MAGNETIC-FLUX LATTICE TRANSITION IN BI-2212 CRYSTALS WITH COLUMNAR DEFECTS

The addition of columnar defects in Bi-2212 crystals shifts the irreve rsibility line to a position which corresponds roughly to one vortex p er defect in a broad range of intermediate temperatures. However, a pr ecise analysis of this shift suggests that there is a continuous melti ng of the vortex lattice. An other transition can be seen on the activ ation energy measured from the temperature dependence of the resistivi ty. At the fast transition, there is a crossover in the angular depend ence of the persistent current from a 2D to a 3D behaviour.