Abstract
We investigate the electronic structure of the ternary iron selenide K y Fe 1.6 Se 2 by considering the spatial symmetry of the √5×√5 vacancy-ordered structure. Based on three orbitals of t 2g , which are believed to play major physics in iron-based superconductors, an effective two-dimensional tight-binding Hamiltonian is constructed with the vacancy-ordered structure being explicitly included. It is shown that the constructed band model, when combined with generalized Hubbard interactions, yields a spin susceptibility that exhibits both the block-checkerboard antiferromagnetism instability and the stripe antiferromagnetism instability. In particular, for large Hund's rule couplings, the block-checkerboard antiferromagnetism wins over the stripe antiferromagnetism, in agreement with the observation in experiments. We argue that such a model with correct symmetry and Fermi surface structures should be the starting point to model K y Fe 1.6 Se 2 . The spin fluctuations at q=(π,π) suggest that interblock fluctuations of spins might play an important role in the mechanism of superconductivity occurring in this system. © 2011 American Physical Society.