Single Crystal Growth and Effect of Doping on Structural, Transport and Magnetic Properties of A1-x K x Fe2As2 (A = Ba, Sr)

We demonstrate the preparation of large, free-standing iron pnictide single crystals with a size up to 20x10x1 mm(3) using solvents in zirconia crucibles under argon atmosphere. Transport and magnetic properties have been investigated to study the effect of potassium doping on the structural and superconducting property of the compounds. The spin density wave (SDW) anomaly at T (s similar to)138 K in BaFe2As2 single crystals from self-flux shifts to T (s similar to)85 K due to Sn solvent growth. We show evidence for an incorporation of Sn on the Fe site. The electrical resistivity data show a sharp superconducting transition temperature T (c) similar to 38.5 K for a single crystal of Ba0.68K0.32Fe2As2. A nearly 100% shielding fraction and the bulk nature of the superconductivity for the single crystal are confirmed by magnetic susceptibility data. A sharp transition T (c) similar to 25 K occurs for the single crystal of Sr0.85K0.15Fe2As2. There is direct evidence for a coexistence of the SDW and superconductivity in the low doping regime of Sr1-x K (x) Fe2As2 single crystals. Structural implications of the doping effects as well as the coexistence of the two order parameters are discussed.