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 clain that this fact is unlikely to be changed by the account of strong correlations. It leads inevitably to the same critical temperature $T_c$ of superconducting transition in any theoretical BCS-like mechanism of Cooper pair formation. We argue that the experimentally observed variations of $T_c$ 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.