METAL-INSULATOR-TRANSITION IN AN ONE-DIMENSIONAL 2-BAND HUBBARD-LIKE MODEL

We consider a one-dimensional two-band model of electrons on a lattice with equal nearest-neighbour hopping, an interband splitting Delta an d a Hubbard-like repulsion U. The model is defined via the SU(4) gener alization of Lieb and Wu's Bethe ansatz solution of the one-dimensiona l Hubbard model. At T = 0 the model has a Mott metal-insulator transit ion at a critical value U-c(Delta) for a band filling of exactly one e lectron per site. U-c decreases with Delta, being zero if the excited- electron band is empty, and U-c = 2.981 when the bands are degenerate. We discuss the ground-state properties, the spectrum of elemental exc itations, the specific heat and the magnetic susceptibility, in both t he metallic and insulating phases, as a function of the crystal-field splitting for exactly one electron per site. There are four branches o f elemental excitations: (i) charge excitations, (ii) crystalline-fiel d excitations and (iii) two branches of spin waves. The Fermi velocity is finite in the metallic phase, diverges as the metal-insulator tran sition is approached from the metallic side and vanishes for the insul ator. Each band contributes to the susceptibility with a term that is inversely proportional to the spin-wave velocity for that band. The lo w-temperature specific heat is proportional to T and to the sum of the inverses of the velocities of the four branches.