Consistent 1/N-expansion for the four-slave-boson approach with an appropriate operator-ordering prescription

Abstract

We study the single-band Hubbard model with arbitrary on-site repulsion U within the four-slave-boson approach. We evaluate 1/N corrections to the mean-field result by properly taking into account the additional terms due to the discretized form of the coherent-state functional integral. We show how the normal-ordering prescription for the bosonic hopping operator Z_i,σ introduced by Kotliar and Ruckenstein can be suitably relaxed leading to additional contributions of$\mathcal{O}$(1/N) to physical quantities. The free-particle result at U = 0 is then recovered for arbitrary doping even with the inclusion of fluctuations. Numerical results are shown for finite values of U.