A. Martinelli

From antiferromagnetism to superconductivity in Fe(1+y)Te(1-x)Se(x) (0 <= x <= 0.20): Neutron powder diffraction analysis

The nuclear and magnetic structure of Fe1+y(Te1-x,Sex) (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>0.05, therefore a significant region where superconductivity and long range antiferromagnetism coexist is present in the pseudo-binary FeTe - FeSe phase diagram.

Synthesis, crystal structure, microstructure, transport and magnetic properties of SmFeAsO and SmFeAs(O0.93F0.07)

SmFeAsO and the isostructural superconducting SmFeAs(O0.93F0.07) phase were prepared and characterized by means of Rietveld refinement of x-ray powder diffraction data, scanning electron microscope observation, transmission electron microscope analysis, and resistivity and magnetization measurements. Sintering treatment strongly improves the grain connectivity, but, on the other hand, induces a competition between the thermodynamic stability of the oxy-pnictide and Sm2O3, thus affecting the sample purity. In the pristine sample both magnetization and resistivity measurements clearly indicate that two different sources of magnetism are present: the former related to Fe ordering at T~140 K and the latter due to the Sm ions that order antiferromagnetically at low temperature. The feature at 140 K disappears in the F-substituted sample, and a superconducting transition appears, at low temperatures. The magnetoresistivity curves of the F-substituted sample probably indicate very high critical field values.

Tuning of the superconducting properties of FeSe0.5Te0.5 thin films through the substrate effect

From recent literature, it became clear that the crystallographic lattice parameters and the superconducting properties of FeSe0.5Te0.5 thin films exhibit a non-trivial dependence on the in-plane lattice constant of the substrates on which they are grown. The strain, which depends on the type of growth, can play an important role both in enhancing the critical temperature Tc and in determining the pinning mechanisms. Here, we present the effects of the substrate on the superconducting properties of FeSe0.5Te0.5 thin films. After a comprehensive overview of the different substrates used, i.e.xa0oxides and fluorides, we compare the superconducting properties of films grown on LaAlO3(001) and SrTiO3(001). We show that the pinning properties of the two types of film are completely different: the angular dependences of the critical current density are opposite due to the presence of extrinsic pinning along the c-axis on films grown on SrTiO3. The presence of strong correlated pinning for a field perpendicular to the surface in films grown on SrTiO3 is confirmed by the analysis of the activation energy for vortex motion U0 and supported by the observation through scanning tunnelling microscopy of nanoscale threading dislocations possibly induced by the lattice mismatch with the substrate, which are not seen on the films deposited on LaAlO3.

Transport and superconducting properties of Fe-based superconductors: a comparison between SmFeAsO1−xFx and Fe1+yTe1−xSex

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: SmFeAsO1 ? xFx with the largest Tc and the largest anisotropy and Fe1 + yTe1 ? xSex, with the largest Hc2, the lowest Tc and the lowest anisotropy. In the case of the SmFeAsO1 ? xFx 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 Fe1 + y(Te1 ? x, Sex) series exhibit a strongly compensated behaviour, which can be described only within a multiband model. In the Fe1 + y(Te1 ? x, Sex) 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 Tc and the upper critical field Hc2. The huge Hc2 values of the Fe1 + yTe1 ? xSex samples are described by a dirty limit law, opposed to the clean limit behaviour of the SmFeAsO1 ? xFx 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.

A new approach for improving global critical current density in Fe(Se0.5Te0.5) polycrystalline materials

A novel method to prepare bulk Fe(Se0.5Te0.5) 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 103 A/cm2 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 mu_{0}H = 7 T it is suppressed only by a factor sim2.

Correlated trends of coexisting magnetism and superconductivity in optimally electron-doped oxypnictides.

We report on the recovery of the short-range static magnetic order and on the concomitant degradation of the superconducting state in optimally F-doped SmFe(1-x)Ru(x)AsO(0.85)F(0.15) for 0.1≤x≲0.5. The two reduced order parameters coexist within nanometer-size domains in the FeAs layers and eventually disappear around a common critical threshold x(c)~0.6. Superconductivity and magnetism are shown to be closely related to two distinct well-defined local electronic environments of the FeAs layers. The two transition temperatures, controlled by the isoelectronic and diamagnetic Ru substitution, scale with the volume fraction of the corresponding environments. This fact indicates that superconductivity is assisted by magnetic fluctuations, which are frozen whenever a short-range static order appears, and totally vanish above the magnetic dilution threshold x(c).

Upper critical field and fluctuation conductivity in the critical regime of doped SmFeAsO

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 criterion-dependence Hc2 values, we suggest a thermodynamic estimation of the upper critical field slope dHc2/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 two-dimensional lowest Landau level (LLL) scaling for applied fields larger than mu_0H_LLL=8T. Finally, we estimate the coherence length values and we observe that they progressively increase with decreasing Tc. 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.

Transport and infrared properties of SmFeAs(O1−xFx): from SDW to superconducting ordering

We report measurements of resistivity, magnetoresistivity, Hall effect, Seebeck coefficient and infrared reflectivity of undoped SmFeAsO and lightly doped SmFeAs(O0.93F0.07) oxypnictides. All the properties measured on SmFeAsO are characterized by clear signatures of the magnetic instability. A self-consistent picture emerges in which carrier condensation occurs below the magnetic transition, due to the opening of a spin density wave (SDW) gap. This is accompanied by the mobility increase of not-gapped carriers due to the suppression of electron–electron scattering. SmFeAs(O0.93F0.07) exhibits an increase of the metallic character on cooling consistent with electron doping, even though at room temperature values of all the properties nearly overlap with those of SmFeAsO. However, with a decrease in temperature all anomalies related to the SDW instability are missing and the superconducting transition occurs. This suggests that doping abruptly breaks the symmetries of the Fermi surface, inhibiting the SDW formation in favor of the superconducting transition, with no substantial changes in the density of states or in the effective mass.

Effect of chemical pressure on spin density wave and superconductivity in undoped and 15% F-doped La 1 − y Y y FeAsO compounds

We present a study concerning the partial substitution of yttrium at the lanthanum site of the undoped LaFeAsO and superconducting LaFeAsO0.85F0.15 compounds. We prepared samples with a nominal yttrium content up to 70% producing simultaneous shrinkage of both the a- and c-lattice parameters by 1.8% and 1.7%, respectively. The chemical pressure provided by the partial substitution with this smaller ion size causes a lowering of the spin density wave temperature in the undoped compounds, as well as an increase of the superconducting transition temperatures in the doped ones. The 15% fluorine-doped samples reach a maximum critical temperature of 40.2 K for the 50% yttrium substitution. Comparison with literature data indicates that chemical pressure cannot be the only mechanism which tunes drastically both TSDW and Tc in 1111 compounds. Our data suggest that structural disorder induced by the partial substitution in the La site or by doping could play an important role as well.

Isoelectronic Ru substitution at the iron site in SmFe 1 − x Ru x AsO 0.85 F 0.15 and its effects on structural, superconducting, and normal-state properties

In this work we present a systematic experimental and theoretical study of the structural, transport, and superconducting properties of