CRITICAL CURRENT-DENSITY IN SUPERCONDUCTING NB-TI UNDER PROXIMITY EFFECT

It has been found that the critical current density J(c) in a multifilamentary superconducting Nb-Ti wire first increases and then reaches a peak, while the critical temperature decreases monotonically as the superconducting filament diameter is decreased by drawing to the submicrometre range. This behaviour originates from a variation in the morphology of the normal alpha-Ti phase precipitated during heat treatment, coupled with the proximity effect between the superconducting matrix and normal alpha-Ti. In this paper the elementary pinning force f(p) of alpha-Ti ribbons is theoretically estimated for idealized multilayers of superconducting and normal regions, and J(c) is calculated from a statistical summation of the elementary pinning interactions. The theoretical result predicts that J(c) increases monotonically with drawing, owing to the increase in density of pin points, in spite of the decrease in f(p). The reason for the observed degradation of J(c) is discussed.