ИСТИНА

We studied a reproducible fine structure observed in dynamic conductance spectra of high transparent Andreev arrays in Sm(1-x)Th(x)OFeAs superconductors with various thorium concentrations (x = 0.08-0.3) and critical temperatures Tc = 26-50 K. This structure is unambiguously caused by those current carriers, excited during the process of multiple Andreev reflections (MAR) in superconductor – normal metal – superconductor (SnS) ballistic contact, which released their excess energy (up to 2Δ_L) through the sequential boson emission of one and the same energy [1]. The determined energy of the bosonic mode reaches E_0 ≈ 15meV for optimal compounds with Tc = 50K and resembles that determined by us earlier for GdO(1-x)F(x)FeAs with similar Tc [2]. One cannot attribute the observed bosonic resonance with the Leggett mode or optic phonon mode. Within the studied range of Tc, this energy as well as the large Δ_L and the small Δ_S superconducting gaps, nearly scales with critical temperature [3], having the characteristic ratio E_0/kTc ≈ 3.2 (while 2Δ_L/kTc ≈ 5.3) and resembles the expected energy (Δ_L + Δ_S) of spin exciton and spectral density enhancement in s± and s++ states, respectively. We thank V. M. Pudalov and H. Kontani for fruitful discussions. The work was supported by RFBR grant 17-02-00805-a. [1] S.A. Kuzmichev, T.E. Kuzmicheva, N.D. Zhigadlo, Europhysics Letters 119, 17007 (2017) [2] S.A. Kuzmichev, T.E. Kuzmicheva, JETP Letters 105, 671 (2017) [3] T.E. Kuzmicheva, S.A. Kuzmichev, K.S. Pervakov, V.M. Pudalov, and N.D. Zhigadlo, Phys. Rev. B 95, 094507 (2017)