Yajing Cui

Peak effect and superconducting properties of SmFeAsO0.8F0.2 wires

Ta-sheathed SmFeAsO0.8F0.2 superconducting wires with Tc = 52.5 K have been fabricated using the powder-in-tube (PIT) method and the superconducting properties of the wires have been investigated. The wires exhibit a very large intragrain critical current density at a temperature below 30 K. A peak effect with maximal Jc = 0.6 MA cm−2 at 10 K under 6 T field was observed. The peak field Hpear is strongly temperature-dependent. A severe weak-link effect depresses the development of global supercurrent owing to a very short coherence length. The wires also show a power law temperature dependence for the irreversibility line with . The H–T phase diagram was found to be similar to that of other superconducting cuprates.

A study of the Fe-based superconductor SmFeAsO1−xFx by x-ray photoelectron spectroscopy

The electronic structure of the Fe-based superconductor SmFeAsO1?xFx (x = 0 and 0.2) is studied with x-ray photoemission spectroscopy (XPS) through comparing the band structures of the nonsuperconducting parent material SmFeAsO and the superconducting SmFeAsO0.8F0.2 (Tc = 52.5?K). A small peak centered at 0.2?eV below the Fermi level, EF, in the valence band is observed in the parent material SmFeAsO, which is due to the low-spin state of the Fe?3d electrons. With fluorine doping, the peak at 0.2?eV disappears and a broad plateau forms near the Fermi level; in the meantime, the density of states at EF is slightly suppressed. The O?1s and Sm?3d core levels shift towards high energy by ~0.55?eV with fluorine doping, but in a sharp comparison, the Fe?2p and As?3d core levels do not shift significantly (the binding energy shift is less than 0.01?eV). It is deduced from the core-level shifts that the Fermi level of the system moves up by 0.55?eV by fluorine doping.

Preparation of Nb3Al by high-energy ball milling and superconductivity

The A15 phase superconductor Nb3Al has been considered as an alternative to Nb3Sn for high field and large scale applications. However, to prepare a stoichiometric Nb3Al with fine grain structures is very difficult. High-energy ball milling is a solid state powder processing technique and is a very useful for preparing Nb-Al alloys (Nb3Al). The effects of ball milling time and annealing temperature on the formation of Nb3Al superconducting phase have been studied. Pure Nb and Al powders with stoichiometric ratio of Nb3Al were mixed and milled, and the charging and milling were performed in an inert atmosphere. Phase formation and structural evolution during high-energy ball milling have been examined by X-ray diffraction. Al disappeared and Nb peaks broadened after about one hour of milling. With increasing milling time, the peaks of Nb became considerably broader and intensities decreased, the Nb-Al solid solution phase was extensive when milled about 3 hours. In order to obtain Nb3Al superconducting phase, a subsequent anneal was required. We have annealed the as-milled powders at 800-900°C for different times to prepared Nb3Al superconducting alloy. The results indicated that Nb3Al with small amount of impurity phase can be obtained on annealing the Nb-Al solid solution phase and the superconducting transition temperature was about 15K, but it is difficult to obtain a homogeneous Nb3Al phase by annealing the amorphous powder.

Multiple magnetic transitions in SmCoAsO

The magnetic properties of SmCoAsO have been investigated. Our results differ from early observations. Complicated magnetism consists of antiferromagnetic, ferromagnetic, ferrimagnetic and paramagnetic, even diamagnetism at low field has been observed. A metamagnetic transition was observed, resulting from a canting of the spins. The interaction between two Co sublattices with canted-structure might take responsibility for the multiple magnetic transitions. Electrical resistivity data indicate that SmCoAsO is metallic conductor with room temperature resistivity of 0.51669 mΩ-cm. Negative magnetoresistance effect suggests a significant suppression of spin-flip scattering by the applied magnetic field. The magnetic phase diagram has been established.