Peak effect and its evolution with defect structure in YBa2Cu3O7-δ thin films at microwave frequencies -: art. no. 174521

The vortex dynamics in YBa2Cu3O7-delta thin films have been studied at microwave frequencies. A pronounced peak in the surface resistance R-s is observed in these films at frequencies of 4.88 and 9.55 GHz for magnetic fields varying from 0.2 to 0.8 T. The occurrence of the peak in R-s is crucially dependent on the depinning frequency omega(p) and on the nature and concentration of growth defects present in these films. Introduction of artificial defects by swift heavy ion irradiation with a 200 MeV Ag ion at a fluence of 4x10(10) ions/cm(2) enhances omega(p) and suppresses the peak at 4.88 GHz but the peak at 9.55 GHz remains unaffected. The peak can be associated with a transformation of the flux line lattice (FLL) from a pinned solid to a depinned liquid in a depinning frequency domain similar to the order-disorder transformation of the FLL associated with the peak effect in critical currents in dc or low frequencies as the temperature or field is increased. A second peak at lower temperature has also been observed at 9.55 GHz. This could be related to twin boundaries from angular dependence studies of R-s. Based on the temperature variation of R-s, vortex phase diagrams have been constructed at 9.55 GHz.