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

For a newly discovered iron-based high T c superconducting parent material KFe 2 Se 2 , we present an effective three-dimensional five-orbital tight-binding model by fitting the band structures. The three t 2 g -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 Neel-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 A y Fe 2 − x Se 2 ( A = Rb , Cs , K , etc.).