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Iron based narrow gap semiconductors such as FeSi, FeSb2, or FeGa3 havereceived a lot of attention because they exhibit a large thermopower, as wellas striking similarities to heavy fermion Kondo insulators. Many proposals havebeen advanced, however, lacking quantitative methodologies applied to thisproblem, a consensus remained elusive to date. Here, we employ realisticmany-body calculations to elucidate the impact of electronic correlationeffects on FeSi. Our methodology accounts for all substantial anomaliesobserved in FeSi: the metallization, the lack of conservation of spectralweight in optical spectroscopy, and the Curie susceptibility. In particular wefind a very good agreement for the anomalous thermoelectric power. Validated bythis congruence with experiment, we further discuss a new physical picture ofthe microscopic nature of the insulator-to-metal crossover. Indeed, we find thesuppression of the Seebeck coefficient to be driven by correlation inducedincoherence. Finally, we compare FeSi to its iso-structural and iso-electronichomologue RuSi, and predict that partially substituted Fe(1-x)Ru(x)Si willexhibit an increased thermopower at intermediate temperatures.

Thermopower of the Correlated Narrow Gap Semiconductor FeSi and Comparison to RuSi