Abstract
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+j X 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= ΔR s/ ΔX 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.
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