Signatures of electronic correlations in iron silicide

TL;DR

This paper uses many-body calculations to explain the temperature-dependent electronic and magnetic behavior of FeSi, showing it transitions from a band insulator to a correlated metal with incoherent fluctuations.

Contribution

It presents a new microscopic scenario for FeSi's behavior, emphasizing correlation-induced incoherence and fluctuating moments, validated by quantitative agreement with experiments.

Findings

FeSi is a band insulator at low temperatures

FeSi becomes metallic with increasing temperature due to correlations

Incoherence is linked to iron fluctuating moments

Abstract

The intermetallic FeSi exhibits an unusual temperature dependence in its electronic and magnetic degrees of freedom, epitomized by the crossover from a low temperature non-magnetic semiconductor to a high temperature paramagnetic metal with a Curie-Weiss like susceptibility. Many proposals for this unconventional behavior have been advanced, yet a consensus remains elusive. Using realistic many-body calculations, we here reproduce the signatures of the metal-insulator crossover in various observables: the spectral function, the optical conductivity, the spin susceptibility, and the Seebeck coefficient. Validated by quantitative agreement with experiment, we then address the underlying microscopic picture. We propose a new scenario in which FeSi is a band-insulator at low temperatures and is metalized with increasing temperature through correlation induced incoherence. We explain that the emergent incoherence is linked to the unlocking of iron fluctuating moments which are almost temperature independent at short time scales. Finally, we make explicit suggestions for improving the thermoelectric performance of FeSi based systems.