Point-contact Andreev-reflection spectroscopy in Fe-based superconductors: multi-gap superconductivity and strong electron-boson interaction

We performed point-contact Andreev-reflection measurements in Ba(Fe1−xCox )2As2 single crystals (x = 0.1, Tc = 24.5 K) and SmFeAsO1−xFx polycrystals (x = 0.2, Tc = 52 K). The spectra indicate the presence of two superconducting gaps with no line nodes on the Fermi surface, but also feature additional structures related to the electron–boson interaction (EBI). From the spectra, it is possible to extract the characteristic energy Ω0 of the mediating boson. In Co-doped Ba-122, we obtain Ω0 = 12 meV that coincides with the spin-resonance energy observed in neutron-scattering experiments. In Sm-1111, Ω0 = 20 meV fulfils the relation Ω0 = 4.65kBTc inferred from neutronscattering results on other Fe-based superconductors. The strong electron–boson coupling may also explain some anomalies in the PCAR conductance curves (e.g., the excess conductance at high energy) which sometimes prevent a good fit of the curves with models based on constant, BCS-like order parameters.