Effect of crystal-field splitting and interband hybridization on the metal-insulator transitions of strongly correlated systems

(Dated: July 3, 2008)We investigate a quarter-filled two-band Hubbard model involving a crystal-field splitting, whichlifts the orbital degeneracy as well as an inter-orbital hopping (inter-band hybridization). Both termsare relevant to the realistic description of correlated materials such as transition-metal oxides. Thenature of the Mott metal-insulator transition is clarified and is found to depend on the magnitudeof the crystal-field splitting. At large values of the splitting, a transition from a two-band to a one-band metal is first found as the on-site repulsion is increased and is followed by a Mott transitionfor the remaining band, which follows the single-band (Brinkman-Rice) scenario well documentedpreviously within dynamical mean-field theory. At small values of the crystal-field splitting, a directtransition from a two-band metal to a Mott insulator with partial orbital polarization is found,which takes place simultaneously for both orbitals. This transition is characterized by a vanishingof the quasiparticle weight for the majority orbital but has a first-order character for the minorityorbital. It is pointed out that finite-temperature effects may easily turn the metallic regime into abad metal close to the orbital polarization transition in the metallic phase.