ИСТИНА

We have studied superconducting (SC) oxypnictides Gd(O,F)FeAs and (Sm,Th)OFeAs (the 1111 family) of optimal and underdoped compositions and Tc ≈ 25–54 K. In order to study the SC energy parameters we used incoherent multiple Andreev reflection effect (IMARE) spectroscopy of SC–thin normal metal–SC (SnS) nanojunctions. SnS junctions were formed by a planar “break-junction” technique. We reproducibly observed two distinct SGS’s and directly determined two SC gaps in the 1111 compounds, with characteristic ratios 2Δ_L(0)/kTc ≈ 5.2, and 2Δ_S(0)/kTc ≈ 1.5, those remained almost constant within the wide range Tc ≈ 25–54 K. Beyond to the parent SGS of the large SC gap, the dI(V)/dV spectra of SnS junctions showed a satellite fine structure below Tc caused by an emission of bosons (existing in the SC state only) with characteristic energy \epsilon_0 by electrons during IMARE. … In the spectra of the most qualitative junctions we observed a multiple bosonic resonance. For Gd(O,F)FeAs and (Sm,Th)OFeAs with Tc ≈ 50 K the boson energy \epsilon_0 ≈ 10–13 meV. Under doping variation, it was scaled with Tc together with Δ_L(0) and Δ_S(0) [6]. At T << Tc, \epsilon_0 does not exceed the indirect SC gap Δ_L(0) + Δ_S(0). We directly measured the temperature dependence of the boson energy. \epsilon_0(T) weakly decreases with temperature and does not resemble the [Δ_L(0) + Δ_S(0)] temperature trend. The observed bosonic mode could be a spin exciton, which indicates an important role of spin fluctuations in the Cooper pairing in the SC oxypnictides. This research was funded by the Ministry of Higher Education and Science of the Russian Federation, project number 075-15-2021-1353.