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 SmFe1-xRuxAsO0.85F0.15 system keeps a multigap character at least up to x = 0.50, and that the gap amplitudes Delta(1) and Delta(2) scale almost linearly with the local critical temperature T-c(A). The gap ratios 2 Delta(i)/k(B)T(c) remain approximately constant only as long as T-c >= 30 K, and increase dramatically when T-c 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 T-c dependence of the gaps and the reduction of T-c 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.