MULTIQUANTUM VORTICES IN CONVENTIONAL SUPERCONDUCTORS WITH COLUMNAR DEFECTS NEAR THE UPPER CRITICAL-FIELD

The equilibrium vortex configuration in conventional type-II supercond uctors containing columnar defects is theoretically investigated. Near the upper critical field a single defect causes a strong local deform ation of the vortex lattice. This deformation has C-3 or C-6 point sym metry, whose character strongly depends on the vortex-defect interacti on. If the interaction is attractive, the vortices can collapse onto t he defect, while in the case of repulsion the regions free of vortices appear near a defect. Increasing the applied magnetic field results i n an abrupt change of the configuration of vortices giving rise to ree ntering transitions between configurations with C-3 or C-6 symmetry. I n the case of a small concentration of defects these transitions manif est themselves as jumps of magnetization.