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We reported a detailed study of Fe 1+ y Te 0.6 Se 0.4 single crystals annealed in the atmosphere of chalcogens (O 2 , S, Se, Te). After annealing with appropriate amount of chalcogens, Fe 1+ y Te 0.6 Se 0.4 single crystals show T c higher than 14 K with a sharp transition width ∼1 K. Critical current density J c for the annealed crystals reach a very high value ∼2–4 × 10 5 A/cm 2 under zero field, and is also robust under applied field at low temperatures. Magneto-optical imaging reveal that the J c is homogeneously distributed in the annealed crystals and isotropic in the ab -plane. Our results show that annealing in the atmosphere of chalcogens can successfully induce bulk superconductivity in Fe 1+ y Te 0.6 Se 0.4 . [ABSTRACT FROM AUTHOR]

Abstract: Cu sheathed polycrystalline Ba0.6K0.4Fe2As2 superconducting wire was prepared by a two-step powder-in-tube method. A pressure of 2GPa was applied to a short sample before heat treatment. Magnetization and transport measurements were performed to investigate the effects of cold high pressure densification on the microstructure and superconductivity. The cold pressed sample shows an improved self-field transport critical current density of 2.8×104 A/cm2 (I c =83A) at 4.2K, which is nearly as twice as the unpressed sample. However, both samples manifest pronounced weak-link behavior, suggesting the technique need to be further optimized. The comparison of properties between pressed and unpressed sample and related mechanism was discussed. [Copyright &y& Elsevier]

Abstract: Polycrystalline NdFeAsO0.88F0.12 superconductors prepared by high pressure (HP) and ambient pressure (AP) methods were comparatively studied by magnetization and transport measurements. Upper critical field H c2, irreversibility field H irr and the anisotropy parameter Γ were estimated from resistance transition curves. The broadening of transition width was observed, and was ascribed to both H c2 anisotropy and superconductivity inhomogeneity of samples. Magnetic hysteresis loops (MHLs) in low fields were measured to detect the trace of weak-link behavior. The reclosed hysteresis loops in low fields indicate that there are weak links in both samples. Magnetization critical current density J cm were derived from MHLs. Sample HP shows higher J cm than sample AP. Direct transport I–V measurements show that the transport critical current density J ct are very low but persist up to 9T, suggesting intrinsic strong-link existing in both samples. [ABSTRACT FROM AUTHOR]