SPIN-ROTATION AND CHARGE-ROTATION INVARIANT APPROACH TO THE HUBBARD-MODEL

We present a slave-boson formulation:for the Hubbard model that preser ves the spin-rotation and particle-hole symmetries of the paramagnetic phase at half filling. Fermion and boson fields are treated on an equ al footing in a functional integral formulation. We show that, while i n two and three dimensions there is a metal-insulator transition as th e interaction U increases, in one dimension our approach always gives an insulating state. Short-range spin-spin correlations are included a t the saddle-point level for all values of U, giving a good descriptio n of the insulating state. We show that the energy gap in the one-part icle spectrum increases from zero as (U-U-c)(a), where U-c is the crit ical value of the on-site interaction and the exponent ct is 1 and 1/2 for 2 and 3 dimensions, respectively.