DETERMINATION OF THE TUNNELING ELECTRON-PHONON SPECTRAL-FUNCTION IN HIGH-T-C SUPERCONDUCTORS WITH ENERGY-DEPENDENCE OF THE NORMAL DENSITY-OF-STATES

In the present paper we discuss the limits and the correct utilization of the standard program for the inversion of the Eliashberg equations , and the determination of the electron-phonon spectral function and t he Coulomb pseudopotential from tunneling measurements in high-T-c sup erconductors. In order to compare the calculated density of states wit h the experimental one, we introduce the results of the inversion proc edure, applied to our recent tunneling data in Bi2Sr2CaCu2O8+x single- crystal break junctions with T-c = 93 K, in a direct program for the s olution of the Eliashberg equations. Most of the observed differences between theoretical and experimental curves at energy greater than the gap can be explained by a smooth energy dependence of the normal dens ity of states that we introduced in the direct solution of the Eliashb erg equations. Finally we show that the effects of the energy-dependen t normal density of states can be simulated by an efficient electron-p honon spectral function but, also, by a negative, nonphysical, Coulomb pseudopotential.