Line shapes of field-dependent nonresonant microwave and RF absorption in high-Tc superconductors

The field-dependent nonresonant absorption at microwave (9.1 GHz) and radio (19.5 MHz) frequencies has been studied in the superconducting state of $YBa_2Cu_3O_{7-x}$ pellets and epitaxially grown c-axis oriented thin films using conventional continuous-wave electron paramagnetic resonance (EPR) and nuclear magnetic resonance (NMR) spectrometers. The line shapes of the signals from the thin-film samples are observed to depend sensitively on the operating frequency; at 19.5 MHz derivative looking signals saturating at low magnetic fields (~10 Oe) are obtained while at 9.1 GHz, signals saturate only at fields of the order of a kOe. Two different possible origins of the losses, namely, diffraction reduction of junction currents in the weak links and the viscous motion of the fluxons are considered and compared. We find that the observed frequency dependence of the line shape can be explained only following the latter mode