Metal-insulator transitions due to self-doping

We investigate the influence of an unoccupied band on the transport properties of a strongly correlated electron system. For that purpose, additional orbitals are coupled to a Hubbard model via hybridization. The filling is one electron per site. Depending on the position of the additional band, both a metal-to-insulator and an insulator-to-metal transition occur with increasing hybridization. The latter transition from a Mott insulator into a metal via ‘‘self-doping’’ was recently proposed to explain the low carrier concentration in ${\mathrm{Yb}}_4$ ${\mathrm{As}}_3$. We suggest a restrictive parameter regime for this transition, making use of exact results in various limits. The predicted absence of the self-doping transition for nested Fermi surfaces is confirmed by means of an unrestricted Hartree-Fock approximation and an exact diagonalization study in one dimension. In the general case metal-insulator phase diagrams are obtained within the slave-boson mean-field and the alloy-analog approximations. © 1996 The American Physical Society.