Electronic and magnetic structures of the ternary iron selenides AFe2Se2 ( A = Cs, Rb, K, or Tl)

We have studied the electronic and magnetic structures of the ternary iron selenides AFe(2)Se(2) (A = Cs, Rb, K, or Tl) using first-principles electronic structure calculations. We find that the ground state of these compounds is bicollinearly antiferromagnetically ordered, with the Fe moments having collinear antiferromagnetic order in each bipartite sublattice. This bicollinear antiferromagnetic order results from the superexchange interactions of Fe moments mediated by the Se 4p orbitals. We have also determined the density of states at the Fermi level, the specific heat coefficient, the Pauli susceptibility, and other related physical properties in both the nonmagnetic and bicollinear antiferromagnetic states for these compounds. The underlying mechanism is discussed according to the electronic structure analysis.