Observation of Vortex Tunneling Between Columnar Defect Tracks in BSCCO Superconductors

We have performed two-sided chemical etching and magnetic decoration experiments on single crystals of Bi₂ Sr₂ Ca Cu₂ O₈ irradiated with 1.1 Gev U^41+ ions parallel to the c axis. Analyses of the positions of defect sites on the opposite sample sides, which were determined by chemical etching and electron microscope, indicate that all the ions go through the 20 μ m thick sample. Moreover, these results have been used to determine the natural splay of the ion tracks and compared with the TRIM Monte Carlo simulations. The vortex positions on both sides of the sample with the applied magnetic field parallel to the c axis were dertermined by decoration. We have found that when the vortex density is smaller than the defect density, each vortex is pinned by at least one columnar defect. In addition, some vortices were found to tunnel between columnar defect tracks as they traversed the sample. Explanations of the tunneling will be presented.