Less than two years after the discovery of high temperature superconductivity in oxypnictide LaFeAs(O,F) several families of superconductors based on Fe layers (1111, 122,11, 111) are available. They share several characteristics with cuprate superconductors that compromise easy applications, such as the layered structure, the small coherence length, and unconventional pairing, On the other hand the Fe-based superconductors have metallic parent compounds, and their electronic anisotropy is generally smaller and does not strongly depend on the level of doping, the supposed order parameter symmetry is s wave, thus in principle not so detrimental to current transmission across grain boundaries. From the application point of view, the main efforts are still devoted to investigate the superconducting properties, to distinguish intrinsic from extrinsic behaviours and to compare the different families in order to identify which one is the fittest for the quest for better and more practical superconductors. The 1111 family shows the highest T c , huge but also the most anisotropic upper critical field and in-field, fan-shaped resistive transitions reminiscent of those of cuprates, while the 122 family is much less anisotropic with sharper resistive transitions as in low temperature superconductors, but with about half the T c of the 1111 compounds. An overview of the main superconducting properties relevant to applications will be presented. Upper critical field, electronic anisotropy parameter, intragranular and intergranular critical current density will be discussed and compared, where possible, across the Fe-based superconductor families. 2 , to the ab-plane. 12 The temperature dependence is very different in the two directions, strongly departing from the WHH behaviour 16 mainly in the direction parallel to c. The anisotropy evaluated as γ = ab c ab c H H H ⊥ = 2 // 2 / γ, is also strongly temperature dependent, reminiscent of the two-gap behaviour seen in MgB 2 . 17,18 However, a different situation is observed in the 122 family. (Ba,K)Fe 2 As 2 single crystals exhibit nearly isotropic μ 0 H c2 with

High purity epitaxial FeSe0.5Te0.5 thin films with different thickness were grown by Pulsed Laser Ablation on different substrates. By varying the film thickness, Tc up to 21K were observed, significantly larger than the bulk value. Structural analyses indicated that the a axis changes significantly with the film thickness and is linearly related to the Tc. The latter result indicates the important role of the compressive strain in enhancing Tc. Tc is also related to both the Fe-(Se,Te) bond length and angle, suggesting the possibility of further enhancement.Tc=21K in epitaxial FeSe0.5Te0.5 thin films with biaxial compressive strain Ver. 04/12/2009 14:29:00 2 Since the discovery of the iron based superconductors 1 , great efforts have been devoted to the preparation of thin films of the various phases 2 -7 and references therein. Between other reasons, the interest in films deposition is motivated by the strong Tc dependence on external and chemical pressure in iron based pnictides and calcogenides 8 -12 , which has suggested the idea of exploring whether a similar effect can be induced by strain in thin films. Indeed, such expectation turned out to be true: in Ba(Fe1-xCox)2As2 thin films 7 deposited on various substrates, Tc has been observed to increase with the ratio c/a. Also in FeSe0.5Te0.5 thin films an increase of Tc has been obtained by two groups 5,6 in particular a maximum Tc of 17K has been measured. Such increase has been attributed to the observed c axis decrease with respect to the bulk value.Here we present preparation as well as structural, morphological and physical characterization of epitaxial FeSe0.5Te0.5 thin films with different thickness, deposited on different substrates. A maximum Tc =21K was obtained, which is a remarkable 30% increase with respect to the bulk value.The films were grown by Pulsed Laser Ablation Deposition (PLD) in ultra high vacuum system 13 starting from a FeSe0.5Te0.5 (Fe(Se,Te)) target prepared by direct synthesis from high purity materials (Fe 99.9+%, Se 99.9% and Te 99.999%) 5 .With the aim of introducing biaxial strain we deposited films on single crystal substrates with different cell parameters, namely magnesium oxide (MgO a=4.217 Å) , strontium titanate (STO a=3.905 Å ), lantanum aluminate (LAO a=3.789 Å), and yttria stabilized zirconia (ZrO:Y, a=3.637 Å); for all the substrates we used the (001) orientation. The deposition conditions were optimized as reported in a previous paper 5 ; namely, we used a deposition temperature of 550°C at a pressure of 5·10 -Tc=21K in epitaxial FeSe0.5Te0.5 thin films with biaxial compressive strain Ver. 04/12/2009 14:29:00 3 9 mBar. The laser repetition rate was 3 Hz (248 nm wavelength) and the laser fluency was 2 J/cm 2 (2 mm 2 spot size). The target-substrate distance was maintained at 5 cm.In order to study the residual strain behaviour, films of different thickness from 1.2 nm to 600 nm were deposited; the thickness was calibrated by X-ray reflectometry.XRD analysis allowed to identify the PbO-like tetragon...

