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

In Nb2O5-x junctions the critical Josephson current j(cJ)(T) is accompanied by a small leakage current j(bl)(proportional to/R-bl) and hence the rf re sidual 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 l eakage current j(bl) of the order of j(cJ) owing to a high density of inter mediate 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)) approximate to constant of weak links at separations a(J), where the R-res (T) dependence is dominated by j(cJ)(T) approximate to j(cJ)(0)(1 - T /T-c). Quantitative experiment-model agreement is obtained with measured va lues of a(J)(approximate to 0.2-20 mu m), of R-bn(approximate to 1000-1 n O mega cm(2)) and of j(cJ) (approximate to 10-10(7) A cm(-2)) with j(cJ)R(bl) (2) approximate to 5 x 10(-12) A Omega(2) cm(2).