Anomalous nonlinear microwave response of YBaCuO thin films on MgO

We report on the nonlinear microwave properties of two high-quality YBaCuO thin films on MgO measured at 8 and 16 GHz using the coplanar resonator technique in zero and finite (up to 12 mT) DC magnetic fields applied perpendicularly to the films in field-cooled regime. In the superconducting state, the surface impedance, Z(s) = R-s + jX(s) of both samples decreases as a function of the microwave magnetic field, H-rf. One of the films exhibits this anomaly in R-s(H-rf) at low temperatures T < 15 K, whereas for the other film pronounced anomalies are only seen in X-S(H-rf) and only at intermediate temperatures (40 < T < 50 K). Analysis of the data in terms of the temperature-, frequency- and field-dependent r-parameter (r = DeltaR(S)/DeltaX(S)) has revealed that the anomalous mechanism is most likely related to the quasiparticle dynamics rather than vortex motion. The uncorrelated functional behavior R-S(H-rf) and X-S(H-rf) observed for both films seems to be related to several nonlinear mechanisms interfering with each other. (C) 2002 Elsevier Science B.V. All rights reserved.