A comparison of high-resolution, angle-resolved photoemission spectroscopy (ARPES) data with ab initio band-structure calculations by density functional theory for the anticipated Kondo insulator FeSi shows that the experimental dispersions can quantitatively be described by an itinerant behavior provided that an appropriate self-energy correction is included, whose real part describes the band renormalization due to interactions of the Fe 3d electrons. The imaginary part of the self-energy, on the other hand, determines the linewidth of the quasiparticle peaks in the ARPES data. We use a model self-energy which consistently describes both the renormalized single-particle dispersion and the energy-dependent linewidth of the Fe 3d bands. These results are clear evidence that FeSi is an itinerant semiconductor whose properties can be explained without a local Kondo-like interaction.

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Fe 1−x Co x Si single crystals with different Co concentrations were grown using the tri-arc Czochralski technique. Samples with 0, 5, and 20 at. % of Co were studied with photon energies of 21.2 and 40.8 eV with spectral resolutions as high as 8 meV at the Fermi level. For FeSi, a full gap of 28 meV has been found, which closes with 5% of Co. In the range between 20 and 150 eV, energy dependent angle-resolved measurements in the ͓111͔ direction were made for FeSi. Through a comparison with theoretical calculations, the states at the Fermi energy can be directly assigned to Fe 3d states. No sign of Kondo resonance is found.

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