Anomalous features in surface impedance of Y-Ba-Cu-O thin films: dependence on frequency, RF and DC fields

Two high-quality Y-Ba-Cu-O thin films on MgO substrates have been investigated using the coplanar resonator technique at 8 and 16 GHz. Both films exhibit an anomalous decrease in their surface impedance Z _s as a function of microwave field H_rf. In zero dc field, H_rf-dependences of R_s and X_s for both the samples are uncorrelated, and only one of the quantities, R_s or X_s, displays anomalous behavior. Here, application of relatively weak (~5 mT) dc magnetic fields H_dc can produce a correlated decrease of R_s(H_rf) and X _s(H_rf). The dependences of Z_s on H_dc in both low and high microwave power regimes were found to be non-monotonic. The frequency dependence of R_s~fⁿ , 1.7<n<2.5, remained the same upon the transition from low to high microwave power ranges. The consequences of the reported findings for microwave device applications are briefly discussed