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

摘要

We have performed photoemission and soft x-ray absorption studies of pyrochlore-type Ru oxides, namely, the filling-control system Sm_(2-x)Ca_xRu_2O_7 and the bandwidth-control system Sm_(2-x)Bi_xRu_2O_7, which show insulator-to-metal transition with increasing Ca and Bi concentration, respectively. Core levels and the O 2p valence band in Sm_(2-x)Ca_xRu_2O_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 Sm_(2-x)Ca_xRu_2O_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 Sm_(2-x)Bi_xRu_2O_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 Ad _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 Sm_(2-x)Bi_xRu_2O_7 than in the filling-control Sm_(2-x)Ca_xRu_2O_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.