 Solution

A concrete example is 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.^[ref]