Block Spin Ground State and Three-Dimensionality of (K, Tl)yFe1.6Se2

The magnetic properties and electronic structure of (K, Tl)(y)Fe1.6Se2 is studied using first-principles calculations. The ground state is checkerboard antiferromagnetically coupled blocks of the minimal Fe-4 squares, with a large block-spin moment similar to 11.2 mu(B). The magnetic interactions could be modeled with a simple spin model involving both the inter-and intrablock, as well as the nearest-neighbor and next-nearest-neighbor couplings. The calculations also suggest a metallic ground state except for y = 0: 8 where a band gap similar to 400-550 meV opens, showing an antiferromagnetic insulator ground state for (K, Tl)(y)Fe1.6Se2. The electronic structure of the metallic (K, Tl)(y)Fe1.6Se2 is highly three dimensional with unique Fermi surface structure and topology. These features indicate that the Fe-vacancy ordering is crucial to the physical properties of (K, Tl)(y)Fe1.6Se2.