Periodic Anderson model from the atomic limit: magnetic and transport properties of FeSi

The exact Green's functions (GF) of the periodic Anderson model (PAM) for U→∞ can be formally expressed within the cumulant expansion in terms of an effective cumulant that was approximated in a previous work by the value it takes for the exactly soluble atomic limit of the PAM. Expressions for the magnetic susceptibility χ(T), static conductivity σ(T) and optical conductivity σ(ω,T) of the PAM have been obtained with these approximate GF and they are employed here to fit the experimental values of FeSi, a compound that behaves like a Kondo insulator with the two first quantities vanishing rapidly for T→0. It was possible to find good agreement between theory and experiment for a set of parameters in the intermediate valence region