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Why is the gap in the density of states of the semiconductor FeSi [1,2] overestimated by band-structure calculations, which usually underestimate gaps? We provide a general framework for situations in which electronic interactions reduce excitation gaps. Studying a minimal model of a correlated covalent band insulator we explore the transition from a band to a Mott insulator and the behavior of charge and spin gaps. While there is only one gap in a weakly correlated band insulator, namely the band gap, this need not be the case in the more strongly correlated regime. The generic properties found in our study may serve as a starting point for a deeper understanding of a broader class of materials like FeSi and FeSb2, the Heusler compounds, or topological insulators. [1] Kunes and Anisimov, Phys. Rev. B 78, 033109 (2008) [2] Klein et al., Phys. Rev. Lett. 101, 046406 (2008) |
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