Electron-phonon properties of pnictide superconductors

In this paper we discuss the normal and superconducting state properties of two pnictide superconductors, LaOFeAs and LaONiAs, using Migdal-Eliashberg theory and density functional perturbation theory. For pure LaOFeAs, the calculated electron-phonon coupling constant lambda = 0.21 and logarithmic-averaged frequency omega(tn) = 206 K, give a maximum T-c of 0.8 K, using the standard Migdal-Eliashberg theory. inclusion of multi-band effects increases the T-c only marginally. To reproduce the experimental T-c a 5-6 times larger coupling constant would be needed. Our results indicate that standard electron-phonon coupling is not sufficient to explain superconductivity in the whole family of Fe-As based superconductors. At the same time, the electron-phonon coupling in Ni-As based compounds is much stronger and its normal and superconducting state properties can be well described by standard Migdal-Eliashberg theory. (C) 2009 Elsevier B.V. All rights reserved.