Critical Field Curve of Superconducting Mercury

The critical field curve of mercury has been precisely measured in a range of reduced temperature, t=T{T₂}, from 1 to 0. 27. The observed H₂ values show an appreciable deviation from a parabolic temperature dependence, lying above the parabola which passes through the experimental values of H₀ and T₂. The general behavior is similar to that previously observed in the case of lead. Values of the temperature coefficient of the normal electronic specific heat, , are derived, but are somewhat uncertain since the H₂ data indicate an appreciable entropy contribution from the superconducting electrons at the lowest temperature of measurement. The qualitative behavior of H₂ (T) for lead and mercury is in accord with recent infrared measurements which give direct indication that the energy gap value for these elements is anomalously large.