Peak effect at microwave frequencies in swift heavy ion irradiated YBa₂Cu₃O_7-δ thin films - Investigation of vortex dynamics

Abstract

The vortex dynamics at microwave frequencies in YBa₂Cu₃O_7-δ(YBCO) films has been studied. We observe a peak in the microwave surface resistance R_s vs. T (temperature) plots at 4.88 GHz in some films in magnetic fields (upto 0.8 T) whereas a few films do not show a pronounced peak in their surface resistance. This is associated with the ‘peak-effect’ phenomenon and reflects the order-disorder transformation of the flux-line lattice near the transition temperature. Introduction of artificial pinning centers like columnar defects as a result of irradiation with 200 MeV Ag ion leads to the suppression of the peak in films exhibiting the ‘peak-effect’. Interestingly, a peak is seen to develop in those films that initially did not show a peak in the surface resistance before irradiation. These results and the dynamics of the vortices at microwave frequencies for the unirradiated and irradiated films will be discussed.