Unrestricted slave-boson mean-field approximation for the   two-dimensional Hubbard model
  The Kotliar-Ruckenstein slave-boson scheme is used to allow for an unrestricted variation of the bosonic and fermionic fields on the saddle-point level. Various inhomogeneous solutions, such as spin polarons and domain walls are discussed within the two-dimensional Hubbard model and compared with results of unrestricted Hartree-Fock (HF) calculations. We find that the present approach drastically reduces the polarization of these states and leads to increased delocalized wave functions as compared to the HF model. The interaction between two spin polarons turns out to be attractive over a wide range of the on-site repulsion U. In addition we obtain the crossover from vertical to diagonal domain walls at a higher value of U than predicted by HF. 
G. Seibold, E. Sigmund, and V. Hizhnyakov