Heat Capacity in the Normal and Superconducting States and Critical Field of Tantalum

The heat capacity of a vacuum-annealed sample of tantalum has been measured in the normal and superconducting state from 1.3 to 25°K. In the normal state the heat capacity $C_N$ can be represented by the expression $C_N=0.00136T+464.4\mathrm{}{\left(\frac{T}{\theta }\right)}^3$, where $\theta$ varies from approximately 255 degrees at the lowest temperature to 220 degrees at the highest. The zero-field transition temperature, $T_c$, and the critical field $H_c$ were determined using a calorimetric method. $T_c$ was found to be 4.39°K. The width of the transition was approximately 0.03°C. The measured critical field was within experimental error comparable to that calculated from the heat-capacity data. The tantalum sample therefore exhibited the properties of an ideal superconductor characteristic of the so called soft superconductors. The critical field at the absolute zero, from an extrapolation of the critical field data, was found to be 780 gauss.