ION IRRADIATION OF LAYERED BSCCO COMPOUNDS - FLUX LINE PINNING AND EVIDENCE FOR 2-D BEHAVIOR

Previous irradiation experiments with high-energy low-Z oxygen ions and fast neutron? on both polycrystalline material and single crystals of the 2212 BSCCO compound showed that the number of effective pinning centres could be increased and j(c) was enhanced particularly at low temperatures but the pinning energies were not significantly changed. Whereas these projectiles create only small sized and randomly distributed defects, high energy high-Z ions like 0. 5 GeV iodine ions produce continuous tracks (5 - 1 0 nm in diameter) along their path in 2212 BSCCO single crystals. Assuming conventional core pinning, the activation energies should be high enough to overcome the thermal depinning problem at elevated temperatures. In contrast to these expectations, the magnetization measurements showed that measured pinning energies at low temperatures and small fields do not exceed 70 meV after irradiation. At T = 1 0 K the magnetization currents could be increased up to 10(7) A cm-2 but they decline rapidly with field. The small pinning energies can be explained theoretically if one assumes that the elementary depinning mechanism is controlled by the thermally activated depinning of individual pancake vortices located within the CUO2 double layers. The 2-D nature is the consequence of the pronounced superconducting anisotropy in the BSCCO compounds. This behaviour is also confirmed in transport current measurements j(ct) of polycrystalline highly textured 2223 BPSCCO silver tapes irradiated with 0.65 GeV Ni ions. The j(ct),(B,T) dependence was investigated for both the field orientations B parallel-to tape and B perpendicular-to tape.