Relationship between anion height and superconducting properties for Fe_1+x Te_0.6 Se_0.4 bulk single crystals

We have synthesized Fe_1+xTe_0.6Se_0.4 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 T_c onset reached the highest value of 14 K for the crystal with x = - 0.005. The maximum critical current densities J_c 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 T_c and J_c were investigated. The T_c decreased as the anion height increased, and it showed the same tendency as other Fe-based superconductors. On the other hand J_c 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. (paper)