Flux line pinning and transport measurements in a Bi2212 crystal in the presence of columnar defects

We have measured the transport properties of a heavy ion (lead) irradiated Bi2212 single crystal. Current-voltage measurements were recorded for magnetic fields below and above the dose equivalent field B-phi (fluence = 10(10) cm(-2), B-phi = 0.2 T), The extracted activation energies U-act(J,B,T) are analyzed using individual (B < B-phi) and collective depinning processes (B > B-phi). The vortex-vortex interactions are described using the analogous Ekman superfluid layer. This leads to a crossover from a field independent activation energy below B-phi to a B-0.5 dependence (as observed in pristine samples) which is only observed in the high field regime. (C) 1997 Elsevier Science B.V.