Specific Heat of Lead and Lead Alloys Between 0.4 and 4.2°K

The specific heat of a pure single crystal and a pure polycrystalline sample of lead has been measured between 0.4 and 4.2°K. For the coefficient of the normal-state electronic term $\gamma$, a value of (3.00±0.04) mJ/mole ${\mathrm{deg}}^2$ is found. The value for the Debye parameter at 0°K, ${\Theta }_0$, is (105.4±0.8)°K. Below 1.5°K, the superconducting-state specific heat $C_s$ could be represented by a $T^3$ law, with a slope 5% larger than that of the normal-state specific heat $C_n$. As a result, below 1.5°K the superconducting-state electronic specific heat $C_{\mathrm{es}}$ followed a $T^3$ law, in disagreement with the exponential behavior predicted by the Bardeen-Cooper-Schrieffer theory. In an effort to investigate the anomalously large values of $C_{\mathrm{es}}$, two samples of lead alloyed with 1.76% indium and 5.93% indium were measured. For the 1.76% In sample the anomaly persists, but it disappears for the 5.93% In sample. Tentatively it can be concluded that energy-gap anisotropy is able to explain the lead results.