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 (Formula presented) or (Formula presented) 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 (Formula presented) or (Formula presented) symmetry. In the case of a small concentration of defects these transitions manifest themselves as jumps of magnetization.