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Superconductivity in the Attractive Hubbard Model: The Double Hubbard--I Approximation

Using the Dyson equation of motion for both the diagonal one-particle Green function, $G(\vec{k},ω)$ and off--diagonal Green function, $F(\vec{k},ω)$, at the level of the Hubbard--I decoupling scheme, we have found that they have four poles symmetric in pairs, justifying a more elaborated calculation done by the Zürich group by means of the $T$-Matrix approach (Pedersen et al, Z. Physik B {\bf 103}, 21 (1997)) and the moment approach of Nolting (Z. Physik {\bf 255}, 25 (1972)). We find that the energy spectra and the weights of $G(\vec{k},ω)$ and $F(\vec{k},ω)$ have to be calculated self-consistently. $G(\vec{k},ω)$ satisfies the first two moments while $F(\vec{k},ω)$ the first sum rule. Our {\it order parameter} $α(T)$ is given by $1/N_s \sum_{\vec{k}} ε({\vec{k}})Δ({\vec{k}})$. Due to the fact that we have a purely local attractive interaction $Δ(\vec{k})$ can be of {\it any} s--type wave. However, for a {\it pure s--wave}, for which $α(T) = 0$, we go back to the mean--field $BCS$ results, with a renormalized chemical potential. In this case, the off--diagonal Green function, $F(\vec{k},ω)$, satisfies the first two off--diagonal sum rules. We explicitly state the range of validity of our approximation.