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

TL;DR

This study investigates how isoelectronic Ru substitution affects the energy gaps in F-doped SmFeAsO superconductors, revealing a multigap structure that scales with critical temperature and is explained by a three-band Eliashberg model.

Contribution

It provides the first detailed analysis of Ru substitution effects on energy gaps in SmFeAsO using point-contact spectroscopy and theoretical modeling.

Findings

Multigap character persists up to x=0.50 Ru doping.

Gap amplitudes scale with local critical temperature.

Gap ratios increase as Tc decreases below 30 K.

Abstract

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 Delta_1 and Delta_2 scale almost linearly with the local critical temperature TcA. The gap ratios 2Delta_i/(kB Tc) remain approximately constant only as long as Tc > 30 K, but 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 turns, modifies the temperature dependence of the characteristic spin-fluctuation energy.