Enhanced far-infrared absorption in ${\mathrm{CePd}}_3$ and ${\mathrm{YbCu}}_2$${\mathrm{Si}}_2$. III. Comparison of a resonant-scattering model with experiment

We present a model calculation for the low-energy (~10-meV) optical conductivity of valence-fluctuation compounds for the case of resonant scattering of conduction electrons from a scattering level lying within 10 meV of the Fermi energy. The model is characterized by an enhancement of the optical absorption in the far infrared over that of a normal metal, and it agrees reasonably well with the measured low-temperature optical conductivity of ${\mathrm{CePd}}_3$ and ${\mathrm{YbCu}}_2$${\mathrm{Si}}_2$ with suitable choices of the model parameters. In the case of ${\mathrm{CePd}}_3$, in which optic phonons are observed, the effects of including the optic-phonon modes in the model calculations are also discussed.