Chalcogen-height dependent magnetic interactions and magnetic order switching in FeSex_xTe1−x_{1-x}

Magnetic properties of iron chalcogenide superconducting materials are investigated using density functional calculations. We find the stability of magnetic phases is very sensitive to the height of chalcogen species from the Fe plane: while FeTe with optimized Te height has the double-stripe-type $(\pi,0)$ magnetic ordering, the single-stripe-type $(\pi,\pi)$ ordering becomes the ground state phase when Te height is lowered below a critical value by, e.g., Se doping. This behavior is understood by opposite Te-height dependences of the superexchange interaction and a longer-range magnetic interaction mediated by itinerant electrons. We also demonstrate a linear temperature dependence of the macroscopic magnetic susceptibility in the single-stripe phase in contrast to a constant behavior in the double-stripe phase. Our findings provide a comprehensive and unified view to understand the magnetism in FeSe$_x$Te$_{1-x}$ and iron pnictide superconductors.Comment: 4 pages, 4 figure

Graphyne: Hexagonal network of carbon with versatile Dirac cones

We study alpha, beta, and gamma graphyne, a class of graphene allotropes with carbon triple bonds, using a first-principles density-functional method and tight-binding calculation. We find that graphyne has versatile Dirac cones and it is due to remarkable roles of the carbon triple bonds in electronic and atomic structures. The carbon triple bonds modulate effective hopping matrix elements and reverse their signs, resulting in Dirac cones with reversed chirality in alpha graphyne, momentum shift of the Dirac point in beta graphyne, and switch of the energy gap in gamma graphyne. Furthermore, the triple bonds provide chemisorption sites of adatoms which can break sublattice symmetry while preserving planar sp2-bonding networks. These features of graphyne open new possibilities for electronic applications of carbon-based two-dimensional materials and derived nanostructures.Comment: 5 pages, 5 figures, 1 tabl