OPTICAL EVIDENCE OF ANDERSON-MOTT LOCALIZATION IN FESI

We have investigated the electrodynamic response of single-crystalline FeSi over a frequency range from d.c. to 10(5) cm-1. At low frequenci es, we found evidence for an Anderson-Mott localization behaviour of t he low-temperature optical conductivity, while at high frequencies the excitation spectrum resembles that of a conventional semiconductor. T he missing spectral weight below the gap energy is redistributed aroun d the gap edge, in disagreement with previous claims based on optical measurements.