Discrete versus continuum modeling of a charged dielectric interface: A first grade test

Two main treatments within classical simulations for modeling a charged surface are using explicit, discrete charges and continuous, uniform charges. The computational cost can be substantially reduced if, instead of discrete surface charges, one uses an electric field to represent continuous surface charges. In addition, many electrolyte theories, including the Poisson--Boltzmann theory, are developed on the assumption of uniform surface charge. However, recent simulations have demonstrated with discrete surface charges, one observes much stronger charge reversal, compared to the surfaces with continuous surface charges, when the lattice constant becomes notably larger than the ion diameter. These examples show that the two treatments for modeling a charged dielectric interface can lead to substantially different results. In this short note, we calculate the electrostatic force for a single point charge above an infinite plane, and compare the differences between discrete and continuous representations of surface charges. Our results show that while the continuous, uniform surface charge model gives a quite simple picture, the discrete surface charge model can offer several different cases even for such a simple problem, depending on the respective values of ion size versus lattice spacing and a self-image interaction parameter.Comment: 3 pages, 3 figure

Development of Powder-in-Tube Processed Iron Pnictide Wires and Tapes

The development of the PIT fabrication process of iron pnictide superconducting wires and tapes has been carried out in order to enhance their transport properties. Silver was found to be the best sheath material, since no reaction layer was observed between the silver sheath and the superconducting core. The grain connectivity of iron pnictide wires and tapes has been markedly improved by employing Ag or Pb as dopants. At present, critical current densities in excess of 3750 A/cm^2 (Ic = 37.5 A) at 4.2 K have been achieved on Ag-sheathed SrKFeAs wires prepared with the above techniques, which is the highest in iron-based wires and tapes so far. Moreover, Ag-sheathed Sm-1111 superconducting tapes were successfully prepared by PIT method at temperatures as low as 900C, instead of commonly used temperatures of 1200C. These results demonstrate the feasibility of producing superconducting pnictide composite wires, even grain boundary properties require much more attention.Comment: 4 pages, 6 figures. Submitted to ASC2010 proceeding

Fabrication and transport properties of Sr0.6K0.4Fe2As2 multifilamentary superconducting wires

Seven-core Ag/Fe sheathed Sr0.6K0.4Fe2As2 (Sr-122) superconducting wires were produced by the ex situ powder-in-tube (PIT) method. The relationship between the cold-work deformation process and the superconducting properties of wires were systematically studied. It was found that flat rolling can efficiently increase the density of the superconducting core and largely improve the transport critical current density (Jc) of as-drawn wires. The Jc of the best sample achieved 21.1 kA/cm^2 at 4.2 K in self field, and showed very weak magnetic field dependence in high fields. Our result suggested a promising future of multifilamentary iron-based superconductors in practical application.Comment: 19 pages, 6 figure

Enhanced critical current properties in Ba0.6K0.4+xFe2As2 superconductor by over-doping of potassium

Phase-pure polycrystalline Ba0.6K0.4+xFe2As2 with were prepared using a one-step solid-state reaction method. We found that over-doping of potassium can improve critical current density (Jc). High-field Jc for samples with x = 0.1 is three times higher than that for samples with x = 0. Over-doping of K has minimal effect on the critical transition temperature (Tc). Less than 0.5 K degradations in Tc was measured for samples with x = 0.1. TEM revealed high concentration of dislocations in samples with x = 0.1, resulting in enhanced flux pining. Further analyses on magnetization loops for powder samples confirm that K over-doping can promote intra-grain Jc. Our results indicate that slight excess of K in Ba0.6K0.4Fe2As2 samples is beneficial to high-field applications.Comment: 13 pages, 4 figure

High transport critical current densities in textured Fe-sheathed Sr1-xKxFe2As2+Sn superconducting tapes

We report the realization of grain alignment in Sn-added Sr1-xKxFe2As2 superconducting tapes prepared by ex-situ powder-in-tube method. At 4.2 K, high transport critical current densities Jc of 2.5x10^4 A/cm^2 (Ic = 180 A) in self-field and 3.5x10^3 A/cm^2 (Ic = 25.5 A) in 10 T have been measured. These values are the highest ever reported so far for Fe-based superconducting wires and tapes. We believe the superior Jc in our tape samples are due to well textured grains and strengthened intergrain coupling achieved by Sn addition. Our results demonstrated an encouraging prospect for application of iron based superconductors.Comment: 14 pages, 4 figure