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Journal Articles

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

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

Physical Review B, 73(3), p.035127_1 - 035127_6, 2006/01

 Times Cited Count:2 Percentile:13.12(Materials Science, Multidisciplinary)

We have performed photoemission and soft X-ray absorption studies of pyrochlore-type Ru oxides, namely, the filling-control system Sm$$_{2-x}$$Ca$$_x$$Ru$$_2$$O$$_7$$ and the bandwidth-control system Sm$$_{2-x}$$Bi$$_x$$Ru$$_2$$O$$_7$$, which show insulator-to-metal transition with increasing Ca and Bi concentration, respectively. Core levels and the O 2$$p$$ valence band in Sm$$_{2-x}$$Ca$$_x$$Ru$$_2$$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 4$$d$$ band in Sm$$_{2-x}$$Ca$$_x$$Ru$$_2$$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 Sm$$_{2-x}$$Bi$$_x$$Ru$$_2$$O$$_7$$, on the other hand, do not show clear energy shifts except for the Ru 3$$d$$ 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 $$4d$$ $$t_{2g}$$ 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$$_x$$Ru$$_2$$O$$_7$$ than in the filling-control Sm$$_{2-x}$$Ca$$_x$$Ru$$_2$$O$$_7$$, in accordance with a recent theoretical prediction. Effects of charge transfer between the Bi 6$$sp$$ band and the Ru 4$$d$$ band are also discussed.

Journal Articles

Antiferromagnetic-to-ferromagnetic transition induced by diluted Co in SrFe$$_{1-x}$$Co$$_x$$O$$_3$$:Magnetic circular X-ray dichroism study

Okamoto, Jun*; Mamiya, Kazutoshi*; Fujimori, Shinichi; Okane, Tetsuo; Saito, Yuji; Muramatsu, Yasuji*; Yoshii, Kenji; Fujimori, Atsushi*; Tanaka, Arata*; Abbate, M.*; et al.

Physical Review B, 71(10), p.104401_1 - 104401_5, 2005/03

 Times Cited Count:20 Percentile:63(Materials Science, Multidisciplinary)

The antiferromagnetic-to-ferromagnetic transition in SrFe$$_{1-x}$$Co$$_x$$O$$_3$$ ($$x_c$$$$simeq$$0.15) induced by Co doping has been studied by magnetic circular X-ray dichroism. The orbital and spin magnetic moments of the Fe and Co 3$$d$$ states under the magenetic field of 2 T are found to show different $$x$$ dependences: The spin polarization of Fe 3$$d$$ gradually increases with Co concentration; On the other hand, a large spin polarization of Co 3$$d$$ is induced already in the antiferromagnetic phase, indicating that the Co moment is nearly fully alligned already in the antiferromagnetic phase. This suggests that the alignment of the Fe magnetic moment in SrFe$$_{1-x}$$Co$$_x$$O$$_3$$ is induced by interaction with doped Co magnetic moment. Possible formation of ferromagnetic Co rich region is discussed.

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