Correlation Effects in Multi-Band Hubbard Model and Anomalous Properties of FeSi

Abstract

The two-band Hubbard model with the density of states obtained from the band calculation is applied for FeSi, which is suggested to be a Kondo insulator or a correlated band insulator. Using this model, the correlation effects on FeSi are investigated in terms of the self-consistent second-order perturbation theory combined with the local approximation. The calculated optical conductivity spectrum reproduces the experiments by Damascelli et al. semiquantitatively and the specific heat explains the anomalous contribution at about 250 K observed in FeSi. Inclusion of the spin fluctuation and the extension to the case of strong correlation are also discussed.