Many authors have suggested that persistent currents in superconducting rings arise from the energy gap in the single-particle spectrum. Indeed, the argument has been put forward many times on this site! It is usually suggested that because there is an energy gap, Cooper pairs are prevented from scattering out of the condensate.
However, this cannot be correct. For one, high temperature superconductors have d-wave symmetry, which implies a node (i.e. it takes zero energy to excite an electron along this direction). This seems to suggest that a complete gap is not necessary for persistent currents. Furthermore, it has been shown by Abrikosov and Gorkov that when one introduces magnetic impurities into an s-wave superconductor, the gap closes before persistent currents are destroyed.
Therefore, the single-particle gap is not a necessary condition for superconductivity and any attempt to explain persistent currents by appealing to an energy gap in the single-particle spectrum cannot be correct.
Is there therefore a simple way to understand why persistent currents exist in a superconductor intuitively? What are the necessary requirements?