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Abstract: Magnetization curves of dense polycrystalline MgB2 have been measured by SQUID magnetometer and calculated by solving the flux creep equation numerically. Considering the anisotropic superconductivity in MgB2 and the grain orientation in polycrystalline samples as well as the flux creep, the experimental data can be fitted very well by a two-exponential model. According to our fitting results, the nonscaling behavior of flux pinning force and the rapid decrease of critical current density of polycrystalline MgB2 at medium and higher magnetic fields originate from the anisotropic superconductivity in MgB2, the dispersion of grain orientations and the flux creep. Through this way, the real dependence of critical current density on temperature and magnetic field can be extracted, and the flux pinning mechanism is discussed. A universal method is therefore proposed for studying flux pinning in polycrystalline samples with intrinsic anisotropic superconductivity. [Copyright &y& Elsevier]

(Ba,Sr) 1- x K x Fe 2 As 2 superconductor is of great interest as a wire for practical applications. In this paper, Ba 0.6 K 0.4 Fe 2 As 2 precursor powder was prepared by high-energy ball-milling aided sintering (12 h/750 °C). The prepared precursor was drawn to powder-in-tube round wire. Partial as-drawn wires were subjected to biaxial cold pressure densification (BCPD) at 0.5 and 1.0 GPa, followed by an annealing for 1 min/770 °C. According to our results, the BCPD improved the grain connectivity of the BCPDed wires without degrading the superconductivity. A (Ba,Sr) 1- x K x Fe 2 As 2 round-wire record transport critical current density, 1.14 × 10 5 A/cm 2 (4.2 K, 2 T), was observed in our BCPDed samples. [ABSTRACT FROM AUTHOR]

Based on the high quality FeSe 1− x S x (0 ≤ x ≤ 1) single crystals synthesized via a hydrothermal method, we carried out systematic measurement of the normal state magnetoresistance and Hall effect. Through the maximum entropy mobility spectrum analysis (MEMSA), we investigate the evolution of mobility spectrum, which reflects the multi-band structures for different doping content x and temperature T. The relationships between T c and carrier properties calculated by MEMSA, as well as bond angles and anion height calculated by first-principle method are investigated. Nonlinear to linear transition of Hall resistivity(ρ xy (B)) is mainly attributed to the decreasing mobilities and merging of mobility peaks with the reductive x or increasing T. The relationship between T c and carrier density n show weak interaction for the orthogonal phases when x ≤ 0.2, but nearly positive correlation for the tetragonal phases when x > 0.2. Bond angle as well as the Se/S anion height change almost linearly with x within the tetragonal phases, and the V-shaped relationships with T c indicate that structural factors play a key role in the regulation of superconductivity. Our researches are helpful for understanding the multi-band and complex phase diagram of iron-based superconductors. • We systematically measured magnetoresistance and Hall resistivity of FeSe 1−x S x (0 ≤ x ≤ 1) single crystals. • Nonlinear to linear transition of Hall resistivity is mainly due to decreasing mobilities and merging of mobility peaks. • The bond angle and Se/S anion height both show V-shaped relationships with T c. • Structural factors play a key role in the regulation of superconductivity in iron-based superconductors. [ABSTRACT FROM AUTHOR]

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]

Highlights: [•] Polycrystalline SmFeAsO1−xF x samples were prepared by different oxide precursors. [•] Phase formation and superconductivity of these samples were comparatively studied. [•] Results show that nano-Fe2O3 and As2O3 promote fluorine doping. [Copyright &y& Elsevier]

Abstract: Polycrystalline iron-based superconducting NdFeAsO0.88F0.12 was prepared via solid-state reaction in ambient pressure. Small amounts of ferromagnetic impurities were detected by Scanning Electron Microscopy (SEM) analysis. To study the influence of magnetism on superconducting properties of the sample, magnetization M(T, H) measurements were performed in fields up to 1.5T and in the temperature range of 5–60K. The abnormal behavior was observed in the χ(T) curves, and the magnetization hysteresis loops (MHLs) can be described by a sum of the contributions of superconductive hysteresis, ferromagnetic hysteresis of the impurities and the paramagnetic background of the isolated Nd3+ ions. The influence of the ferromagnetic impurities and the paramagnetic background were investigated and discussed. After the corrections of the magnetism contributions, the intrinsic superconductive MHLs as well as the critical current density were obtained. [Copyright &y& Elsevier]

Abstract: Resistivities of polycrystalline iron-based NdFeAsO0.88F0.12 superconductors prepared via solid-state reaction in ambient pressure (AP) and high pressure (HP) were measured in various magnetic fields ranging from 0 to 9T. Different resistivity broadening behaviors of the AP and HP samples, which may originate from the anisotropic superconductivity, the vortex motion, or the connectivity, were investigated in detail. The effective pinning barriers of both the AP and HP samples were obtained from the Arrhenius plot of the resistivity data by two analytic methods of which one assuming the prefactor 2ρ c U/T of the thermally activated flux flow (TAFF) resistivity ρ=(2ρ c U/T)exp(−U/T) is a constant, while the other assuming the prefactor is temperature dependent. The results determined from the two methods were compared and analysed, and the true effective pinning barriers were obtained. The origin of different pinning barriers of the AP and HP samples is discussed. [Copyright &y& Elsevier]

Abstract: The superconducting properties of polycrystalline NdFeAsO0.88F0.12 oxypnictide were studied by both DC and AC magnetization measurements. The zero-field cooled (ZFC) magnetic susceptibility, χ(T), measured under the magnetic field of 0.5T shows a dramatic decrease at about 11K. The imaginary component of the first harmonics of the AC magnetic susceptibility, χ″(T), increases with the increasing DC field H dc below 11K. These results indicate the onset of robust intergranular superconductivity at low temperatures. The magnetic hysteresis loops show an anomalous double central peak effect at low temperatures, with one peak at positive fields in decreasing positive external fields. This phenomenon suggests the granular character of this system. The paramagnetism of Nd3+ ions tilts the magnetic hysteresis loops and broadens the hysteresis width ΔM. After correction for the paramagnetism, the temperature and field dependence of the average magnetization critical current density J cm was obtained. The related mechanism was discussed. [ABSTRACT FROM AUTHOR]

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]