Superconducting phase transitions in thin mesoscopic rings with enhanced surface superconductivity

The superconducting phase transitions between different vortex states with the magnetic field for a thin mesoscopic superconducting ring surrounded by a medium that enhanced its superconductivity near the boundary are investigated by the phenomenological Ginzburg-Landau theory. The transitions between different giant vortex states and between the giant vortex and multivortex states with Delta L > 1 (L is the vorticity of the vortex state) are found for a small ring with increasing surface enhancement. The influences of the surface enhancing superconducting effect and the inner radius as well as the temperature on phase transition are studied by examining the H-parallel to xi/b parallel to, H-R-i, and H-T phase diagrams, respectively. Further increasing the effect of enhanced surface superconductivity, we find the reentrant transition with the same vorticity and the transitions between the stable multivortex states with Delta L > 1 for different ring inner radii. We also investigate the vortex configurations for a relatively large ring, and the vortex state with two stable vortex shells can be found as the ground state due to the enhanced surface superconductivity.