  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.
