Vorticity and vortex-core states in type-II superconductors

The origin of the vortex-core states in s-wave and d_x²−y²-wave superconductors is investigated by means of some selected numerical experiments. By relaxing the self-consistency condition in the Bogoliubov–de Gennes equations and tuning the order parameter in the core region, it is shown that the suppression of the superfluid density in the core is not a necessary condition for the core states to form. This excludes “potential well” types of interpretations for the core states. The topological defect in the phase of the order parameter, however, plays a crucial role. This fact is explained by considering the effect of the vortex supercurrent on the Bogoliubov quasiparticles and illustrated by comparing conventional vortices to multiply quantized vortices and vortex-antivortex pairs. The core states are also found to be extremely robust against random disorder of the phase.