Surface pinning of superconducting Nb–Ta alloy

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Abstract
Surface irreversibility of cold-rolled Nb–50 at. % Ta was investigated by using an ac measuring method. Deformation was varied from 0% to 95%. Both the surface and bulk critical current densities increased remarkably and were then saturated to almost the same value as the deformation was increased. A change in the surface critical current density over almost three decades due to dislocations nucleated by deformation cannot be explained by mechanisms other than surface flux pinning. Close correspondence of the deformation dependence of the two critical current densities also confirms this hypothesis.