= Solution
A concrete example is \b[<samarium hexaboride>], $\mathrm{SmB}_6$: narrow, nearly localized $4f$-derived states mix with a much more dispersive $5d$-derived <conduction band>. Such <band hybridization> is the low-energy mechanism behind a bulk <Kondo insulator> gap. The two-band <Hamiltonian> is an effective illustration; correlations help set its narrow-band energy and renormalized mixing.https://www.nature.com/articles/ncomms3991[Introduction: nearly localized flat f bands hybridize with a dispersive d-derived conduction band in SmB6]{ref}
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