Experimental study of flux pinning in NbN films and multilayers: ultimate limits on critical currents in superconductors

A flux-pinning model is presented which predicts the maximum critical current density attainable in superconductors. That such a limit must exist comes from the realization that flux pinning is strongest in regions of weak superconductivity, but these regions cannot carry a large supercurrent. Since the same regions within the superconductor cannot be used for both pinning and supercurrent conduction, there must be an optimum mix, leading to a maximum J_c. Measurements on films and multilayers of NbN have verified many details of the model including anisotropy effects and a strong reduction in J_c for defect spacings smaller than the flux core diameter. In an optimized multilayer the pinning force reached ~22% of the theoretical maximum. The implications of these results on the practical applications of NbN films and on the maximum critical current density in the high-temperature oxide superconductors are also discussed