Kondo insulator

In solid-state physics, Kondo insulators (also referred as Kondo semiconductors and heavy fermion semiconductors) are understood as materials with strongly correlated electrons, that open up a narrow band gap (in the order of 10 meV) at low temperatures with the chemical potential lying in the gap, whereas in heavy fermion materials the chemical potential is located in the conduction band.

[1] In 1969, Menth et al. found no magnetic ordering in SmB6 down to 0.35 K and a change from metallic to insulating behavior in the resistivity measurement with decreasing temperature.

At low temperatures, the local magnetic moments are screened by the sea of conduction electrons, forming a so-called Kondo resonance.

If the chemical potential lies in the hybridization gap, an insulating behavior can be seen in the dc-resistivity at low temperatures.

In recent times, angle-resolved photoemission spectroscopy experiments provided direct imaging of band-structure, hybridization and flat band topology in Kondo insulators and related compounds.

Dispersion relation of conduction band and localized states.
Hybridization and forming of an indirect energy (hybridization) gap due to the coherent Kondo screening of the local moments by the sea of conduction electrons.
In case of Kondo insulators the Fermi level (chemical potential) is located in the hybridization gap.