Magnetic Properties of Superconducting Tantalum-Niobium Alloys

Abstract

The superconducting properties of Ta-rich Ta-Nb alloys have been investigated by magnetization measurements. The Ginzburg-Landau parameter κ of pure tantalum is estimated to be 0.42 and the critical value \(\kappa{=}1/\sqrt{2}\) is attained at a composition Ta–2.4 at. % Nb. The dependence of the thermodynamic critical field on temperature T follows the BCS prediction and all the alloys are considered to be weak-coupling superconductors. The lower critical field accords with the microscopic theory in the vicinity of the transition temperature T c , but the discrepancy between experiment and theory increases with decreasing T . The temperature dependence of the upper critical field and the generalized Ginzburg-Landau parameter κ 2 is stronger than the prediction and the disparity becomes large as the electron mean free path l increases. These properties may be ascribed to the effect of the anisotropy of the electronic structure in the state where the electron mean free path l is considerably larger than the intrinsic coherence length ξ 0 .