Large and high quality single crystals of the new unconventional superconductor LiFeAs were grown by a new approach using the self-flux technique. Both energy dispersive X-ray spectroscopy and inductively coupled plasma mass spectroscopy revealed a stoichiometric Li/Fe/As composition. Measurements of the magnetic susceptibility reveal the superconducting transition at T C = 17 K with a very sharp ΔT C and a 100% shielding fraction and, thus, bulk superconductivity. This sharp transition is also found by measurements of the specific heat and by measurements of the temperature dependence of the resistivity. Nuclear quadrupole resonance (NQR) spectroscopy reveals a very sharp resonance line, with a much smaller line width than reported for all other FeAs superconductors, confirming the high ordering of the LiFeAs single crystals also on a local scale.

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The effect of palladium-and ruthenium-based clusters on nanocrystalline tin dioxide interaction with oxygen was studied by temperature-programmed oxygen isotopic exchange with mass-spectrometry detection. The modification of aqueous sol−gel prepared SnO 2 by palladium and, to a larger extent, by ruthenium, increases surface oxygen concentration on the materials. The revealed effects on oxygen exchangelowering the threshold temperature, separation of surface oxygen contribution to the process, increase of heteroexchange rate and oxygen diffusion coefficient, decrease of activation energies of exchange and diffusionwere more intensive for Ru-modified SnO 2 than in the case of SnO 2 /Pd. The superior promoting activity of ruthenium on tin dioxide interaction with oxygen was interpreted by favoring the dissociative O 2 adsorption and increasing the oxygen mobility, taking into account the structure and chemical composition of the modifier clusters.

The superconducting state of LiFeAs single crystals with the maximum critical temperature T c ≈ 17 K in the 111 family has been studied in detail by multiple Andreev reflections (MAR) spectroscopy implemented by the breakjunction technique. The three superconducting gaps, ∆ Γ = 5.1 -6.5 meV, ∆ L = 3.8-4.8 meV, and ∆ S = 0.9 -1.9 meV (at T << T c ), as well as their temperature dependences, have been directly determined in a tunneling experiment with these samples. The anisotropy degrees of the order parameters in the k space have been estimated as <8, ~12, and ~20%, respectively. Andreev spectra have been fitted within the extended Kummel-Gunsenheimer-Nikolsky model with allowance for anisotropy. The relative electron-boson coupling constants in LiFeAs have been determined by approximating the ∆(T) dependences by the system of the two-band Moskalenko and Suhl equations. It has been shown that the densities of states in bands forming ∆ Γ and ∆ L are approximately the same, intraband pairing dominates in this case, and the interband coupling constants are related as λ ΓL ≈ λ LΓ << λ SΓ , λ SL . Iron-containing superconductors [1] have been studied for almost five years. However, interest in these compounds remains. The main unanswered question is: What is the mechanism of the formation of Cooper pairs responsible for such high critical temperatures in superconductors with magnetic atoms in the structure? Although the iron isotopic effect was observed experimentally [2] with the coefficient α ≈ 0.4 < 0.5, the strong electron-phonon interaction [3] does not describe the observed T c values [4]. In view of the closeness of the main antiferromagnetic state with the spin density wave and wave vector Q AFM = (π/a, π/a) [5] and the congruence of the Fermi surfaces in the Γ-M direction (the so-called nesting on the Q Γ-M ≡ Q AFM vector) [6], the dynamic magnetic susceptibility has a peak (so-called magnetic resonance) at the energy E res ≈ 1.5∆ L according to calculations reported in [7], and the ratio E res /k B T c ≈ 5.5 according to the spectroscopic data on the inelastic scattering on neutrons presented in [8]. The facts listed above initiate the theoretical idea that spin fluctuations play a significant role in the