MODELS OF CORRELATED FERMIONS IN THE SLAVE BOSON APPROXIMATION FOR THE COPPER-OXIDE SYSTEMS

In this short communication we discuss results obtained for the single and multi-orbital Hubbard models within the mean-field-slave-boson description of correlated fermions on a square lattice. For appropriate paramete values the results can be used to mimic some of the normal state properties of the copper oxide systems. Results for the single and multi-orbital models differ in a very characteristic manner. In the single band case the phase diagram consists of metallic (U < U(c)) and strongly correlated (U > U(c)) regions, which are clearly seperated at half filling (U(c) being the critical value of the Brinkman-Rice transition1). Upon doping the transition between these two phases smoothes out and for large enough doping (delta > 0.2, delta = 1 - n) the system behaves as a weakly correlated metal. In the multi-orbital case depending on the input energies for the orbitals the transition to the insulating state can be either of the Brinkman-Rice or the charge-transfer type. Similar to the single band case doping causes rapid loss of correlation effects. We tentatively conclude that the Kondo-type of effect is absent in the multi-orbital model.