Two-band hopping model is treated on a two-dimensional square lattice. The atoms are located at the corners and the middles of the edges of the squares. In addition to the strongly overlapping orbitals of the atoms, there are extra orbitals at the corners, which are weakly hybridized. The assumption is made that the Fermi level is inside the broad band and is very near to the narrow band formed by the extra orbitals. The Hamiltonian is Hubbard type, but the off-diagonal part of the two-site interaction is kept also where one creation or annihilation operator acts on the extra orbital and the others on one of its neighbors. The weak coupling is enhanced by the on-site Coulomb repulsion at the corners, which enhancement is a power function of the ratio of the broad band width and the narrow band position measured from the Fermi level. That enhancement is obtained by summation of logarithmic Kondo-type corrections of orbital origin, which reflects the formation of a ground state of new type with strong orbital and spin correlations. Interaction between the particles of the broad band is generated by processes with one heavy and one light particle in the intermediate state. That interaction strongly depends on the momenta of the particles. The generated interaction can result in attractive interaction, but particular parity of the different orbitals are required in the given approximation. The model clearly demonstrates that repulsive interaction can result in superconductivity.
ASJC Scopus subject areas
- Atomic and Molecular Physics, and Optics
- Mathematical Physics
- Condensed Matter Physics