Observation of individual vortices trapped along columnar defects in high-temperature superconductors

Many superconductors do not entirely expel magnetic flux-rather, magnetic f lux can penetrate the superconducting state in the form of vortices. Moving vortices create resistance, so they must be 'pinned' to permit dissipation less current flow. This is a particularly important issue for the high-tran sition-temperature superconductors, in which the vortices move very easily( 1). Irradiation of superconducting samples by heavy ions produces columnar defects, which are considered(2) to be the optimal pinning traps when the o rientation of the column coincides with that of the vortex line. Although c olumnar defect pinning has been investigated using macroscopic techniques(3 ,4), it has hitherto been impossible to resolve individual vortices interse cting with individual defects. Here we achieve the resolution required to i mage vortex lines and columnar defects in Bi2Sr2CaCu2O8+delta (Bi-2212) thi n films, using a 1-MV field-emission electron microscope(5). For our thin f ilms, we rnd that the vortex lines at higher temperatures are trapped and o riented along tilted columnar defects, irrespective of the orientation of t he applied magnetic field. At lower temperatures, however, vortex penetrati on always takes place perpendicular to the film plane, suggesting that intr insic 'background' pinning in the material now dominates.