Low-temperature magnetic, thermal, and transport properties of FeSi

We have measured the ac susceptibility, specific heat, resistivity, and thermal conductivity of single-crystalline samples of FeSi in various temperature ranges between 0.04 and 300 K. The unequivocal result emerges from the ac susceptibility data, and is that there is no evidence for any magnetic ordering, at least down to 40 mK. The specific heat contains a linear contribution visible between 1.5 and 3.5 K, and a slow upturn below 1.5 K. This latter feature, and the observed Curie behavior of the ac susceptibility can both be quantitatively accounted for within a model of Anderson-localized states associated with impurities, at the level ${10}^{19}$ ${\mathrm{cm}}^{\mathrm{-}3}$. However, the electrical conductivity, which is nowhere activated, appears to saturate below about 5 K, which would suggest a metallic state. We draw attention to numerous similarities between our data and previously published data for the rare-earth compound ${\mathrm{SmB}}_6$.