A Generalized Antiferromagnetic Model for Bonding Electrons

The antiferromagnetic (AF) density matrix is generalized for bonding electrons. The covalent-correlation operators are constructed based on the energy contribution of the two AF parts in the covalent component, and the density-fluctuation operators are introduced to include the charge variation resulting from the coexistence of the ionic and covalent components. Under the adjacent hopping, the effective mass tends to infinity in the covalent limit just as that in the Mott insulator. On the other hand, the effective mass becomes finite when the ionic component coexists with the covalent one. By extending the density matrix, a family of random-hopping Hamiltonians can be taken into account for the disorder-induced insulating behaviors under such coexistence. Therefore, our study reveals a unified way to understand different insulating behaviors.