ versus frequency to be made. Results for R/sub s/ using 17 harmonics in the frequency range 1-20 GHz are presented. Initial measurements of the field dependence of R/sub s/ are also given. The results show that the frequency dependence of the ceramic material's R/sub s/ is not omega /sup 2/, as expected from BCS (Bardeen-Cooper-Schrieffer) and the London model. The results also show that R/sub s/ is dependent on even very-low-RF magnetic fields. There have been a number of possible explanations for this deviation from the BCS or London theories of R/sub s/.