BEYOND THE GUTZWILLER APPROXIMATION IN THE SLAVE-BOSON APPROACH - INCLUSION OF FLUCTUATIONS WITH THE CORRECT CONTINUUM-LIMIT OF THE FUNCTIONAL INTEGRAL

We study the single-band Hubbard model via the functional-integral four-slave-boson formulation introduced by Kotliar and Ruckenstein, and show that a proper treatment of the continuum limit of the functional integral removes the inconsistencies due to an incorrect handling of this limit so far performed in the literature. It follows from our analysis that the Kotliar and Ruckenstein mean-field solution has to be abandoned in order to include fluctuation corrections in a systematic way. We show explicitly how to construct a suitable alternative solution which admits the inclusion of fluctuations.