Surface impedance of R-1(NdxBa2-x)Cu3O7-delta (R = Nd, Y) thin films

We have studied the effect of Nd/Ba substitution on the superconducting properties of R-1(Nd4Ba2-lambda)Cu3O7 (R = Nd, Y) thin films by using microwave penetration depth and surface resistance measurements. Data on samples characterised by a Nd-excess of 4-5% have been collected by using a microstrip resonator at frequencies of 1.44 and 9.7 GHz and a symmetric dielectric resonator technique in TE011 mode at 19.8 GHz. A quadratic bT(2) low temperature dependence of the penetration depth is observed for the Nd-rich samples, with the coefficient b decreasing with the frequency. The data can be reasonably explained in a d-wave framework supposing that Nd at Ba site behaves, both in Nd1+x Ba2-lambda Cu3O7 and Y-iota(NdxBa2-lambda)Cu3O7, as a resonant impurity, as confirmed by scanning tunnelling spectroscopy. The changes of the temperature dependence of the penetration depth with the frequency can be explained with a two fluid approach taking into account the frequency dependence of the complex conductivity.