Multiquantum vortices in conventional superconductors with columnar defects near the upper critical field

The equilibrium vortex configuration in conventional type-II superconductors containing columnar defects is theoretically investigated. Near the upper critical field a single defect causes a strong local deformation of the vortex lattice. This deformation has C-3 or C-6 point symmetry, whose character strongly depends on the vortex-defect interaction. If the interaction is attractive, the vortices can collapse onto the defect, while in the case of repulsion the regions free of vortices appear near a defect. Increasing the applied magnetic field results in an abrupt change of the configuration of vortices giving rise to reentering transitions between configurations with C-3 or C-6 symmetry. In the case of a small concentration of defects these transitions manifest themselves as jumps of magnetization.