Superconductivity in Th-Nb composites
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Heat capacities, critical temperatures and critical currents have been measured for directionally solidified eutectic Th-Nb wires in order to determine whether superconductor-normal metal boundary is a strong pinning center for quantized vortices. The heat capacity data show that there is a large change in pair potential at the super-normal boundaries as predicted by de Gennes boundary conditions. The T(,c) data were used to evaluate the extrapolation length b, and to show that Ginsburg-Landau predicts the changes in the superconductors rather well. Hence, the equilibrium thermodynamical properties are predicted from the filament size and spacing rather well. Critical currents on the other hand, seem to be independent of the density of super-normal boundaries and some other factors must control J(,c). The magnetic field dependence of C(,p) and J(,c) tend to confirm this conclusion. A field as small as 20 mT will decrease the pair potential in the normal metal by factors of 100 with no apparent change in J(,c). The critical current data was insensitive to Nb filament diameter over the range of 8 to 116 nm indicating that the superconductor-normal metal interfaces were not effective pinning centers. Transition temperature data was used to calculate the extrapolation length, b, which was a smooth function of Nb filament diameter with a sharp increase in b for filament diameters below the coherence length. Heat capacity data showed that the composites with Nb filament diameters of 500 nm had a critical field curve close to that of bulk Nb with the Th essentially normal for fields of 0.02 T and above.