  This work investigates a d-p Hubbard model by the n-pole approximation in the
hole-doped regime. In particular, the spectral function $A(\omega,\vec{k})$ is
analyzed varying the filling, the local Coulomb interaction and the $d-p$
hybridization. It should be remarked that the original n-pole approximation
(Phys. Rev. 184 (1969) 451) has been improved in order to include adequately
the $\vec{k}$-dependence of the important correlation function $<
\vec{S}_j\cdot\vec{S}_i>$ present in the poles of the Green's functions. It has
been verified that the topology of the Fermi surface (defined by
$A(\omega=0,\vec{k})$) is deeply affected by the doping, the strength of the
Coulomb interaction and also by the hybridization. Particularly, in the
underdoped regime, the spectral function $A(\omega=0,\vec{k})$ presents very
low intensity close to the anti-nodal points $(0,\pm \pi)$ and $(\pm \pi,0)$.
Such a behavior produces an anomalous Fermi surface (pockets) with pseudogaps
in the region of the anti-nodal points. On the other hand, if the $d-p$
hybridization is enhanced sufficiently, such pseudogaps vanish. It is precisely
the correlation function $< \vec{S}_j\cdot\vec{S}_i>$ present in the poles of
the Green's functions which plays the important role in the underdoped
situation. In fact, antiferromagnetic correlations coming from $<
\vec{S}_j\cdot\vec{S}_i>$ strongly modify the quasi-particle band structure.
This is the ultimate source of anomalies in the Fermi surface in the present
approach.
