C. Ferdeghini

Large transport critical currents in unsintered MgB2 superconducting tapes

The powder-in-tube process has been employed to fabricate tape-like conductors with a strong metallic sheath and based on the newly discovered MgB2 superconducting phase. Long superconducting tapes have been prepared by packing reacted MgB2 powders inside pure Ag, Cu, and Ni tubes which are then cold worked by drawing and rolling. Such tapes have shown transport properties as good as bulk MgB2 samples sintered in high pressure and high temperature conditions. At 4.2 K, the highest critical current density of 105 A/cm2 has been achieved on nickel-sheathed single-filament conductors. A direct correlation between the sheath tensile strength and the critical current of the unsintered tape has been observed.

New Fe-based superconductors: properties relevant for applications

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, and 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 behaviors 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 Tc, huge but also the most anisotropic upper critical field and in-field, fan-shaped resistive transitions reminiscent of those of cuprates. On the other hand, the 122 family is much less anisotropic with sharper resistive transitions as in low temperature superconductors, but with about half the Tc of the 1111 compounds. An overview of the main superconducting properties relevant to applications will be presented. Upper critical field, electronic anisotropy parameter, and intragranular and intergranular critical current density will be discussed and compared, where possible, across the Fe-based superconductor families.

Tc=21 K in epitaxial FeSe0.5Te0.5 thin films with biaxial compressive strain

Epitaxial FeSe0.5Te0.5 thin films with different thickness were grown by pulsed laser ablation deposition on different substrates. High purity phase and fully epitaxial growth were obtained. By varying the film thickness, superconducting transition temperatures up to 21 K were observed, significantly larger than the bulk value 16.2 K. Structural analyses indicated that the c-axis is smaller than the bulk value but it is almost independent of the film thickness and 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.

Significant enhancement of upper critical fields by doping and strain in iron-based superconductors

We report measurements up to 85 Tesla of the upper critical fields H(c2)(T) on Ba(1-x)K(x)As(2)Fe(2) single crystals and FeSe(1-x)Te(x) films tuned by doping and strain. We observed an H(c2) enhancement by more than 25 T at low temperatures for the optimally doped Ba(1-x)K(x)As(2)Fe(2) as compared to the previous measurements and extraordinarily high slopes of dH(c2)/dT = 250-500 T/K near T(c) in FeSe(1-x)Te(x), indicating almost-complete suppression of orbital pair breaking. Theoretical analysis of H(c2)(T) suggests an inhomogeneous Fulde-Ferrel-Larkin-Ovchinnikov state below 10 K for H//ab in the optimally doped Ba(1-x)K(x)As(2)Fe(2) and below 3K for H//c and 9K for H//ab in FeSe(1-x)Te(x). The analysis also shows that H(c2) in a multiband Fe-based superconductor can be significantly enhanced by doping and strain by shrinking and expanding different pockets of the Fermi surface, which can be more effective than the conventional way of increasing H(c2) by nonmagnetic impurities.

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.

Observation of the crossover from two-gap to single-gap superconductivity through specific heat measurements in neutron-irradiated MgB2.

We report specific heat measurements in neutron irradiated MgB2 samples, for which the critical temperature has been suppressed down to 8.5 K, but the superconducting transition remains extremely sharp, indicative of a defect structure extremely homogeneous. Our results demonstrate that the two-gap feature is evident in the temperature range above 21 K, while the single-gap superconductivity is well established as a bulk property not associated to local disorder fluctuations when Tc is decreased down to 11 K.

