Effects of isoelectronic Ru substitution at the Fe site on the energy gaps of optimally F-doped SmFeAsO

We studied the effects of isoelectronic Ru substitution at the Fe site on the energy gaps of optimally F-doped SmFeAsO by means of point-contact Andreev-reflection spectroscopy. The results show that the SmFe_(1-x)Ru_(x)AsO_(0.85)F_(0.15) system keeps a multigap character at least up to x =0.50, and that the gap amplitudes D1 and D2 scale almost linearly with the local critical temperature TcA. The gap ratios 2Di/kBTc remain approximately constant only as long as Tc >=30 K, and increase dramatically when Tc decreases further. This trend seems to be common to many Fe-based superconductors, irrespective of their family. Based on first-principle calculations of the bandstructure and of the density of states projected on the different bands, we show that this trend, as well as the Tc dependence of the gaps and the reduction of Tc upon Ru doping, can be explained within an effective three-band Eliashberg model as being due to a suppression of the superfluid density at finite temperature that, in turn, modifies the temperature dependence of the characteristic spin-fluctuation energy.