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Quasiparticles in Iron Silicides: GW versus LDA

The angle-resolved photoemission spectroscopy, ARPES, is able to measure both the spectra and spectral weights of the quasiparticles in solids. Although it is common to interpret the band structure obtained within the density-functional-theory based methods as quasi-particle spectra, these methods are not able to provide the changes in spectral weights of the electron excitations. We use Vienna Ab initio Simulation Package, VASP, for evaluation of the quasiparticle spectra and their spectral weights within Hedin GW approximation for Fe3Si and FeSi2, providing, thus, a prediction for the ARPES experiments. Comparison of the band structures shows that both theories reflect peculiarities of the crystal structures in similar shape of the bands in certain k-directions, however, in general the difference is quite noticeable. We find that the spectral weight of quasiparticles in these compounds deviates from unity everywhere and shows non-monotonic behavior in those parts of bands where the delocalized states contribute to their formation. Both methods lead to the same conclusion: those of iron ions in Fe3Si which occupy the positions, where they are surrounded by only Fe ions, have the d-electrons localized and large magnetic moment whereas Fe ions with the Si nearest neighbors have d-electrons delocalized and the magnetic moments quenched. The Si influence on the Fe ion state is even more pronounced in the FeSi2 where all iron ions have the Si ions as nearest neighbors: both approaches produce zero moment on iron ions. The advantages and disadvantages of both approaches are discussed.