FLUX-LINE PINNING IN BI2SR2CA1CU2OX CRYSTALS - INTERPLAY OF INTRINSIC 2D BEHAVIOR AND IRRADIATION-INDUCED COLUMNAR DEFECTS

We report on flux-line pinning in Bi2Sr2Ca1Cu2Ox crystals by a well-defined defect structure produced through 0.5-GeV iodine irradiation. Along their path these ions create columnar damage tracks of 5 to 10 nm in diameter. Even when flux lines are aligned parallel to these tracks, the measured activation energies at low temperatures and small fields do not exceed 70 meV and the critical currents decline rapidly with increasing temperature. The small pinning energies are explained quantitatively by pinning of two-dimensional pancake vortices instead of complete flux lines.