Composite operator candidates for a study of the p-d model

We study the two-dimensional p-d model by means of a four-pole approximation within the Composite Operator Method framework. Many results of numerical simulations have been correctly reproduced by considering as basic composite field a four-component spinor field, including the p field, the two Hubbard operators for the d field, and a composite operator describing the p field dressed by the spin excitations of the d field. In this manuscript, we propose an alternative choice for the latter field, which takes into account charge and pair fluctuations too. The results of this study show that near half-filling one can safely neglect the contribution of charge and pair fluctuations and safely use the previously proposed field.