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Using multiple Andreev reflections (MAR) effect spectroscopy of SnS junctions, we directly determine temperature dependences of the superconducting order parameters, excess Andreev current, and zero-bias conductance (ZBC) in polycrystalline samples of superconducting oxypnictides Sm0.7Th0.3OFeAs and NdO0.6H0.36FeA with almost optimal critical temperatures. It is shown that the results obtained are self-consistent and could be described in the framework of a two-band model. We estimate a dominating contribution 70–85% of the bands with the large gap to the total conductance.

We studied SNS and S-N-S-N-...-S contacts (where S is a superconductor and N is a normal metal) formed by “break-junction” technique in polycrystalline Sm1 − x Th x OFeAs superconductor samples with critical temperatures T C = 34–45 K. In such contacts (intrinsic) multiple Andreev reflections effects were observed. Using spectroscopies based on these effects, we detected two independent bulk order parameters and determined their magnitudes. Theoretical analysis of the large and the small gap temperature dependences revealed superconducting properties of Sm1 − x Th x OFeAs to be driven by intraband coupling, and $$\sqrt {V_{11} V_{22} } /V_{12} \approx 14$$ (where V ij are the electron-boson interaction matrix elements), whereas the ratio between density of states for the bands with the small and the large gap, N 2/N 1, correspondingly, was roughly of an order. We estimated “solo” BCS-ratio values in a hypothetic case of zero interband coupling (V i ≠ j = 0) for each condensate as 2ΔL, S/k B T C L,S ≤ 4.5. The values are constant within the range of critical temperatures studied, and correspond to a case of strong intraband electron-phonon coupling.

The vortex structure in the iron pnictide single crystals Ba 1– x K x Fe 2 As 2 and Sr 1– x K x Fe 2 As 2 of the 122 type and SmFeAsO 1– xFx of the 1111 type has been investigated using the decoration method. In all of the crystals under investigation, no regular vortex lattice has been revealed in the magnetic field range up to 200 Oe. The disordered vortex structure is discussed in view of the vortex pinning in single crystals.

The influence of precursors on the single-crystal growth of BaCrO4 in the controlled reactions of BaCl2 with K2Cr2O7, (NH4)2Cr2O7, and K2CrO4 in aqueous solutions is studied. The conditions of the reaction of BaCl2 with K2Cr2O7 are found to be most favorable for single-crystal growth. A growth texture is observed in the (011) and (100) planes in the samples prepared by the reactions with K2Cr2O7 and K2CrO4, respectively. A more complex type of growth preference was observed in the reaction with (NH4)2Cr2O7.

An investigation was made of how the partial replacement of oxygen with fluorine and of strontium with potassium influences the formation and properties of the superconducting phases in a (Bi,Pb)-Sr-Ca-Cu-O system. It was established that simultaneous anion-cation substitutions have a positive influence, speeding up the formation of the 2223 high-temperature phase.

The influence of synthesis temperature and time on the properties and formation of superconducting phases in Bi1.6Pb0.4Sr1.98K0.02Ca2Cu3F0.8Oy samples has been studied by x-ray phase analysis and by measuring the electrical resistance. It is found that the ratio of the 2223 and 2212 phases formed during synthesis at 845 °C for 240 h remains almost constant in the range 845–855 °C, regardless of the additional synthesis time. Synthesis at higher temperatures leads to breakdown of the 2223 phase and enhances the content of the 2212 phase and impurity phases. The highest values Tc(R=0)=112.8 K were obtained for samples synthesized at 845 °C for 240 h.