In this work we present a systematic experimental and theoretical study of the structural, transport and superconducting properties of Sm(Fe 1-x Ru x )As (O 0.85 F 0.15 ) polycrystalline samples as a function of Ru content (x) ranging from 0 to 1. The choice of Ru as isoelectronic substitution at Fe site of Fdoped compounds allows to better clarify the role of structural disorder in modifying the normal and superconducting properties of these newly discovered multiband superconductors. Two different regions are identified: the Fe-rich phase (x<0.5) where superconducting and normal state properties are strongly affected by disorder induced by Ru substitution; the Ru-rich phase (x>0.5) where the system is metallic and strongly compensated and the presence of Ru frustrates the magnetic moment on Fe ions. Here the lack of magnetic features and related spin fluctuations may be the cause for the suppression of superconductivity. 1.IntroductionThe recent discovery of high critical temperature superconductivity in iron based compounds 1 has attracted a great deal of attention as these compounds appear to be a glaring case of proximity between superconductivity and magnetisms. The parent compounds exhibit antiferromagnetic spindensity-wave (SDW) order that disappears upon doping, giving rise to superconductivity. It has been suggested by many authors that superconductivity in pnictides could be mediated by magnetic excitations which couple electron and hole pockets of the Fermi surface, favoring s-wave order parameters with opposite sign on different sheets of the Fermi surface (s ± coupling). 2The interplay between superconductivity and magnetisms can be investigated by varying magnetic and superconducting properties of the compounds through suitable substitutions. Moreover, scattering induced by substitutions is expected to affect superconductivity in very differently ways in the cases of conventional or unconventional coupling. 3 As a consequence, a thorough study of the behavior of T c vs structural disorder is crucial in order to probe different theoretical models. Similarly to cuprates, the pnictide compounds have a layered structure characterized by the stacking of insulating and FeAs-conducting layers with general formulas REFeAsO (RE being a rare earth)

We report on the isotropic pinning obtained in epitaxial Fe(Se,Te) thin films grown on CaF2(001) substrate. High critical current density values - larger than 1 MA/cm(2) in self field and liquid helium - are reached together with a very weak dependence on the magnetic field and a complete isotropy. Analysis through transmission electron microscopy evidences the presence of defects looking like lattice disorder at a very small scale, between 5 and 20 nm, which are thought to be responsible for such isotropic behavior in contrast to what was observed on SrTiO3, where defects parallel to the c-axis enhance pinning in that direction

Abstract.A novel method to prepare bulk Fe(Se 0.5 Te 0.5 ) samples is presented, based on a melting process and a subsequent annealing treatment. With respect to the standard sintering technique, it produces much more homogeneous and denser samples, characterized by large and well interconnected grains. The resulting samples exhibit optimal critical temperature values, sharp resistive and magnetic transitions, large magnetic hysteresis loops and high upper critical fields are observed. Interestingly, the global critical current density is much enhanced as compared to the values reported in literature for bulk samples of the same 11 family, reaching about 10 3 A/cm 2 at zero field at 4.2 K as assessed by magnetic, transport and magneto-optical techniques. Even more importantly, its field dependence turns out to be very weak, such that at 0 H = 7 T it is suppressed only by a factor 2.

The synthesis of a series of layered iron arsenides Ca1-xRExFeAs2(112) was attempted by heating at 1000 degrees C under a high pressure of 2 GPa. The 112 phase successfully forms with rare earth (RE) = La, Ce, Nd, Sm, Eu, and Gd, while RE-free samples do not contain the 112 phase. While a Ce-doped sample does not show superconductivity, La-, Pr-, Nd-, Sm-, Eu-, and Gd-doped samples show superconducting transition temperatures of 24.5, 13.2, 11.9, 11.6, 9.3, and 12.6 K, respectively, in magnetization measurements. Their T-c and Fe-Fe interlayer distances decreased with decreasing ionic radii of RE, but there are exceptions

FeSe 0.5 Te 0.5 thin films were grown by pulsed laser deposition on CaF 2 , LaAlO 3 and MgO substrates and structurally and electro-magnetically characterized in order to study the influence of the substrate on their transport properties. The in-plane lattice mismatch between FeSe 0.5 Te 0.5 bulk and the substrates shows no influence on the lattice parameters of the films, whereas the type of substrates affects the crystalline quality of the films and, therefore, the superconducting properties. The film on MgO showed an extra peak in the angular dependence of critical current density J c (θ) at θ = 180° (H || c), which arises from c-axis defects as confirmed by transmission electron microscopy. In contrast, no J c (θ) peaks for H || c were observed in films on CaF 2 and LaAlO 3 . J c (θ) can be scaled successfully for both films without c-axis correlated defects by the anisotropic Ginzburg-Landau (AGL) approach with appropriate anisotropy ratio γ J . The scaling parameter γ J is decreasing with decreasing temperature, which is different from what we observed in FeSe 0.5 Te 0.5 films on Fe-buffered MgO substrates.