Far-Infrared Energy Gap Measurements in Bulk Superconducting In, Sn, Hg, Ta, V, Pb, and Nb

Measurements of the onset of absorption due to excitation of electrons across the energy gap were made on a variety of superconductors. Using the onset of the main absorption edge as a measure of the energy gap, the values of $E_g\mathrm{}\left(0\right)\mathrm{}$, the energy gap at absolute zero, were found to be $4.1\pm 0.2k{T}_c$ for indium, $3.6\pm 0.2k{T}_c$ for tin, $4.6\pm 0.2k{T}_c$ for mercury, $\leqq 3.0k{T}_c$ for tantalum, $3.4\pm 0.2k{T}_c$ for vanadium, $4.1\pm 0.2k{T}_c$ for lead, and $2.8\pm 0.3k{T}_c$ for niobium. The deviations of these values of the energy gap from the Bardeen-Cooper-Schrieffer value of $3.5k{T}_c$ are a fairly smooth function of the Debye temperature alone, but a less smooth one of the ratio of the Debye temperature to the critical temperature. Structure was found on the absorption curves of lead and mercury. Its interpretation is discussed in terms of possible states in the gap, anisotropy of the gap, and of a possible dielectric anomaly. The position of the main absorption edge seemed to scale with temperature according to the temperature dependence of the energy gap predicted by the Bardeen-Cooper-Schrieffer theory, and the structure appeared to have a roughly similar temperature dependence. The absorption curves for pure superconductors are compared with predictions based on the theory of the skin effect in the normal and superconducting states. Two alloy superconductors were measured. They appeared to have less well-defined gaps than the pure superconductors.