Critical current density and mechanism of vortex pinning in KxFe2-ySe2   doped with S
  We report the critical current density Jc in KxFe2-ySe2-zSz crystals. The Jc can be enhanced significantly with optimal S doping (z=0.99). For K0.70(7)Fe1.55(7)Se1.01(2)S0.99(2), the weak fishtail effect is found for H||c. The normalized vortex pinning forces follow the scaling law with a maximum position at 0.41 of the reduced magnetic field. These results demonstrate that the small size normal point defects dominate the vortex pinning mechanism. 
Hechang Lei and C. Petrovic
cond-mat.supr-con cond-mat.mtrl-sci