Saddle-point states and energy barriers for vortex entrance and exit in superconducting disks and rings

The transitions between the different vortex states of thin mesoscopic superconducting disks and rings are studied using the nonlinear Ginzburg-Landau functional. They are saddle points of the free energy representing the energy barrier which has to be overcome for transition between the different vortex states. In small superconducting disks and rings the saddle point state between two giant vortex states, and in larger systems the saddle point state between a multivortex state and a giant vortex state and between two multivortex states is obtained. The shape and the height of the nucleation barrier is investigated for different disk and ring configurations.