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We report the first observation of the intrinsic multiple Andreev reflections effect (IMARE) in S-n-S-…-S-arrays (S = superconductor, n = normal metal) formed by "break-junction" technique in GdO(F)FeAs superconductor (T C = 48 ÷ 53 K). We show that superconducting gap peculiarities at dI/dV-spectra sharpen dramatically in the arrays as compared with that in the single-contact spectra; this enables to improve significantly accuracy of the bulk superconducting parameters determination. Using IMARE, we determined the large and the small gap values Δ L = 11.0 ± 1.1 meV and Δ S = 2.6 ± 0.4 meV. The BCS-ratio 2Δ L /k B T C local = 5.0 ÷ 5.9 > 3.52 (T C local is the contact area critical temperature) evidences for a strong electron-boson coupling. The results obtained agree well with our previous data by Andreev spectroscopy for single SnS-contacts.

Using intrinsic multiple Andreev reflections effect (IMARE) spectroscopy, we studied SnS contacts in the layered oxypnictide superconductors Sm1−xThxOFeAs with various thorium doping and critical temperatures TC = 21-54 K. We observe a scaling between both superconducting gaps and TC. The determined BCS-ratio for the large gap 2∆L/kBTC = 5.0-5.7 and its eigen BCS-ratio (in a hypothetical case of zero interband coupling) 2∆L/kBT L C = 4.1-4.6 both exceeding the weakcoupling limit 3.52, and for the small gap 2∆S/kBTC = 1.2-1.6 remain nearly constant within all the TC range studied. The temperature dependences ∆L,S(T ) agree well with a two-band BCS-like Moskalenko and Suhl model. We prove intraband coupling to be stronger than interband coupling, whereas and Coulomb repulsion constants µ * are finite in Sm-based oxypnictides.

We report comprehensive study of physical properties of the binary superconductor compound SnAs. The electronic band structure of SnAs was investigated using both angle-resolved photoemission spectroscopy (ARPES) in a wide binding energy range and density functional theory (DFT) within generalized gradient approximation (GGA). The DFT/GGA calculations were done including spin-orbit coupling for both bulk and (111) slab crystal structures. Comparison of the DFT/GGA band dispersions with ARPES data shows that (111) slab much better describes ARPES data than just bulk bands. Superconducting properties of SnAs were studied experimentally by specific heat, magnetic susceptibility, magnetotransport measurements and Andreev reflection spectroscopy. Temperature dependences of the superconducting gap and of the specific heat were found to be well consistent with those expected for the single band BCS superconductors with an isotropic s-wave order parameter. Despite spin-orbit coupling is present in SnAs, our data shows no signatures of a potential unconventional superconductivity, and the characteristic BCS ratio 2∆/Tc = 3.48 − 3.73 is very close to the BCS value in the weak coupling limit.

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