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 λ=0.21 and logarithmic-averaged frequency ω$_{ln}$ = 206 K, give a maximum Tc of 0.8 K, using the standard Migdal–Eliashberg theory. Inclusion of multi-band effects increases the Tc only marginally. To reproduce the experimental Tc, 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.