Magnetic Properties of Superconducting Niobium-Tantalum Alloys

Abstract

The superconducting transition temperature and isothermal magnetization have been measured for a series of Nb-Ta alloys ranging from Nb-13 at. % Ta to Nb-83 at. % Ta. These alloys are characterized as dirty type II superconductors; the electron mean free path l lies in a range from 0.1 ξ 0 to 0.3 ξ 0 , where ξ 0 is the intrinsic coherence length. The Ginzburg-Landau parameter κ takes the maximum value of 4.1 at the composition, Nb-50 at. % Ta. In concentrated alloys with compositions 20∼80 at. % Ta, the temperature dependence of the generalized Ginzburg-Landau parameter κ 1 ( T ) accords with the theory for the dirty limit. Especially, the reduced parameter at 0°K, κ 1 (0)/κ, is in excellent agreement with the theoretical value 1.20. This agreement between experiment and theory is attributed to the fact that the temperature dependence of both the thermodynamic critical field H c ( T ) and the upper critical field H c 2 ( T ) follows the microscopic theory.