ELECTRONIC-STRUCTURE AND OPTICAL-PROPERTIES OF FESI, A STRONGLY CORRELATED INSULATOR

Kondo insulators are a class of materials with a nonmagnetic ground state characterized by extremely narrow charge-excitation gaps induced by strong electron correlations. Prompted by suggestions that FeSi may be strongly correlated in a similar class to the Kondo insulators, we present results for the electronic structure and optical properties for FeSi calculated with a self-consistent augmented spherical wave method. The calculated indirect gap (approximate to 0.1 eV) is roughly consistent with resistivity measurements which indicate a gap of about 50 meV. We calculate the optical response incorporating simple finite-temperature effects, namely, electron-phonon scattering of intrinsic carriers. We find that this provides a poor direct fit to recent infrared measurements. The number of intrinsic carriers is low compared to experimental observations, even with an ad hoc renormalization of the gap, and a strong scattering mechanism with attendant disordering of the band structure is required as well.