Materials science and surface impedanceZ(T,f,H) of Nb and YBCO and their quantitative modelling by the leakage current of weak links

In Nb 2 O 5- x junctions the critical Josephson current j cJ ( T ) is accompanied by a small leakage current j bl ( 1/ R bl ) and hence the rf residual losses R res caused by weak links are small. HTS in-plane weak links show very much reduced j cJ (0) values compared with theory and a large leakage current j bl of the order of j cJ owing to a high density of intermediate states in disordered HTS junction material. The finite R res ( T , f , H ) is due to the finite normal conducting leakage current with R bl ( T <0.9 T c ) constant of weak links at separations a J , where the R res ( T ) dependence is dominated by j cJ ( T ) j cJ (0)(1- T / T c ). Quantitative experiment-model agreement is obtained with measured values of a J ( 0.2-20 µm), of R bn ( 1000-1 n cm 2 ) and of j cJ ( 10-10 7 A cm -2 ) with