Gapped electron state in the symmetric Anderson lattice, Kondo insulator state

The Kondo insulator state (KIS) realized in the symmetric Anderson model at half filling is studied in the framework of a mean field approach. It is shown that a Kondo insulator state with doubled lattice period is stabilised, where the gapped electron liquid behaves like a gapless Majorana spin liquid. Local moments of the d-electrons form a static Z₂-field in which the band electrons move. The gap  in the quasi-particle excitation spectrum decreases with increasing  external magnetic field and closes at its critical value. The behavior of the electron liquid is studied for an arbitrary dimensionality of the model. The proposed approach leads to a description of KIS without the need to resort to any artificial symmetry breaking.