Electronic structure of the Hg-O double-layer compoundHg2Ba2YCu2O8

Abstract

${\mathrm{Hg}}_{2}{\mathrm{Ba}}_{2}{\mathrm{Y}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{Cu}}_{2}{\mathrm{O}}_{8\ensuremath{-}\ensuremath{\delta}}$ contains a double HgO layer with a rock-salt structure. For $x=0.3,$ 0.4, and 0.5 the samples have metallic resistance and become superconductors at low temperatures. The $x=0$ prototype is an insulator. In order to provide the basic ingredients for the understanding of the normal and superconducting properties, the electronic structure of body-centered tetragonal ${\mathrm{Hg}}_{2}{\mathrm{Ba}}_{2}{\mathrm{YCu}}_{2}{\mathrm{O}}_{8}$ has been calculated using the full-potential linear augmented plane wave method, within the local approximation to density-functional theory. States close to the Fermi level ${(E}_{F})$ are characterized by two antibonding $\mathrm{Cu}\ensuremath{-}{d}_{{x}^{2}\ensuremath{-}{y}^{2}}\ensuremath{-}\mathrm{O}1\ensuremath{-}p$ bands (one per Cu-O plane) and the presence of another pair, derived from $\mathrm{Hg}\ensuremath{-}d\ensuremath{-}\mathrm{O}2\ensuremath{-}\mathrm{O}3\ensuremath{-}p.$ The states of $\mathrm{Hg}\ensuremath{-}p$ character, characteristic of mono Hg layer superconductors, are pushed well above ${E}_{F}.$ The $\mathrm{Hg}\ensuremath{-}d\ensuremath{-}\mathrm{O}\ensuremath{-}p$ band is not totally occupied and the resulting hole doping of the Cu-O planes is dimished from that expected from ionic considerations. Our results are compared with the previously obtained electronic structure for the isostructural ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8}$ and ${\mathrm{Tl}}_{2}{\mathrm{Ba}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8}$ compounds, to the stoichiometric $(\ensuremath{\delta}=0)$ one-layer parent compounds of the Hg high-${T}_{c}$ superconductor and in particular with the nearly optimally doped ${\mathrm{HgBa}}_{2}{\mathrm{Ca}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{8+\ensuremath{\delta}},$ $\ensuremath{\delta}=0.5.$ Finally, the electric-field gradients at each atomic site are reported.