High quality epitaxial FeSe0.5Te0.5 thin films grown on SrTiO3 substrates by pulsed laser deposition

Superconducting epitaxial FeSe0.5Te0.5 thin films are prepared on SrTiO3(001) substrates by pulsed laser deposition. The high purity of the phase, the quality of the growth and the epitaxy are studied with different experimental techniques: x-rays diffraction, reflection high energy electron diffraction, scanning tunneling microscopy and atomic force microscopy. The substrate temperature during the deposition is found to be the main parameter governing sample morphology and superconducting critical temperature. Films obtained under optimal conditions show an epitaxial growth with the c axis perpendicular to the film surface and the a and b axes parallel to the substrate, without evidence of any other orientation. Moreover, such films exhibit a metallic behavior over the whole measured temperature range and the critical temperature is above 17?K, which is higher than the target value.Superconducting epitaxial FeSe0.5Te0.5 thin films were prepared on SrTiO3 (001) substrates by pulsed laser deposition. The high purity of the phase, the quality of the growth and the epitaxy were studied with different experimental techniques: X-rays diffraction, reflection high energy electron diffraction, scanning tunnelling microscopy and atomic force microscopy. The substrate temperature during the deposition was found to be the main parameter governing sample morphology and superconducting critical temperature. Films obtained in the optimal conditions show an epitaxial growth with c axis perpendicular to the film surface and the a and b axis parallel to the substrates one, without the evidence of any other orientation. Moreover, such films show a metallic behavior over the whole measured temperature range and critical temperature above 17K, which is higher than the target one.

Fabrication and properties of monofilamentary MgB2 superconducting tapes

Nickel-sheathed MgB2 tapes were fabricated by means of the powder-in-tube method with a monocore configuration of the superconductor. The metallic tubes were filled with commercial reacted powders and cold-worked, respectively, by groove rolling, drawing, and rolling to flat tapes of about 4 mm in width and 0.35 mm in thickness. Portions of the manufactured conductors were heat-treated in an argon atmosphere at temperatures up to 940 °C. Conductors were characterized by transport measurements to determine the magnetic field dependence of the critical current density, the irreversibility line, as well as to establish an initial relationship between the thermo-mechanical treatment and the current carrying capacity of the manufactured MgB2 samples. Our optimized samples for the low field operation carry a critical current density of at least 4 × 105 A cm−2 at 4.2 K, 1 T. The main outcome of this study, however, is that the optimal heat treatment condition changes if the MgB2 conductor has to be employed respectively for a low- or high-field operation. Finally, the first successful demonstration of the current carrying capability of longer MgB2 tapes is also reported in this paper.

Effects of neutron irradiation on polycrystalline Mg 11 B 2

We studied the influence of the disorder introduced in polycrystalline MgB2 samples by neutron irradiation. To circumvent self shielding effects due to the strong interaction between thermal neutrons and 10B we employed isotopically enriched 11B which contains 40 times less 10B than natural B. The comparison of electrical and structural properties of different series of samples irradiated in different neutron sources, also using Cd shields, allowed us to conclude that, despite the low 10B content, the main damage mechanisms are caused by thermal neutrons, whereas fast neutrons play a minor role. Irradiation leads to an improvement in both upper critical field and critical current density for an exposure level in the range 1-2x1018 cm-2. With increasing fluence the superconducting properties are depressed. An in-depth analysis of the critical field and current density behaviour has been carried out to identify what scattering and pinning mechanisms come into play. Finally the correlation between some characteristic lengths and the transition widths is analysed.

Effect of two bands on critical fields in MgB 2 thin films with various resistivity values

Upper critical fields of four MgB2 thin films were measured up to 28 Tesla at Grenoble High Magnetic Field Laboratory. The films were grown by Pulsed Laser Deposition and showed critical temperatures ranging between 29.5 and 38.8 K and resistivities at 40 K varying from 5 to 50 mWcm. The critical fields in the perpendicular direction turned out to be in the 13-24 T range while they were estimated to be in 42-57 T the range in ab-planes. In contrast to the prediction of the BCS theory, we did not observe any saturation at low temperatures: a linear temperature dependence is exhibited even at lowest temperatures at which we made the measurements. Moreover, the critical field values seemed not to depend on the normal state resistivity value. In this paper, we analyze these data considering the multiband nature of superconductivity in MgB2 We will show how the scattering mechanisms that determine critical fields and resistivity can be different.