AFe₂As₂ (A = Ca, Sr, Ba, Eu) and SrFe_2-xTM_xAs₂ (TM  = Mn, Co, Ni): crystal structure, charge doping, magnetism and superconductivity

The electronic structure and physical properties of the pnictide compound families REOFeAs (RE=La, Ce, Pr, Nd, Sm), AFe₂As₂ (A=Ca, Sr, Ba, Eu), LiFeAs and FeSe are quite similar. Here, we focus on the members of the AFe₂As₂ family whose sample composition, quality and single-crystal growth are more controllable compared with the other systems. Using first-principles band structure calculations, we focus on understanding the relationship between the crystal structure, charge doping and magnetism in AFe₂As₂ systems. We will elaborate on the tetragonal to orthorhombic structural distortion along with the associated magnetic order and anisotropy, the influence of doping on the A site and the Fe site and the changes in the electronic structure as a function of pressure. Experimentally, we investigate the substitution of Fe in SrFe_2−xTM_xAs₂ by other 3d transition metals, TM=Mn, Co or Ni. In contrast to a partial substitution of Fe by Co or Ni (electron doping), a corresponding Mn partial substitution does not lead to the suppression of the antiferromagnetic order or the appearance of superconductivity. Most of the calculated properties agree well with the measured properties, but several of them are sensitive to the As z position. For a microscopic understanding of the electronic structure of this new family of superconductors, this structural feature related to the Fe–As interaction is crucial, but its correct ab initio treatment still remains an open question.