On the Nature of Coupling of In Situ Superconductors

Abstract

Recently alternative fabrication methods have been developed to produce A15 superconducting materials with properties comparable to those of bronze-processed multifilamentary materials. First of all this concerns the so-called in situ processed or microcomposite superconductor.¹ Such superconductors differ from the conventional multifilamentary ones by initially discontinuous superconducting filaments running through the whole sample. In the in situ composite, the filaments have finite length and form a lacy three-dimensional structure in the Cu matrix. The filaments are formed from the initial Nb dendrites in the Cu matrix by severe drawing of the sample. Under strong deformation the Nb filament thickness and the interfilamentary spacing in the Cu matrix may achieve values of the order of 100 Å, which is comparable to the Cooper pair size. Consequently, it is interesting to investigate the superconducting properties of Cu-Nb in situ composites (with low and high Nb percentage). Several theoretical models were proposed to explain the experimental data. Prevalent models are based on the percolation and/or proximity effects. Some proximity-induced features of the Cu-Nb in situ composites with Nb concentration over the percolation threshold are investigated in the present work.