Far-Infrared Study of Superconductors Containing Magnetic Impurities

A new far-infrared method has been developed to study the energy-gap properties of a thin superconducting film containing magnetic impurities, and which is thus capable of exhibiting gapless superconductivity. The method allows the direct determination of the real part ${\sigma }_1$ of the conductivity of the superconductor by measuring calorimetrically the infrared radiation absorbed by the film. The systems studied were quenched films of lead containing either Gd or Mn impurities. The experimentally observed frequency dependence of ${\sigma }_1$ (which exhibits the properties of the energy gap) is found to be distinctly different from that expected for nonmagnetic impurities. In the case of lead films containing Gd impurities, whose spins are due to $4f$ electrons, the behavior of ${\sigma }_1$ is in good quantitative agreement with the behavior predicted by Skalski et al. on the basis of the Abrikosov-Gorkov theory. In the case of lead films containing Mn impurities, whose spins are due to $3d$ electrons, the behavior of ${\sigma }_1$ is affected more drastically than predicted by this theory. These results agree with conclusions obtained in previous tunneling measurements on these systems.