Anomalous properties of the normal state of a strongly correlated electron system described by an attractive extended Hubbard model are investigated. The equations of motion of the Green's functions are calculated with the two-pole approximation which gives rise to quasiparticle renormalized bands. The two-pole approximation leads to a set of correlation functions. In particular, the antiferromagnetic correlation function S i • S j plays an important role as a source of anomalies in the normal state of the model. The uniform static magnetic susceptibility as a function of occupation n T and temperature is calculated. At low temperatures, the susceptibility presents a peak for n T 0.80. The results suggest that it is the onset of short-range antiferromagnetic correlations, which could be a mechanism for the pseudogap. The Fermi surface, defined by the spectral function A(ω = 0, k), is presented for different dopings. It has been observed that above n T 0.80 the ordinary Fermi surface evolves to a hole pocket with pseudogaps near the antinodal points (0, π ) and (π, 0).

We present a discussion where the choice of the regularization procedure and the routing for the internal lines momenta are put at the same level of arbitrariness in the analysis of Ward identities involving simple and well-known problems in quantum field theory. They are the complex self-interacting scalar field and two simple models where the scalar-vector-vector and axial-vector-vector process are pertinent. We show that, in all these problems, the conditions to symmetry relations preservation are put in terms of the same combination of divergent Feynman integrals, which are evaluated in the context of a very general calculational strategy, concerning the manipulations and calculations involving divergences. Within the adopted strategy, all the arbitrariness intrinsic to the problem are still maintained in the final results and, consequently, a perfect map can be obtained with the corresponding results of traditional regularization techniques. We show that, when we require an universal interpretation for the arbitrariness involved, in order to get consistency with all stated physical constraints, a strong condition is imposed for regularizations which automatically eliminates the ambiguities associated to the routing of the internal lines momenta of loops. The conclusion is clean and sound: the association between ambiguities and unavoidable symmetry violations in Ward identities cannot be maintained if an unique prescription is required for identical situations in the evaluation of divergent physical amplitudes.

The specific heat of an attractive (interaction $G<0$) non-local Hubbard model is investigated. We use a two-pole approximation which leads to a set of correlation functions. In particular, the correlation function $\ <\vec{S}_i\cdot\vec{S}_j\ >$ plays an important role as a source of anomalies in the normal state of the model. Our results show that for a giving range of $G$ and $\delta$ where $\delta=1-n_T$ ($n_T=n_{\uparrow}+n_{\downarrow}$), the specific heat as a function of the temperature presents a two peak structure. Nevertehelesss, the presence of a pseudogap on the anti-nodal points $(0,\pm\pi)$ and $(\pm\pi,0)$ eliminates the two peak structure, the low temperature peak remaining. The effects of the second nearest neighbor hopping on the specific heat are also investigated.Comment: 5 pages, 7 figure