Superconducting properties and critical current density of Nb-Ti/Timultilayers

摘要

Multilayers of Nb47Ti superconductor (S) and titanium pins (N) have been made with a critical temperature Tc and an upper critical field Hc2 approaching bulk values. There is no proximity effect suppression of Tc and Hc2 for layer thickness dS=dN=10 nm, in contrast to the strong suppression for comparable multilayers made with Cu, Cu-alloy, or Nb pins. This may be because the proximity length of the Ti layers, ~10 nm, is less than or equal to their thickness. The critical current density Jc exhibits, multiple peaks in Jc(H), which suggests that matching effects contribute to the overall pinning. Jc for 20 nm bilayers is comparable to that of Nb47Ti tapes, which have a higher number density of pins but a lower pin volume fraction. The weak proximity coupling suggests that much higher Jc can be obtained if smaller bilayer periods can be made with good adhesion. The results are compared to other Nb-Ti/Ti multilayer experiments, and stability limitations are also discussed