Surface vortex depinning in an irradiated single crystal microbridge ofBi2Sr2CaCu2O8+δ: Crossover from individual to collective bulk pinning

Current--voltage characteristics have been performed on an irradiated microbridged ${\text{Bi}}_{2}{\text{Sr}}_{2}{\text{CaCu}}_{2}{\text{O}}_{8+\ensuremath{\delta}}$ single crystal at low temperature $(T=5\text{ }\text{K})$ in a magnetic field applied parallel to the $c$ axis. The evolution of the I--V characteristics that leads to a peak in the differential resistance ${r}_{d}(I)=dV/dR(I)$ in the vicinity of the ``peak effect'' in the critical current was investigated. The analysis of the current-dependent differential resistance curves shows that the dynamics of the vortices in the vicinity of ${B}_{\ensuremath{\phi}}$ is dominated by the plastic flow of interstitially pinned vortex around regions of flux lines strongly pinned at the columnar defects. The pinning is found to be individual at low magnetic fields and collective when the vortex--vortex interactions are involved.