Intermediate-Valence Behavior in the Transition-Metal Oxide CaCu$_3$Ru$_4$O$_{12}$

A detailed study of the electronic properties of strongly correlated CCRO will be presented. Along with transport, $\chi(T)$ and specific heat, we focus on NMR/NQR. CCRO is a metallic system with perovskite structure showing non-Fermi-liquid behavior below 2${\,}$K which is inferred from $C_p/{\,}T\propto -ln(T)$ and a deviation of $1/{\,}T_1 (T)$ from the Korringa law at the copper site [PRB \textbf{78}, 165126 (2008)]. On closer inspection, a volume change could be detected by neutron diffraction which comes along with a corresponding anomaly in the specific heat around 150${\,}$K. Furthermore, the $^{99}$Ru Knight shift shows a cross-over between paramagnetic behavior of localized moments at high $T$ and itinerant band states at low $T$, respectively. Complementary density-functional calculations (LDA+DFT) relate these phenomena to the ruthenium \emph{d}-electron number. We conclude that dynamic charge fluctuations originating from the strong electronic correlations are present in CCRO and give rise to the intermediate valence of the ruthenium ions [PRB \textbf{80}, 121101(R) (2009)].