Experimental study of the ultimate limit of flux pinning and critical currents in superconductors

Abstract

Abstract The effectiveness of planar surface and grain boundary pinning structures in films and multilayers has been determined from measurements of the critical temperature, Tc, critical magnetic field, Bc2, and critical current density, Jc. The effects of these were separated using a Kramer-like analysis near Bc2. The dramatic increases in Jc of the multilayers of NbN over single NbN films, measured at 4.2 K and 20 T parallel field, are found to be a result of both increased Bc2 and increased pinning due to the alternating barrier layers. Both pinning structures exhibited strong pinning forces, which were significantly reduced for defect spacings smaller than the flux core diameter, and in an optimized multilayer the pinning force reached ∼22% of the theoretical maximum. A large anisotropy, similar to that observed in high-temperature superconductors, is found in the critical properties of the NbN multilayers, and an anisotropic model is developed. This model predicts a maximum potential Jc value of at least 2 x 108 A/cm2 in YBa2Cu3O7 at low reduced temperature and 20 T field. This would be reduced to ∼4 x 107 A/cm2 if a pinning strength equal to the highest effectiveness found in NbN could be realized in YBa2Cu3O7.