Size and proximity effects in multifilamentary superconducting wires

The longitudinal resistance of a copper matrix in commercial multifilamentary superconducting wires with various numbers and diameters of Nb-Ti filaments has been measured. This was done at a temperature, T ≈ 12 K when PNbzTi, ≈103Pcu so that the longitudinal resistance of a sample is entirely determined by copper. For all samples the resistivity, ϱ∥, in the core consisting of both Cu and NbTi, is greater than the resistivity, Pcu, in the surface superconductor-free layer. Size effect makes a significant contribution to the value of ϱ∥. If a current is passed through a multifilamentary superconducting wire perpendicular to the superconducting filaments, a state specific to two-dimensional superconductors is created. The temperature dependence of the transverse resistance of the samples used was analogous to that of the resistance of a two-dimensional system. Thus, after the superconducting transition of the NbTi-50 filaments, a decrease in the temperature leads to a monotonic decrease of the resistance due to the proximity effect. This is in good agreement with a mathematical model developed for such two-dimensional superconductors.