A site-selective antiferromagnetic ground state in layered   pnictide-oxide BaTi2As2O
  The electronic and magnetic properties of BaTi$_{2}$As$_{2}$O have been investigated using both the first-principles and analytical methods. The full-potential linearized augmented plane-wave calculations show that the most stable state is a site-selective antiferromagnetic (AFM) metal with a $\text{2}\times \text{1}\times \text{1}$ magnetic unit cell containing two nonmagnetic Ti atoms and two other Ti atoms with antiparallel moments. Further analysis to Fermi surface and spin susceptibility shows that the site-selective AFM ground state is driven by the Fermi surface nesting and the Coulomb correlation. Meanwhile, the charge density distribution remains uniform, suggesting that the phase transition at $200$ K in experiment is a spin-density-wave (SDW) transition. 
Xiang-Long Yu, Da-Yong Liu, Ya-Min Quan, Ting Jia, Hai-Qing Lin, and   Liang-Jian Zou
cond-mat.supr-con