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. 
Chunlei Wang, Lei Wang, Zhaoshun Gao, Chao Yao, Dongliang Wang,   Yanpeng Qi, Xianping Zhang, and Yanwei Ma
cond-mat.supr-con