A three-dimensional tight-binding model and magnetic instability of iron selenide KFe2Se2

For a newly discovered iron-based high Tc superconducting parent material KFe2Se2, we present an effective three-dimensional five-orbital tight-binding model by fitting the band structures. The three t2g-symmetry orbitals of the five Fe 3d orbitals mainly contribute to the electron-like Fermi surface, which has a three-dimensional character, in agreement with recent angle-resolved photoemission spectroscopy experiments. To understand the groundstate magnetic properties, the two- and three-dimensional spin and charge susceptibilities within the random phase approximation are investigated. It obviously shows a sharp peak at wave vector q∼(π,π), indicating the magnetic instability of Néel-type antiferromagnetic rather than (π/2, π/2)-type or stripe (π, 0)-type antiferromagnetic ordering, while it exhibits a ferromagnetic instability between the nearest neighboring FeSe layers along c axis. The three-dimensional spin fluctuation may play an important role in pairing symmetry in superconducting materials AyFe2−xSe2(A=Rb,Cs,K, etc.).