Electronic structure of ${\mathrm{CaCu}}_3{\mathrm{Ru}}_4{\mathrm{O}}_{12}$ studied by x-ray photoemission spectroscopy

We have studied the electronic structure of the $d$-electron heavy-fermion system ${\mathrm{CaCu}}_3{\mathrm{Ru}}_4{\mathrm{O}}_{12}$ using x-ray photoemission spectroscopy and a cluster model calculation. The $\mathrm{Ru}3d$ core level spectrum shows a double-peak structure as commonly observed in metallic Ru oxides. In ${\mathrm{CaCu}}_3{\mathrm{Ru}}_4{\mathrm{O}}_{12}$, the well-screened peak has dominating intensity, indicating that the $\mathrm{Ru}4d$ electrons in ${\mathrm{CaCu}}_3{\mathrm{Ru}}_4{\mathrm{O}}_{12}$ are highly itinerant. On the other hand, the $\mathrm{Cu}2p_{3∕2}$ core level peak is accompanied by a satellite and shows that the valence state of Cu is close to $3d^9$ $\left({\mathrm{Cu}}^{2+}\right)$ with localized character. In addition, the main $\mathrm{Cu}2p_{3∕2}$ peak shows an asymmetric line shape due to the screening effect, suggesting the hybridization effect between the $\mathrm{Cu}3d$ and $\mathrm{Ru}4d$ orbitals. The present results show that, among the $d$-electron heavy-fermion materials, the electronic structure of ${\mathrm{CaCu}}_3{\mathrm{Ru}}_4{\mathrm{O}}_{12}$ best resembles that of the $f$-electron Kondo system.

Figure 1

Schematic drawing of the crystal structure of ${\mathrm{CaCu}}_3{\mathrm{Ru}}_4{\mathrm{O}}_{12}$. The arrow indicates the charge transfer from the $\mathrm{O}2p$ to $\mathrm{Cu}3d$ orbitals.

Figure 2

Valence band XPS spectra of ${\mathrm{CaCu}}_3{\mathrm{Ru}}_4{\mathrm{O}}_{12}$ (closed circles) and ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ (open squares).

Figure 3

$\mathrm{Ru}3d$ core level XPS spectra of ${\mathrm{CaCu}}_3{\mathrm{Ru}}_4{\mathrm{O}}_{12}$ (closed circles) and ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ (open squares).

Figure 4

$\mathrm{Cu}2p$ photoemission spectrum of ${\mathrm{CaCu}}_3{\mathrm{Ru}}_4{\mathrm{O}}_{12}$ (closed circles) and the cluster model analysis (dotted curve).