Relationship between anion height and superconducting properties for Fe1+x Te0.6 Se0.4 bulk single crystals

Abstract

We have synthesized Fe1+xTe0.6Se0.4 large bulk single crystals by the melting method with a homogenization heat treatment. The relationship between the crystal structures and the superconducting properties was investigated. Critical temperature Tc onset reached the highest value of 14 K for the crystal with x = - 0.005. The maximum critical current densities Jc in the a-b plane at 4.2 K achieved 1.53 and 0.83×105 A/cm2 at 0 T and 5 T for the crystal with x = 0, respectively. The crystal lattice and structural parameters were refined by Rietveld analysis. The anion height was estimated by using the obtained parameters. The anion height dependence of Tc and Jc were investigated. The Tc decreased as the anion height increased, and it showed the same tendency as other Fe-based superconductors. On the other hand Jc increased initially and then decreased with the increase in the anion height. We believe that it comes from the trade-off between the increase in flux pinning centres of excess Fe sites and the deterioration of superconductivity.