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 γ, a value of (3.00±{}0.04) mJ/mole deg² is found. The value for the Debye parameter at 0^∘{}K, Θ₀, is (105.4±{}0.8)^∘{}K. Below 1.5^∘{}K, the superconducting-state specific heat Cₛ could be represented by a T³ law, with a slope 5% larger than that of the normal-state specific heat Cₙ. As a result, below 1.5^∘{}K the superconducting-state electronic specific heat Cₑₛ followed a T³ law, in disagreement with the exponential behavior predicted by the Bardeen-Cooper-Schrieffer theory. In an effort to investigate the anomalously large values of Cₑₛ, 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.