Far-Infrared Absorption in Superconducting Lanthanum

Measurements have been made of far-infrared absorption in pure, face-centered cubic, superconducting lanthanum at 1.3^∘{}K. The results indicate a gap width 2Δ=12.0±1.0 cm^-1. In terms of the measured value of the critical temperature, Tc=6.06^∘K, 2Δ=2.85±0.24kTc. The data indicate the possible presence of a small amount of structure in the absorption curve at an energy about twice the gap width. These results are compared with theoretical predictions which have been made recently from a proposed new mechanism for superconductivity in lanthanum.