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

パイロクロアRu酸化物におけるフィリング・バンド幅制御による電子状態の変化

岡本 淳*; 藤森 伸一   ; 岡根 哲夫  ; 藤森 淳*; Abbate, M.*; 吉居 俊輔*; 佐藤 正俊*

Okamoto, Jun*; Fujimori, Shinichi; Okane, Tetsuo; Fujimori, Atsushi*; Abbate, M.*; Yoshii, Shunsuke*; Sato, Masatoshi*

絶縁体SmRuOにおいてフィリング制御(SmCaRuO)とバンド幅制御(SmBiRuO)による金属絶縁体転移が電子構造に及ぼす影響を光電子分光と軟X線吸収分光を用いて系統的に比較した。Fermi準位近傍の状態密度は双方とも抵抗の変化に応じた変化を示したが、Ru 4バンド全体ではフィリング制御ではリジッドバンド的なシフトが観測されたのに対し、バンド幅制御ではスペクトル強度の移行が観測された。内殻準位の化学ポテンシャルのシフトにおいては、フィリング制御では正孔のドープに対応した単調なシフトが見られたのに対し、バンド幅制御ではRu 3準位以外は単調な変化を示さなかった。

We have performed photoemission and soft X-ray absorption studies of pyrochlore-type Ru oxides, namely, the filling-control system SmCaRuO and the bandwidth-control system SmBiRuO, which show insulator-to-metal transition with increasing Ca and Bi concentration, respectively. Core levels and the O 2 valence band in SmCaRuO 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 4 band in SmCaRuO is also shifted toward the Fermi level () with hole doping and the density of states (DOS) at increases. The core levels in SmBiRuO, on the other hand, do not show clear energy shifts except for the Ru 3 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 band with Bi concentration, which is expected for a bandwidth-control Mott-Hubbard system. The increase of the DOS at is more abrupt in the bandwidth-control SmBiRuO than in the filling-control SmCaRuO, in accordance with a recent theoretical prediction. Effects of charge transfer between the Bi 6 band and the Ru 4 band are also discussed.