Experimental tests of proposed relations between the critical field and dislocation cell structure of superconducting niobium

Abstract

Experimental tests were made of the suggestion that the anomalously large magnitude and anisotropy of the resistive critical field of severely deformed niobium is due to the development of dense dislocation cell walls. The basis for the tests was the theoretical prediction that as the cell size approaches the coherence length ξ_Nb for superconductivity, the superconducting properties of the cell walls would be smeared out over the entire micro-structure, destroying the anisotropy and a good part of the parallel critical field. Resistive measurements were made on swaged niobium wire and powder composites with true strains ranging up to 10.61, at temperatures from 4.2‡K to T_c. Two approaches were used: either by raising the temperature above 4.2‡K, ξ_Nb could be enlarged, or by increasing strain the cell size could be reduced. The predicted effects were indeed observed; from the temperatures at which the anisotropy disappeared, values of ξ_Nb were obtained which agree well with the expected magnitudes and variation of the cell size with cold work in drawn or swaged niobium.