Superconducting Properties of Tin, Indium, and Mercury below 1° K

The superconducting critical field curves for pure Sn, In, and Hg have been measured down to about 0.3^∘{}K. In the low-temperature region where the superconducting electronic entropy is negligible, the entropy difference (and hence the normal electronic entropy) is linear in temperature, as expected for a free electron gas. The measurements provide a sensitive test for details of the BCS theory in that the accuracy obtainable is much higher than in other experiments. A simple scaling of the energy-gap-to-temperature ratio brings the theoretical predictions into good agreement with experiment. The results are critically analyzed in the light of a possible lattice contribution to the entropy change at the superconducting transition of In, but no evidence for such a lattice contribution is found.