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

It has been found that the critical current density J(c) in a multifil amentary superconducting Nb-Ti wire first increases and then reaches a peak, while the critical temperature decreases monotonically as the s uperconducting filament diameter is decreased by drawing to the submic rometre range. This behaviour originates from a variation in the morph ology 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) o f alpha-Ti ribbons is theoretically estimated for idealized multilayer s of superconducting and normal regions, and J(c) is calculated from a statistical summation of the elementary pinning interactions. The the oretical result predicts that J(c) increases monotonically with drawin g, owing to the increase in density of pin points, in spite of the dec rease in f(p). The reason for the observed degradation of J(c) is disc ussed.