Vortex dynamics at subcritical currents at microwave frequencies inDyBa2Cu3O7−δthin films

Abstract

We have investigated the dynamics of vortices at subcritical microwave currents in dc magnetic fields (up to 0.8 T) in epitaxial ${\mathrm{DyBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ (DBCO) thin films. Microwave measurements were performed using microstrip resonators as test vehicles at 4.88 GHz and 9.55 GHz on laser ablated DBCO thin films in the thickness range 1800--3800 \AA{}. Experimental evidence indicates that the peak effect (PE) observed in surface resistance vs temperature ${(R}_{s}$ vs $T)$ plots in applied dc magnetic fields up to 0.8 T is primarily due to the extended defects in thinner films (1800 \AA{}) such as twin boundaries at the ${\mathrm{s}\mathrm{u}\mathrm{b}\mathrm{s}\mathrm{t}\mathrm{r}\mathrm{a}\mathrm{t}\mathrm{e}(\mathrm{L}\mathrm{a}\mathrm{A}\mathrm{l}\mathrm{O}}_{3})$-film interface; whereas, the high density of point defect disorder in thicker $(>~3000\mathrm{\AA{}})$ films is responsible for low ${R}_{s}$ and high depinning frequency ${\ensuremath{\omega}}_{p}.$ This has been confirmed by generation of columnar defects using 200 MeV Ag ion irradiation which showed that even thicker DBCO films show PE in ${R}_{s}$ after the introduction of columnar defects. Further, DBCO films grown on low-twinned ${\mathrm{LaAlO}}_{3}$ substrates (which cause low density of substrate-related extended defects in the film) have shown PE only at 9.55 GHz but not at 4.88 GHz. Values of ${\ensuremath{\omega}}_{p}$ have been calculated from experimental ${R}_{s}$ data. ${\ensuremath{\omega}}_{p}$ vs T plots obtained for the thinner films show a peak which is a result of the peaks in ${R}_{s}$ vs T plots of these films at 4.88 GHz and 9.55 GHz.