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.

IntroductionUnexpected discovery in 2006 [1] of the first layered Fe-based high-temperature superconductor (HTSC) -LnOFePn (where Ln -lanthanide, Pn -pnictide; hereafter referred to as "1111") becomes a key issue of modern solid state physics. Since 2008, the class of iron-based superconductors has much expanded: several families of iron pnictides and chalcogenides have been synthesized [2][3][4]. The crystal structure of oxypnictides recalls that of cuprates and is in fact a stack of the superconducting Fe-As layers alternating along the c-direction with spacers, the nonsuperconducting oxide blocks, Ln-O. In spite of the pronounced layered structure and anisotropic physical properties, the electron subsystem in Fe-based superconductors is less quasi-two-dimensional in comparison with that in cuprate HTSC, because the height of the Fe-As blocks exceeds the thickness of the Cu-O planes, whereas the distance between superconducting blocks in iron-based superconductors is significantly less than that in cuprates. The latter seems to be a reason [5] why the critical temperature of Fe-based superconductors, though being as high as T C ≈ 57.5 K [6] still does not reach the cuprate one. Superconductivity in novel materials emerges with the suppression of spin density wave ground state under doping of the superconducting Fe-As layers or under external pressure [7]. The key distinction from cuprates, however, is the multiband nature of newly-discovered superconductivity in iron-based materials. Band structure calculations showed (for a review, see [8]) the coexistence of the electron and the hole quasi-two-dimensional bands in these compounds, whereas the Fermi surfaces consist of slightly warped along the c-direction cylinders (near Γ and M points), where several superconducting condensates could form. Contrary to the observation of the strong isotope effect [9], an early theoretical study showed [10] that the high temperature superconductivity in Fe-based compounds could not to be based on the electron-phonon interaction solely. Although the latter plays an important role, it is incapable [10] to explain the observable values of T C in the framework of the Eliashberg theory [11]. Taking into account the nesting of the Fermi surface sheets along the Γ-M-direction, the vicinity of the antiferromagnetic state [12,13], and the appearance of the experimentally observed [14] peak of dynamic spin susceptibility ("magnetic resonance"), Mazin et al. suggested the theoretical description of the mechanism of superconductivity in iron pnictides and chalcogenides based on sign-changing (in different bands) order parameter -the so-called s ± -model. The simplest initial model considers two isotropic order parameters (in the electron and the hole bands, correspondingly), which have equal amplitudes but are in antiphase (i.e. having opposite signs). The strong interband interaction mediated by spin fluctuations along the Γ-M-direction plays the key role in this model, whereas the intraband electron-phonon coupling is an order of magnit...

We have studied current-voltage characteristics of Andreev contacts in polycrystalline GdO0.88F0.12FeAs samples with bulk critical temperature Tc = (52.5 ± 1) K using break-junctions technique. The data obtained cannot be described within the single-gap approach and suggests the existence of a multi-gap superconductivity in this compound. The large and small superconducting gap values estimated at T = 4.2K are ∆L = 10.5 ± 2 meV and ∆S = 2.3 ± 0.4 meV, respectively.

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Using intrinsic multiple Andreev reflection effect (IMARE) spectroscopy we studied superconducting properties of nearly optimal oxygen-deficient GdFeAsO 0.88 polycrystalline samples (bulk critical temperatures T bulk C = 49÷52 K). Temperature dependences for two superconducting gaps ∆ L,S (T ) (T local C = 48 ÷ 50 K) have been measured in the range from 4.2 to 50 K. The ∆ L,S (T ) dependences were found to deviate from the BCS-like function; this suggests an importance of the k-space (internal) proximity effect between the two condensates.GdFeAsO firstly reported in [1] belongs to oxypnictide high-temperature superconductors [2]. The stoichiometric compound is an antiferromagnetic metal with spin-density-wave (SDW) ground state [3], which can be turned into superconductivity under electron doping. The maximal critical temperatures up to T C = 56 K are achieved by O-deficiency, F introducing in-T.E. Shanygina · A.S. Usol'tsev Lebedev Physical Institute RAS,