s- and d-wave symmetries in nonadiabatic theory of superconductivity

High-T-c superconductors have Fermi energies E-F much smaller than conventional metals comparable to phonon frequencies. In such a situation nonadiabatic effects are important. A. generalization of Eliashberg theory in the nonadiabatic regime has previously been shown to reproduce some anomalous features of the high-T-c superconductors as for istance the enhancement of T-c or the isotopic effects on T-c and m*. In this contribution we address the issue of the symmetry of the gap in the context of nonadiabatic superconductivity. We show that vertex corrections have a momentum structure which favours d-wave superconductivity when forward scattering is predominant. An additional increase of T-c is also found.