Periodic Anderson model from the atomic limit: The optical conductivity of FeSi

The optical conductivity σ(ω,T) can be obtained with the same approximations employed in a previous work to calculate the static conductivity σ(T) and magnetic susceptibility χ(T) of FeSi, a compound that behaves like a Kondo insulator with both quantities vanishing rapidly for T⃗0; the periodic Anderson model (PAM) for U⃗∞ was employed to model FeSi, assuming that the system is in the intermediate valence region. The same treatment is employed in the present paper to describe the three properties for the same compound, and a fairly good agreement with the experimental results is obtained. The sum rule of σ(ω,T), appropriate for the PAM, is well satisfied in the range of the experimentally measured temperatures. Some quantities derived from our results are of the same order of magnitude that was estimated from recent measurements of angle-resolved low-temperature photoemission in FeSi.