The nuclear and magnetic structure of Fe 1+y (Te 1-x ,Se x ) (0 ≤ x ≤ 0.20) compounds was analyzed between 2 K and 300 K by means of Rietveld refinement of neutron powder diffraction data.Samples with x ≤ 0.075 undergo a tetragonal to monoclinic phase transition at low temperature, whose critical temperature decreases with increasing Se content; this structural transition is strictly coupled to a long range antiferromagnetic ordering at the Fe site. Both the transition to a monoclinic phase and the long range antiferromagnetism are suppressed for 0.10 ≤ x ≤ 0.20. The onset of the structural and of the magnetic transition remains coincident with the increase of Se substitution. The low temperature monoclinic crystal structure has been revised. Superconductivity arises for x ≥ * Corresponding author: amartin@chimica.unige.it 0.05, therefore a significant region where superconductivity and long range antiferromagnetism coexist is present in the pseudo-binary FeTe -FeSe phase diagram.

We present Muon spectroscopy investigations on SmFeAsO 1−x F x showing coexistence of magnetic order and superconductivity only in a very narrow F-doping range. The sharp crossover between the two types of order is similar to that observed in LaFeAsO 1−x F x , suggesting a common behavior for the 1111 pnictides. The analysis of the muon asymmetry demonstrates that the coexistence must be nanoscopic, i.e., the two phases must be finely interspersed over a typical length scale of few nm. In this regime both the magnetic and the superconducting transition temperatures collapse to very low values. Our data suggest a competition between the two order parameters.

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 measure magnetotransport of F doped SmFeAsO samples up to 28T and we extract the upper critical fields, using different criteria. In order to circumvent the problem of criteriondependence H c2 values, we suggest a thermodynamic estimation of the upper critical field slope dH c2 /dT based on the analysis of conductivity fluctuations in the critical regime. A high field slope as large as -12T/K is thus extracted for the optimally doped sample. We find evidence of a twodimensional lowest Landau level (LLL) scaling for applied fields larger than μ 0 H LLL ∼8T. Finally, we estimate the coherence length values and we observe that they progressively increase with decreasing T c . In all cases, the coherence length values along the c axis are smaller than the interplanar distance, confirming the two-dimensional nature of superconductivity in this compound.

Ac susceptibility and static magnetization measurements were performed in the optimally doped SmFeAsO0.8F0.2 superconductor. The field -temperature phase diagram of the superconducting state was drawn and, in particular, the features of the flux line lattice derived. The dependence of the intra-grain depinning energy on the magnetic field intensity was derived in the thermallyactivated flux creep framework, enlightening a typical 1/H dependence in the high-field regime. The intra-grain critical current density was extrapolated in the zero temperature and zero magnetic field limit, showing a remarkably high value Jc0(0) ∼ 2 · 10 7 A/cm 2 , which demonstrates that this material is rather interesting for the potential future technological applications.

In this paper we carry out a direct comparison between transport and superconducting properties-namely resistivity, magnetoresistivity, Hall effect, Seebeck effect, thermal conductivity, upper critical field-of two different families of Fe-based superconductors, which can be viewed in many respects as end members: SmFeAsO 1−x F x with the largest T c and the largest anisotropy and Fe 1+y Te 1−x Se x , with the largest H c2 , the lowest T c and the lowest anisotropy. In the case of the SmFeAsO 1−x F x series, we find that a single-band description allows us to extract an approximate estimation of band parameters such as carrier density and mobility from experimental data, although the behaviour of the Seebeck effect as a function of doping demonstrates that a multiband description would be more appropriate. On the contrary, experimental data for the Fe 1+y (Te 1−x , Se x ) series exhibit a strongly compensated behaviour, which can be described only within a multiband model.In the Fe 1+y (Te 1−x , Se x ) series, the role of the excess Fe, tuned by Se stoichiometry, is found to be twofold: on one hand it dopes electrons in the system and on the other hand it introduces localized magnetic moments, responsible for Kondo like scattering and likely pairbreaking of Cooper pairs. Hence, Fe excess also plays a crucial role in determining superconducting properties such as the T c and the upper critical field H c2 . The huge H c2 values of the Fe 1+y Te 1−x Se x samples are described by a dirty limit law, opposed to the clean limit behaviour of the SmFeAsO 1−x F x samples. Hence, magnetic scattering by excess Fe seems to drive the system in the dirty regime, but its detrimental pairbreaking role seems not to be as severe as predicted by theory. This issue has yet to be clarified, addressing the more fundamental issue of the interplay between magnetism and superconductivity.