Electronic structure of CaCu3 Ru4 O12 studied by x-ray photoemission spectroscopy

We have studied the electronic structure of the d -electron heavy-fermion system CaCu3 Ru4 O12 using x-ray photoemission spectroscopy and a cluster model calculation. The Ru 3d core level spectrum shows a double-peak structure as commonly observed in metallic Ru oxides. In CaCu3 Ru4 O12, the well-screened peak has dominating intensity, indicating that the Ru 4d electrons in CaCu3 Ru4 O12 are highly itinerant. On the other hand, the Cu 2 p3 2 core level peak is accompanied by a satellite and shows that the valence state of Cu is close to 3 d9 (Cu2+) with localized character. In addition, the main Cu 2 p3 2 peak shows an asymmetric line shape due to the screening effect, suggesting the hybridization effect between the Cu 3d and Ru 4d orbitals. The present results show that, among the d -electron heavy-fermion materials, the electronic structure of CaCu3 Ru4 O12 best resembles that of the f -electron Kondo system.