Coexistence of superconductivity and antiferromagnetism in single   crystals $A_{0.8}Fe_{2-y}Se_2$ (A= K, Rb, Cs, Tl/K and Tl/Rb): evidence from   magnetization and resistivity
  We measure the resistivity and magnetic susceptibility in the temperature range from 5 K to 600 K for the single crystals $A$Fe$_{2-y}$Se$_2$ ($A$ = K$_{0.8}$, Rb$_{0.8}$, Cs$_{0.8}$, Tl$_{0.5}$K$_{0.3}$ and Tl$_{0.4}$Rb$_{0.4}$). A sharp superconducting transition is observed in low temperature resistivity and susceptibility, and susceptibility shows 100% Meissner volume fraction for all crystals, while an antiferromagnetic transition is observed in susceptibility at Neel temperature ($T_N$) as high as 500 K to 540 K depending on A. It indicates the coexistence of superconductivity and antiferromagnetism. A sharp increase in resistivity arises from the structural transition due to Fe vacancy ordering at the temperature slightly higher than $T_{\rm N}$. Occurrence of superconductivity in an antiferromagnetic ordered state with so high $T_{\rm N}$ may suggest new physics in this type of unconventional superconductors. 
R. H. Liu, X. G. Luo, M. Zhang, A. F. Wang, J. J. Ying, X. F. Wang, Y.   J. Yan, Z. J. Xiang, P. Cheng, G. J. Ye, Z. Y. Li a...
cond-mat.supr-con cond-mat.mtrl-sci