Determination of the tunneling electron-phonon spectral function in High- Tc superconductors with energy dependence of the normal density of states

Abstract

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 the Coulomb pseudopotential from tunneling measurements in high- T c superconductors. In order to compare the calculation density of states with the experimental one, we introduce the results of the inversion procedure, applied to our recent tunneling data in Bi 2Sr 2CaCu 2O 8+ x single-crystal break junctions with T c = 93 K, in a direct program for the solution 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 density of states that we introduced in the direct solution of the Eliashberg equations. Finally we show that the effects of the energy-dependent normal density of states can be simulated by an efficient electron-phonon spectral function but, also, by a negative, nonphysical, Coulomb pseudopotential.