Critical current enhancement in NbN/AlN multilayers

Measurements are presented of the critical current density, J_c, and parallel upper critical field, H_c2∥, for the refractory superconductor/insulator multilayer system NbN/AlN. A dramatic increase in J_c, together with an increase of H_c2∥, is observed with decreasing NbN layer thickness. The J_c enhancement arises from flux pinning in the AlN or at the NbN-AlN interface, not at defects in the NbN structure. The results indicate that the development of superconductor/insulator composites is a viable strategy to increase the critical current. Thermal stabilization which does not degrade the very high J_c values was also accomplished in a novel structure by addition of copper layers within the superlattice.