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[en] 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.
[en] We report our studies on the crystal structures, morphologies, and superconductivity in CeO1-xFxFeAs compounds which were fabricated by solid state reaction. The crystal structures were refined using Rietveld refinement. Superconducting properties such as critical temperature (Tc), critical current density (Jc), and upper critical field (Hc2) were determined using magneto-transport and magnetic measurement over a wide range of temperature below Tc, and in magnetic fields up to 13 T. Jc is 2 x 103 A cm-2 for the x = 0.1 sample. However, the Jc exhibited a weak dependence on magnetic field for B > 1 T and T = 5 and 10 K. A peak effect in the Jc as a function of field was observed at 20 K in the x = 0.1 sample. We estimate Hc2ab of 185 T for CeO0.9F0.1FeAs compound. The broadening of the superconducting transition near Tc with increasing field can be well understood using the thermal activated flux flow model. The pinning potential scales as U0/KB∝B-n with n = 0.2 for B < 3 T and n = 0.71 for B > 3 T in the x = 0.1 sample.
[en] In this paper we present the results of critical current (I _c) measurements of MgB_2 wires made with two different set-ups of the four-point probe method: current sweep type—constant magnetic field and increasing current, and field sweep type—constant current and rapidly increasing magnetic field. Results from magnet field sweep type measurements can be interpreted by a new physical concept—a jump of the electric field in low magnetic fields. This physical concept can be correlated with damages in the Nb-barrier existing in the MgB_2 wire and be employed as a detection scheme. The damage in Nb barrier reduces critical current density (J _c) and complicates the study on critical temperature (T _c), upper critical field (B _c_2), irreversible magnetic field (B _i_r_r), pinning force (F _p), and pinning centers in superconducting MgB_2 wires. Our proposed method to detect damages in Nb barrier would benefit efforts in development and applications of MgB_2 wires. (paper)
[en] Inhomogeneity of a high Tc superconducting sample is described by alternating weak and strong regions, which are depicted by different critical temperatures Tcw and Tcs, or different critical current densities jcw and jcs. Based on flux creep simulation, a step and two peaks in ac susceptibility (ACS) curves, as reported in the references, are observed. The ACS response in the two inhomogeneous models is analyzed through the calculated field distributions and their evolution with temperature. The turning point, according to the dissipation peak at low temperature, is not a critical temperature of any part of the sample, and the step in the real part χ' is induced by the slow movement of flux lines in the strong region. The different influence of ac magnetic field amplitudes Bac and frequencies f on ACS curves for two inhomogeneous models can be used as a practical criterion to distinguish Tc inhomogeneity from jc inhomogeneity in sintered samples.
[en] We report a study on the effect of sintering temperature on the microstructure and superconducting properties of Sr0.6K0.4Fe2As2 bulk samples, which were synthesized by the one-step solid reaction method. The heat treatment temperatures were varied from 700 to 900 0C in an argon atmosphere. It was found that the annealing temperature had little influence on the critical temperature Tc. However, the irreversibility field Hirr and critical current density Jc were significantly affected by the sintering temperature; for instance, the Jc at both 5 and 20 K increased with increasing the synthesis temperatures up to 850 0C, and then further increasing the temperature hardly affected the Jc improvement. In addition, samples heat treated at temperatures over 850 0C exhibited a very weak Jc-field dependence, especially at 20 K. Characterizations revealed that high temperature sintering resulted in large grains with fewer impurities.
[en] We have investigated the influence of MgB2 layer thickness on the superconducting properties of MgB2 /Ni multilayer thin films. Multilayer thin films were prepared using an alternate growth of MgB2 (31, 22, and 15 nm in thickness) and Ni (∼1 nm) layers. The critical temperature, Tc, upper critical field, Bc2, and magnetic irreversibility field, Birr, values decreased as the MgB2 layer become thinner. In the multilayer thin films, the critical current density, Jc, was higher in fields parallel to the substrate than in perpendicular fields, reflecting the layered nanostructure parallel to the substrate. On decreasing the MgB2 layer thickness, the peak position of the global pinning force, Fp, curves moved to higher magnetic field. These peak shifts could be explained by a matching effect of the flux lines with the periodic multilayer nanostructure.
[en] Iron-based superconductors (IBSs) with ultrahigh upper critical fields and low anisotropies have attracted significant attention in terms of the novel mechanism of superconductivity and high-field applications. A major concern for practical research is the fabrication of long wires with enhanced critical current density and low cost. In this paper, Cu/Ag composited sheathed Ba0.6K0.4Fe2As2 (Ba122) tapes were fabricated through a hot isostatic pressing method, which is feasible for long-wire manufacturing. The Cu/Ag composite sheath can lower the tape cost by reducing the use of expensive Ag. A high-transport critical current density (J c) up to 5.8 × 104 A cm−2 under 10 T at 4.2 K was achieved in our tapes. Evidence has shown that these tapes have pure Ba122 phase, homogeneous element distribution, orientated grains and good grain connectivity. Our work shows that low-cost Cu/Ag-sheathed IBS tapes have great promise for practical applications in the future. (paper)
[en] Comprehensive measurements are reported of the critical current density (JC) of internal-tin and bronze-route Nb3Sn superconducting wires as a function of magnetic field (B≤23 T), temperature (4.2 K ≤T≤12 K) and axial strain (-1.6%≤εI≤0.40%). Electric field-temperature characteristics are shown to be equivalent to the standard electric field-current density characteristics to within an experimental uncertainty of ∼20 mK, implying that JC can be described using thermodynamic variables. We report a new universal relation between normalized effective upper critical field (BC2*(0)) and strain that is valid over a large strain range for Nb3Sn wires characterized by high upper critical fields. A power-law relation between BC2*(0,εI) and TC*(εI) (the effective critical temperature) is observed with an exponent of ∼2.2 for high-upper-critical-field Nb3Sn compared to the value ≥3 for binary Nb3Sn. These data are consistent with microscopic theoretical predictions and suggest that uniaxial strain predominantly affects the phononic rather than the electronic properties of the material. The standard Summers scaling law predicts a weaker strain dependence than is observed. We propose a scaling law for JC(B,T,εI) based on microscopic theory and phenomenological scaling that is sufficiently general to describe materials with different impurity scattering rates and electron-phonon coupling strengths. It parametrizes complete datasets with a typical accuracy of ∼4%, and provides reasonable predictions for the JC(B,T,εI) surface from partial datasets
[en] The reversible axial strain dependence of the critical current of MgB2 conductors is shown to vary with the temperature and magnetic field. The measured critical temperature and irreversibility field are also found to change reversibly with the axial strain. Combining these effects, we show empirically how the strain dependence of the whole critical surface can be scaled with just three parameters: the strain dependences of its three corner points
[en] The transport ac loss Q of an elliptical thin strip of critical current I c with a power-law relation is accurately computed as a function of current amplitude I m and frequency f. The resulting is normalized to following the Norris critical-state formula, and converted to at a critical frequency f c based on a transport scaling law. Having a set of at several values of n as a base, a general expression of is obtained, which can be used to easily calculate for any practical purposes. (paper)