Surface impedance studies of YBCO

This paper summarizes recent measurements of the penetration depth and surface resistance of YBCO, with particular emphasis on the asymptotic behaviour at low temperatures and its dependence on hole doping and cation substitution. A complete set of measurements of the penetration depth tenser indicates that lambda(T) has a predominantly linear temperature dependence in both the a and b directions at low temperatures, but the c-axis penetration depth is much flatter, without a clear linear term. Ni and Ca substitution are found to have little effect on the linear behaviour of lambda(T), but Zn impurities alter the behaviour; from T linear to T-2 and then to a non-monotonic temperature dependence at a Zn concentration of 1%. The surface resistance in the ab-plane also seems to vary linearly with temperature at low temperatures, except when the sample is doped with Zn. Substitution of Zn for Cu leads to upturns in the surface resistance below 4 K that are seen at 3.8 GHz, but not at 35 GHz, suggesting impurity effects at a very low energy scale.