Maurizio Vignolo

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.

Improvement of Magnetic Field Behavior of Ex-Situ Processed Magnesium Diboride Tapes

MgB2 tapes have been synthesized through the ex-situ powder-in-tube (PIT) method. This technique involves the cold working of tubes of various metals-in this case Ni-previously filled by suitably reacted MgB powders, followed by proper heat treatments. In particular, it allows the use of different starting powders and the control over their properties. We studied the influence of the starting powders on the superconducting properties of the final conductors, and we tried to improve their behavior in magnetic field by mastering their grain size and by inserting appropriate doping. In order to improve the pinning properties of the tapes, the granulometry has been varied through ball-milling of MgB, and different dopants-such as carbon or SiC nanoparticles-have been introduced either on the precursors or on MgB. Critical current measurements on the tapes are presented, both magnetic from the SQUID and transport in high magnetic field. In particular, IV characteristics have been measured up to 13 Tesla at GHMFL (Grenoble) in order to extract the critical current in two directions, i.e. with the field perpendicular and parallel to the tape surface.

Study of the MgB2 grain size role in ex?situ multifilamentary wires with thin filaments

The MgB2 superconductor has already demonstrated its potential, in particular for DC applications such as magnetic resonance imaging (MRI) magnets, thanks to the low cost of the raw materials and to its simple production process. However, further efforts have still to be made in order to broaden its employment in AC applications such as superconducting fault current limiters (SFCLs), motors, transformers etc. The main issues are related to the reduction of AC losses. Some of these can be faced by obtaining multifilamentary conductors with a large number of very fine filaments and, in this context, the powders granulometry can play a crucial role. We have prepared MgB2 starting powders with different granulometries and by the ex situ powder-in-tube (PIT) method we have realized multifilamentary wires with a number of filaments up to 361 and an average size of each filament lowered down to 30 µm. In particular we have studied the relationship between grain and filament size in terms of transport properties and have shown that the optimization of this ratio is possible in order to obtain suitable conductors for AC industrial applications.

Effect of external pressure on the magnetic properties of LnFeAsO (Ln = La, Ce, Pr, Sm)

We investigate the effect of external pressure on magnetic order in undoped LnFeAsO (Ln =La, Ce, Pr, Sm) by using muon spin relaxation measurements and ab initio calculations. Both the magnetic transition temperature Tm and the Fe magnetic moment decrease with external pressure. The effect is observed to be lanthanide dependent, with the strongest response for Ln = La and the weakest for Ln = Sm. The trend is qualitatively in agreement with our density functional theory calculations. The same calculations allow us to assign a value of 0.68(2) μB to the Fe moment, obtained from an accurate determination of the muon sites. Our data further show that the magnetic lanthanide order transitions do not follow the simple trend of Fe, possibly as a consequence of the different f-electron overlap.

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.

Magnetotransport in La(Fe,Ru)AsO as a probe of band structure and mobility

In this work we investigate the Ru substituted LaFeAsO compound, by studying the magnetotransport behaviour and its relationship with the band structure, in different regimes of temperature, magnetic field and Ru content. In particular we analyse the magnetoresistance of LaFe1-xRuxAsO (0 <= x <= 0.6) samples with the support of ab initio calculations and we find out that in the whole series: (i) the transport is dominated by electron bands only; (ii) the magnetoresistance exhibits distinctive features related to the presence of Dirac cones; indeed, ab initio calculations confirm the presence of anisotropic Dirac cones in the band structure; (iii) the low temperature mobility is exceptionally high and reaches 18.6 m2/(Vs) in the Ru-free sample at T=2K, in the extreme limit of a single Landau level occupied in the Dirac cones; (iv) the mobility drops abruptly above 10K-15K; (v) the disorder has a very weak effect on the band mobilities and on the transport properties; (vi) there exists a correlation between the temperature ranges of Dirac cones and SDW carrier condensation. These findings may be of crucial importance in the investigation of the pairing mechanism in the F-doped superconducting La(Fe,Ru)As(O,F) compounds related to this series of parent compounds.

Role of the Grain Oxidation in Improving the In-Field Behavior of

The grain size and the grain boundaries play an important role in determining the pinning properties and the current carrying capability of MgB2. Here we present an experimental study on the effect of grain oxidation on the superconducting properties of MgB2 ex-situ Powder-in-Tube (PIT) tapes. We fabricated Fe-sheathed monofilamentary tapes using a variety of powders synthesized in inert (IA) or oxidizing (OA) atmosphere. In order to understand the role of magnesium oxide (MgO) we studied the effect of the presence of the oxidation layer on the field dependence of the critical current density. To this purpose, we developed a process for the powder synthesis and tube filling and sealing in controlled atmosphere to reduce the oxygen contamination; with this process the connectivity of the grains in the final tapes proved to be increased. Then, we introduced a controlled oxidation step. We will show how the presence of the MgO layer enhances the critical current field dependence. High energy X-ray diffraction measurements were performed on the samples, and the grain size of the unsintered and sintered powders in the tapes was measured by means of Scanning Electron Microscopy (SEM). The pinning force was studied, and different mechanisms were found for the samples prepared in IA and OA. The different behavior shown by the two different typologies of tapes was correlated with the presence or absence of the MgO layer.

{\rm MgB}_{2}

MgB2 monofilamentary nickel-sheathed tapes and wires were fabricated by means of the ex-situ powder-in-tube method using either high-energy ball milled and low temperature synthesized powders. In the first part of the paper it will be shown the effect of milling on tapes realized with powders synthesized at 900degC. The milling time and the milling revolution speed were tuned in order to maximize the critical current density (Jc) in field: the maximum Jc value of 6times104 A/cm2 at 5 K and 4 T was obtained for the tape prepared with powders milled for 144 h at 180 rpm. In the second part of the paper we study the effect of the powders synthesis temperature in a wire configuration. Various synthesis temperature were investigated (730degC-900degC) finding a best Jc value for the wire prepared with powders synthesized at 745degC. We speculate that this optimal temperature is due to the fluidifying effect of unreacted magnesium content before the sintering process which could better connect the grains.

Ex-Situ Tapes

A high-purity MgB4 phase has been synthesized and used as the precursor powder for the realization of in situ wires. Various final heat treatments, from 550 to 1100 ° C for 20 min each, have been carried out to convert the inner mixture to MgB2. Critical current densities up to 1.75 × 106 and 0.98 × 106 A cm−2 at 4.2 and 20 K, with 0.2 T, up to 0.76 × 105 A cm−2 at 4.2 K and 4 T, and up to 0.78 × 105 A cm−2 at 20 K and 2 T were measured. Critical temperatures up to 38.5 K were measured. These results suggest that this could be a good alternative to both ex situ and classical in situ routes.

High-Energy Ball Milling and Synthesis Temperature Study to Improve Superconducting Properties of

In this paper, we report a new synthesis route to produce boron powders characterized as being amorphous and having very fine particle size. This route has been developed to improve the performances of superconducting MgB2 powders, which can be directly synthesized from this nanostructured boron precursor by following the ex-situ or the in-situ PIT method during manufacturing of tapes, wires, and cables. All the procedure steps are explained, and the chemical-physical characterization of the boron powder, using X-ray diffraction, scanning electron microscopy, and transmission electron microscopy techniques, is reported. Furthermore, a comparison with commercial boron is given. Preliminary results of the magnetic and electrical characterization, such as critical temperature TC and transport critical current density JC t, for the MgB2 tape are reported and compared with those for the tape prepared with commercial boron.