Upper fields and critical current density of K0.58Fe1.56Se2 single   crystals grown by one step technique
  Single crystals of K0.58Fe1.56Se2 were successfully synthesized by a new single step process with the onset superconducting transition temperatures 31.9 K. The x-ray diffraction patterns suggest that they have high crystalline quality and c-axis orientation. A possible modulation structure of Fe-vacancy along c axis was observed. The upper critical field has been determined with the magnetic field along ab-plane and c-axis, yielding an anisotropy of about 3.3. It has also been shown that the critical current density of the K0.58Fe1.56Se2 is about 1.7x10^4 A/cm^2 at 5 K. 
Zhaoshun Gao, Yanpeng Qi, Lei Wang, Chao Yao, Dongliang Wang, Xianping   Zhang, Yanwei Ma
cond-mat.supr-con