Flux pinning by thin α-Ti ribbons in superconducting Nb-Ti

It has been found by Lee and Larbalestier that the critical temperature reduces monotonically, while the critical current density first increases and is followed by a decrease, according as the diameter of superconducting multifilamentary Nb-Ti wire is decreased. This behavior is mainly caused by the proximity effect between superconducting matrix and normal α-Ti ribbons that becomes more remarkable as the thicknesses of two regions become smaller. The critical temperature and the elementary pinning strength of α-Ti ribbons are theoretically estimated by solving the phenomenological Ginzburg-Landau equations for multilayered structure with superconducting and normal layers. The critical current density is calculated from a statistical summation of the elementary pinning forces. The obtained critical temperature decreases monotonically, while the critical current density increases, with decreasing thicknesses of the two layers. The decrease in Jc observed by Lee and Larbalestier is considered to result from constriction of superconducting layers by heavy cold work, which may be associated with the sausaging of superconducting filaments in the multifilamentary wire. If a development of drawing technology that does not disorder the laminar structure inside the filament is achieved, a further increase in Jc will be realized by a further reduction of the wire. (author). 5 refs, 5 figs