Two permitted states of superconducting ring observed at measurements of   its critical current
  Magnetic dependencies of the critical current of superconducting, aluminium rings, with homogeneous section and with small narrow spot, are measured. Only permitted state at each magnetic flux value is observed in the conventional ring because of strong discreteness of its spectrum. But two states are observed because of the small narrow spot. 
V.L. Gurtovoi, S.V.Dubonos, A.V. Nikulov, and V.A. Tulin
cond-mat.supr-con