High-temperature superconductivity in transition metal oxypnictides: A rare-earth puzzle?

Extensive ab initio LDA and LSDA + U calculations of an electronic structure of newly discovered high-temperature superconducting series ReO(1-x)F(x)FeAs (Re = La, Ce, Pr, Nd, and Sm and the hypothetical case of Re = Y) have been performed. In all cases, almost identical electronic spectrum (both energy dispersions and the densities of states) has been obtained in a rather wide energy interval (about 2 eV) around the Fermi level. This fact is unlikely to be changed by strong correlations. This leads inevitably to the same critical temperature T(c) of a superconducting transition in any theoretical BCS-like mechanism of the Cooper pair formation. The experimentally observed variations of the T(c) for different rare-earth substitutions are either due to the disorder effects or less probably because of possible changes in the spin-fluctuation spectrum of FeAs layers caused by magnetic interactions with rare-earth spins in ReO layers.