The ground state of the Kondo insulator

The Kondo insulator (KI) is the state of an electron liquid in the Kondo lattice at half filling, studied within the mean field approach. We demonstrate, that the Z 2 -field, which is formed by interaction between electrons and local moments, leads to an insulator state in a lattice with a double cell lattice. In the ground state, electrons and local moments form singlets; in this case, no spin or charge density waves are realized in a lattice with a double cell. The Majorana-type gap spectrum of the quasi-particle excitations is realized in the Z 2 -field. The gap in the spectrum decreases with increasing external magnetic field; it closes at a critical value at the insulator-metal phase transition point. In metal phase the lattice remains with a double cell. Thus, the introduction of the Z 2 -field allows us to answer the key question what is the ground state of KI.