rf residual losses, surface impedance, and granularity in superconducting cuprates

摘要

The rf residual surface resistanceRresis 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 rf surface resistance valuesRresobserved but also there is strong field dependence ofRres(T,ohgr;,Hrf) and the penetration depth lgr;res(T,ohgr;,Hrf), even at very low field levels (of order 1 Oe). These observations can be explained in terms of weak links and insulating interfaces, which provide additional degrees of freedom due to fluxoid motion and electromagnetic striphyphen;line modes. These degrees of freedom cause additional interaction mechanisms with applied rf fields. These mechanisms allow a coherent description of the observedT, ohgr;,Hrf, andHdcdependence ofRresand lgr;resfor 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.