Superconducting Critical Field of Tantalum as a Function of Temperature and Pressure

The results of precise critical field measurements on tantalum samples which show "soft" superconducting behavior are given along with direct measurements of the pressure effect, ${\left(\frac{\partial H_c}{\partial P}\right)}_T$, as a function of temperature. The Bardeen-Cooper-Schrieffer theory is used as a guide in the extrapolation of these data to absolute zero from 1.1°K. The advantages of using an $H^2$ vs $T^2$ representation for both the critical-field and pressure-effect data are discussed, and it is shown that if both sets of data can be represented in terms of power series [$H^2$ or ${\left(\frac{\partial {H_c}^2}{\partial P}\right)}_T$ vs $T^2$] over a limited range of temperature, it is then possible to write down explicit power series expressions for the differences in the thermodynamic functions between the normal and superconducting states over this same temperature range. The electronic contributions to the specific heats and the thermal expansions for tantalum are calculated from the experimental data.