title: Block spin ground state and three-dimensionality of (K,Tl)(y)Fe(1.6)Se2.
abstract: The magnetic properties and electronic structure of (K,Tl)(y)Fe(1.6)Se(2) 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 ∼11.2 μ(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 ∼400-550 meV opens, showing an antiferromagnetic insulator ground state for (K,Tl)(0.8)Fe(1.6)Se(2). The electronic structure of the metallic (K,Tl)(y)Fe(1.6)Se(2) 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)Fe(2-x)Se(2).
authors.affiliation: Condensed Matter Physics Group, Department of Physics, Hangzhou Normal University, Hangzhou 310036, China.
authors.forename: Chao
authors.initials: C
authors.lastname: Cao
authors.forename: Jianhui
authors.initials: J
authors.lastname: Dai