High Temperature Superconductivity in Transition Metal Oxypnictides: a   Rare-Earth Puzzle?
  We have performed an extensive ab initio LDA and LSDA+U calculations of electronic structure of newly discovered high-temperature superconducting series ReO(1-x)F(x)FeAs (Re=La,Ce, Pr, Nd, Sm and hypothetical case of Re=Y). In all cases we obtain almost identical electronic spectrum (both energy dispersions and the densities of states) in rather wide energy interval (about 2 eV) around the Fermi level. We also debate that this fact is unlikely to be changed by the account of strong correlations. It leads inevitably to the same critical temperature Tc of superconducting transition in any theoretical BCS-like mechanism of Cooper pair formation. We argue that the experimentally observed variations of Tc for different rare-earth substitutions are either due to disorder effects or less probably because of possible changes in spin-fluctuation spectrum of FeAs layers caused by magnetic interactions with rare-earth spins in ReO layers. 
I.A. Nekrasov, Z.V. Pchelkina, M.V. Sadovskii