Novel effects observed in heavy ion irradiated Bi₂Sr_2-xLa_xCuO_y thin films

The superconducting properties of Bi₂Sr_2-xLa_xCuO_y thin films irradiated by 1 GeV heavy ions have been investigated. Firstly, irradiation produces dramatic changes in T_c and in R(T). This fact is attributed to a self-doping effect of the sample by oxygen liberated from the ion tracks. Secondly, the pinning properties of the columnar defects allowed us to probe different vortex behaviours for films with increasing La content (0 ≤ x ≤ 0.4). From the angular dependence of R(H) and J_c(H) in the mixed state, for a given column density, a directional pinning effect, or linear vortex behaviour, is observed for low x values, whereas no directional effect is seen for large x values and 2D scaling properties, characteristic of pancake vortices, are then observed. Importantly this reveals that changes in x induce large changes in the material anisotropy.