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) irradi ated 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 ener gies U-act(J,B,T) are analyzed using individual (B < B-phi) and collec tive depinning processes (B > B-phi). The vortex-vortex interactions a re 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 ob served in the high field regime. (C) 1997 Elsevier Science B.V.