Anomalies in the Microwave Power-Dependent Surface Impedance of YBa2CU3o7-X Thin Films

Nonlinear microwave properties of two high-quality YBaCuO thin films on MgO substrate have been studied in zero and finite dc magnetic field (up to 12 mT) at 8 and 16 GHz using the coplanar resonator technique. Anomalous decrease in the surface impedance Z(s)=R-s + jX(s) as a function of increasing microwave magnetic field H-rf was found for both the samples. In one of the films anomalous behavior is only seen in R-s(H-rf) and only at low temperatures Tless than or equal to15 K, whereas in the other one only X-s(H-rf) exhibits anomalies and mostly at intermediate T (40-45 K). Impedance plane analysis in terms of T, frequency f, and H-rf dependences of the r parameter revealed that the anomalous behavior is most certainly governed by dynamics of quasiparticles, rather than vortices. At higher H-rf (greater than or equal to10 kA/m), however, a noticeable deviation from the quasiparticle scaling towards the vortex motion mechanisms was found. This indicates that in high microwave fields the anomalous affects in Z(s)(H-rf) may be masked by or interfered with the vortex dynamics. Also some evidence is presented that the origin of the anomalies in Z(s)(H-rf) is seemingly related to the oxygen content of the films.