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A Kondo insulator has a bulk insulating state associated with mixing between relatively localized correlated electronic states and itinerant conduction states. A renormalized two-band picture captures a narrow hybridization gap, although an uncorrelated orbital-mixing model does not explain the formation and temperature dependence of the coherent low-energy state. Samarium hexaboride is an example.

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]

Samarium hexaboride is a Kondo insulator whose narrow samarium $4f$-derived states mix with more dispersive conduction states. It motivates effective models of band hybridization between a nearly flat band and a light dispersive band.