Peak effect and its evolution with defect structure inYBa2Cu3O7−δthin films at microwave frequencies

Abstract

The vortex dynamics in ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ 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 ${\ensuremath{\omega}}_{p}$ 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 $4\ifmmode\times\else\texttimes\fi{}{10}^{10} {\mathrm{i}\mathrm{o}\mathrm{n}\mathrm{s}/\mathrm{c}\mathrm{m}}^{2}$ enhances ${\ensuremath{\omega}}_{p}$ 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.