Absolute Measurement of the Far-Infrared Surface Resistance of Pb

The far-infrared surface resistance of Pb has been measured absolutely using a new technique of parallel-plate waveguides. These are made from two rolled-metal foils and a dielectric spacing material or by evaporating the metal directly on both surfaces of the dielectric and thus preventing the air from oxidizing the measured metal surface. Measurements of gold and lead at room temperature show good agreement with the Drude theory and illustrate the accuracy of the new technique. The low-temperature measurements of lead carried out between 6 and 280 ${\mathrm{cm}}^{-1\mathrm{}}$ in the superconducting and normal state show the onset of the Holstein process (a simultaneous absorption of a photon and emission of a phonon by an electron) and the superconducting energy gap and fit a new sum rule for the surface resistance. The surface resistance of normal Pb agrees well with calculations by Scher, if impurity scattering is taken into account. The data are also compared with Shaw and Swihart's calculations for strong coupling superconductors. The far-infrared properties of the spacing materials, Teflon and polyethylene, used in this work are also given.