Flux creep and the nature of a flux bundle in high-Tcthin films

We describe measurements of the temperature and magnetic-field dependence of the resistance and critical current in laser-deposited thin films of the high-temperature superconductor ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathit{x}}$. We identify two regimes: At low current densities and magnetic fields the sample is Ohmic. We attribute dissipation in this regime to activated flux motion of individual flux quanta in a vortex glass. In the high current and magnetic-field regime we show that collective vortex motion is occurring, and we propose a model for the structure of a flux bundle.