SURFACE IMPEDANCE AND GRANULARITY IN SUPERCONDUCTING CUPRATES

The rf residual surface resistance R(res) is an important figure of merit for superconducting microwave devices and for establishing the homogeneity of superconducting surfaces. In granular superconductors not only are high residual surface resistance values R(res) observed, but also strong field dependencies of R(res) (T, omega, H) and the penetration depth lambda-res (T, omega, H) are encountered even at very low field levels (of order 1 Oe). These observations can be explained in terms of granularity consisting of weak links and of insulating interfaces, which provide additional degrees of freedom due to fluxoid motion and electromagnetic strip line modes. These degrees of freedom cause additional interaction mechanisms with applied fields. These mechanisms allow a coherent description of the observed T, omega, H(rf) and H(dc) dependence of R(res) and lambda-res for the granular cuprates and for similar effects in Pb, Nb, Nb3Sn and NbN. In addition, on the basis of such rf data, it is possible to quantify the density of weak links and their critical fields and currents.