We theoretically investigate the effect of various symmetries of the superconducting order parameter Delta(omega) on the normalized tunneling conductance of SIN functions (with no spatial variation of the order parameter in the S electrode) by directly solving the real-axis Eliashberg equations (EEs) for a half-filled infinite band, with the simplifying assumption mu* = 0. We analyze six different symmetries of the order parameter. s, d, s + id, s + d, extended s and anisotropic s, by assuming that the spectral function alpha(2)F(Omega) contains an isotropic part alpha(2)F(Omega)(is) and an anisotropic one, alpha(2)F(Omega)(an), such that alpha(2)F(Omega)(an) = galpha(2)F(Omega)(is), where g is a constant. We compare the resulting conductance curves at T = 2 K to those obtained by analytical continuation of the imaginary-axis solution of the EEs, and we show that the agreement is not equally good for all symmetries. Then, we discuss the effect of non-magnetic impurities on the theoretical tunneling conductance curves at T = 4 K for all the symmetries considered Finally, as an example, we apply our calculations to the case of optimally-doped high-T-c superconductors. Surprisingly, although the possibility of explaining the very complex phenomenology of HTSC is probably beyond the limits of the Eliashberg theory, the comparison of the theoretical curves calculated at T = 4 K with the experimental ones obtained in various optimally-doped copper-oxides gives fairly good results. (C) 2002 Elsevier Science B.V. All rights reserved.
