Kondo insulating state

The Kondo insulating state occurs in certain materials where strongly interacting electrons cause the material to behave as an insulator at low temperatures. In these materials, magnetic atoms with localized electrons create a strong interaction with conduction electrons. This interaction leads to a many-body quantum effect called the Kondo effect, where conduction electrons effectively "screen" the magnetic moments, opening an energy gap. This gap prevents electrons from moving freely, making the material an insulator despite its metallic composition at higher temperatures. Essentially, the complex interplay of electron interactions transforms the material into an insulator through a collective quantum phenomenon.