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

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 great interest due to the still controversial theoretical understanding of their origin. On one side the purely bandlike mechanism of the gap formation in FeSi at low temperature is well established; on the 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 nonmagnetic ground state. This explains the results of high-field experiments, which found a first-order metamagnetic phase transition into a metallic ferromagnetic state. Our results suggest a strongly correlated nature of the low-energy excitations in FeSi. From our supercells calculations we reveal that these excitations are local and exhibit a Kondo-like behavior since a strong antiferromagnetic screening is present.

Figure 1

Ground DOS of FeSi around Fermi level calculated with PBE (black) and GW (red) approximation.

Figure 2

Total energy dependence on the magnetic moment in FeSi calculated with PBE (black) and EXX+RPA@PBE (red) approximations. The energy is given per Fe atoms. The stars show the energies calculated in PBE+U method with $U=2.0\mathrm{eV}$.

Figure 3

Contributions to the EXX+RPA@PBE total energy (per Fe) in B20 FeSi: black–Hartree-Fock contribution, red–RPA correlation energy.

Figure 4

Isosurfaces of the spin polarizations for 64-atom cell of FeSi as described in the text. The yellow surface is positive spin polarization, blue is negative. The brown spheres are Fe atoms, the dark blue spheres are Si atoms. The Fe atoms surrounded by large positive isosurface are excited (see text) Fe atoms. (a) One excited Fe in the supercell, (b) three excited Fe atoms per supercell, and (c) five Fe excited atoms per supercell.

Figure 5

“Schematic representation” of the spin polarization around the central Fe excited atoms (red) in 64-atom supercell with one excited atom. White and yellow circles represent the Fe atoms in the first and second nearest-neighbor shell of the excited Fe site. The numbers in the figure are the average atomic spin in the shell.

Figure 6

Calculated DOS around the Fermi level for a 64-atom supercell with one to five excited Fe atoms.