Surface impedance of R1(NdxBa2−x)Cu3O7−δ (R=Nd, Y) thin films

We have studied the effect of Nd/Ba substitution on the superconducting properties of R 1 (Nd x Ba 2− x )Cu 3 O 7 (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 TE 0 1 1 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 Nd 1+ x Ba 2− x Cu 3 O 7 and Y 1 (Nd x Ba 2− x )Cu 3 O 7 , 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.