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

TL;DR

This paper uses advanced many-body calculations to understand the electronic correlations in FeSi, explaining its thermopower and insulator-metal crossover, and predicts enhanced thermopower in Fe-Ru substituted compounds.

Contribution

It provides a quantitative, realistic many-body analysis of FeSi's anomalous properties and introduces a new physical picture of its insulator-metal transition, also comparing it to RuSi.

Findings

Good agreement between calculations and experimental thermopower.

Correlation effects cause incoherence, suppressing the Seebeck coefficient.

Predicted increased thermopower in Fe(1-x)Ru(x)Si at intermediate temperatures.

Abstract

Iron based narrow gap semiconductors such as FeSi, FeSb2, or FeGa3 have received a lot of attention because they exhibit a large thermopower, as well as striking similarities to heavy fermion Kondo insulators. Many proposals have been advanced, however, lacking quantitative methodologies applied to this problem, a consensus remained elusive to date. Here, we employ realistic many-body calculations to elucidate the impact of electronic correlation effects on FeSi. Our methodology accounts for all substantial anomalies observed in FeSi: the metallization, the lack of conservation of spectral weight in optical spectroscopy, and the Curie susceptibility. In particular we find a very good agreement for the anomalous thermoelectric power. Validated by this congruence with experiment, we further discuss a new physical picture of the microscopic nature of the insulator-to-metal crossover. Indeed, we find the suppression of the Seebeck coefficient to be driven by correlation induced incoherence. Finally, we compare FeSi to its iso-structural and iso-electronic homologue RuSi, and predict that partially substituted Fe(1-x)Ru(x)Si will exhibit an increased thermopower at intermediate temperatures.