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The physical properties of the semiconductor FeSi with very narrow band gap,anomalous behavior of the magnetic susceptibility and metal-insulatortransition at elevated temperatures attract gross interest due to the stillcontroversial theoretical understanding of their origin. On one side the purelyband like mechanism of the gap formation in FeSi at low temperature is wellestablished, on other side a number of experiments and their theoreticalinterpretation suggest a rich physics of strong correlations at finitetemperature. In this work we use an ab-initio scheme based on the Random PhaseApproximation and Local Spin Density Approximation (RPA@LSDA) to reveal therole of the electron correlation effects in FeSi extending it by applying afixed spin moment constraint. In the parameter free framework we show thatcorrelation effects essentially alter the one-electron LSDA results leading tothe formation of an additional state with finite magnetic moment on Fe, whoseenergy is almost degenerate with the non-magnetic ground state. This explainsthe results of high field experiments, which found a first-order meta-magneticphase transition into a metallic ferromagnetic state. Our results suggest astrongly correlated nature of the low-energy excitations in FeSi. From oursuper-cells calculations we reveal that these excitations are local and exhibita Kondo-like behavior since a strong antiferromagnetic screening is present.

Correlated excited states in the narrow gap band semiconductor FeSi and antiferromagnetic screening of local spin moments