HOPPING CONDUCTIVITY IN FESI

We measured the ac conductivity of polycrystalline FeSi at temperature s 80 mK less than or equal to T less than or equal to 450 K and freque ncies 20 Hz less than or equal to nu less than or equal to 1 GHz. At t emperatures T < 25 K the ac response is governed by hopping of localiz ed charge carriers. However, the complex conductivity in FeSi cannot b e described by models developed for conventional semiconductors. At hi gher temperatures no signs of hopping conductivity could be found. Obv iously itinerant charge carriers dominate the charge transport for T > 25 K and deviations of the temperature dependence of the de resistivi ty from a thermally activated behavior which occur below 90 K are not due to Anderson localization but probably to Kondo-like behavior. Belo w 4 K the dielectric constant increases by an order of magnitude signa ling a reentrant transition into a metallic state.