Evolution of the electronic structure across the filling-control and bandwidth-control metal-insulator transitions in pyrochlore-type Ru oxides

Abstract

We have performed photoemission and soft x-ray absorption studies of pyrochlore-type Ru oxides, namely, the filling-control system ${\mathrm{Sm}}_{2\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{Ru}}_{2}{\mathrm{O}}_{7}$ and the bandwidth-control system ${\mathrm{Sm}}_{2\ensuremath{-}x}{\mathrm{Bi}}_{x}{\mathrm{Ru}}_{2}{\mathrm{O}}_{7}$, which show insulator-to-metal transition with increasing Ca and Bi concentration, respectively. Core levels and the O $2p$ valence band in ${\mathrm{Sm}}_{2\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{Ru}}_{2}{\mathrm{O}}_{7}$ show almost the same amount of monotonous upward energy shifts with Ca concentration, which indicates that the chemical potential is shifted downward due to hole doping. The Ru $4d$ band in ${\mathrm{Sm}}_{2\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{Ru}}_{2}{\mathrm{O}}_{7}$ is also shifted toward the Fermi level $({E}_{F})$ with hole doping and the density of states (DOS) at ${E}_{F}$ increases. The core levels in ${\mathrm{Sm}}_{2\ensuremath{-}x}{\mathrm{Bi}}_{x}{\mathrm{Ru}}_{2}{\mathrm{O}}_{7}$, on the other hand, do not show clear energy shifts except for the Ru $3d$ core level, whose line shape change also reflects the increase of metallic screening with Bi concentration. We observe pronounced spectral weight transfer from the incoherent to the coherent parts of the Ru $4d\phantom{\rule{0.3em}{0ex}}{t}_{2g}$ band with Bi concentration, which is expected for a bandwidth-control Mott-Hubbard system. The increase of the DOS at ${E}_{F}$ is more abrupt in the bandwidth-control ${\mathrm{Sm}}_{2\ensuremath{-}x}{\mathrm{Bi}}_{x}{\mathrm{Ru}}_{2}{\mathrm{O}}_{7}$ than in the filling-control ${\mathrm{Sm}}_{2\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{Ru}}_{2}{\mathrm{O}}_{7}$, in accordance with a recent theoretical prediction. Effects of charge transfer between the Bi $6sp$ band and the Ru $4d$ band are also discussed.