Block Antiferromagnetism and Checkerboard Charge Ordering in   Alkali-doped Iron Selenides $R$$_{1-x}$Fe$_{2-y}$Se$_2$
  By performing first-principles electronic structure calculations and analyzing effective magnetic model of alkali-doped iron selenides, we show that the materials without iron vacancies should approach a novel checkerboard phase in which each four Fe sites group together in tetragonal structure. The checkerboard phase is the ground state with a block antiferromagnetic (AFM) order and a small charge density wave order in the absence of superconductivity. Both of them can also coexist with superconductivity. The results explain mysterious $2\times 2$ ordered patterns and hidden orders observed in various different experiments, clarify the missing link between AFM and superconducting phases, suggest that the block-AFM state is the parent state, and unify the understanding of various observed phases in alkali-doped iron selenides. 
Wei Li, Shuai Dong, Chen Fang and Jiangping Hu
cond-mat.supr-con cond-mat.str-el