Slave-boson functional-integral approach to the Hubbard model with orbital degeneracy

A slave-boson functional-integral method has been developed for the Hubbard model with arbitrary, orbital degeneracy D. Its saddle-point mean-field theory is equivalent to the Gutzwiller approximation, as in the case of single-band Hubbard model. Our theory is applied to the doubly degenerate (D = 2) model, whose paramagnetic state has been studied by numerical calculations. The effect of the exchange interaction on the metal-insulator (MI) transition is discussed. The critical interaction for the MI transition is analytically calculated as functions of orbital degeneracy and electron occupancy.