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<img src='content_image/348157.jpg'>

π between the gaps at zone-centers and the gaps at zone corners. It appears possible that for appropriate band-structure AFM fluctuations may even promote triplet pairing. This may be implicated in the triplet superconductivity in Sr$_{2}$RuO$_{4}$ [20], which shows no FM correlations but modest AFM correlations. 

One can ph ysically understand for the simple square lattice band-structure why a steep increase of J ( q ) near the zone-boundary favors d-wave. The wave-function for this interac- tion favors anti-parallel correlations of spins on nearest neighbors. Such a spin-correlation is also produced by the d − wave BCS wave-function, for such a band-sturcture. 

A generalization of magnetic fluctuations due to spin-moment correlations to magnetic fluctuations due to orbital-moment correlations will be discussed in connection with the physics of the cuprates in Sec. (IV). 

While the arguments above may provide valid grounds for discussing pairing symmetry, they are no help in thinking about the value of T$_{c}$ . For that one must turn, as we do next, to the frequency dependence of the pairing vertex, which in most cases of interest is given by the frequency dependence of the collective modes exchanged by the fermions. 

