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/sub s/ as a function of microwave field H/sub rf/. In zero dc field, H/sub rf/-dependences of R/sub s/ and X/sub s/ for both the samples are uncorrelated, and only one of the quantities, R/sub s/ or X/sub s/, displays anomalous behavior. Here, application of relatively weak (/spl sim/5 mT) dc magnetic fields H/sub dc/ can produce a correlated decrease of R/sub s/(H/sub rf/) and X/sub s/(H/sub rf/). The dependences of Z/sub s/ on H/sub dc/ in both low and high microwave power regimes were found to be non-monotonic. The frequency dependence of R/sub s//spl sim/f/sup n/, 1.7