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

TL;DR

This study uses advanced ab-initio methods to show that electron correlations in FeSi create local magnetic excitations with Kondo-like behavior, explaining its complex magnetic and electronic properties.

Contribution

The paper introduces a correlation-aware ab-initio approach revealing the formation of magnetic states and local excitations in FeSi, advancing understanding of its strongly correlated physics.

Findings

Correlation effects induce a magnetic state near the non-magnetic ground state.

Local excitations in FeSi exhibit Kondo-like antiferromagnetic screening.

Results explain high-field experiments and the metal-insulator transition.

Abstract

The physical properties of the semiconductor FeSi with very narrow band gap, anomalous behavior of the magnetic susceptibility and metal-insulator transition at elevated temperatures attract gross interest due to the still controversial theoretical understanding of their origin. On one side the purely band like mechanism of the gap formation in FeSi at low temperature is well established, on other side a number of experiments and their theoretical interpretation suggest a rich physics of strong correlations at finite temperature. In this work we use an ab-initio scheme based on the Random Phase Approximation and Local Spin Density Approximation (RPA@LSDA) to reveal the role of the electron correlation effects in FeSi extending it by applying a fixed spin moment constraint. In the parameter free framework we show that correlation effects essentially alter the one-electron LSDA results leading to the formation of an additional state with finite magnetic moment on Fe, whose energy is almost degenerate with the non-magnetic ground state. This explains the results of high field experiments, which found a first-order meta-magnetic phase transition into a metallic ferromagnetic state. Our results suggest a strongly correlated nature of the low-energy excitations in FeSi. From our super-cells calculations we reveal that these excitations are local and exhibit a Kondo-like behavior since a strong antiferromagnetic screening is present.