Competition between heavy-fermion and Kondo interaction in isoelectronic A-site ordered perovskites
With current research efforts shifting towards the 4$d$ and 5$d$ transition metal oxides, understanding the evolution of the electronic and magnetic structure as one moves away from 3$d$ materials is of critical importance. Here we perform X-ray spectroscopy and electronic structure calculations on $A$-site ordered perovskites with Cu in the $A$-site and the $B$-sites descending along the 9th group of the periodic table to elucidate the emerging properties as $d$-orbitals change from partially filled 3$d$, 4$d$, to 5$d$. The results show that when descending from Co to Ir the charge transfers from the cuprate like Zhang-Rice state on Cu to the t$_{2g}$ orbital of the B site. As the Cu $d$-orbital occupation approaches the Cu$^{2+}$ limit, a mixed-valence state in CaCu$_3$Rh$_4$O$_{12}$ and heavy fermion state in CaCu$_3$Ir$_4$O$_{12}$ are obtained. The investigated d-electron compounds are mapped onto the Doniach phase diagram of the competing RKKY and Kondo interactions developed for f-electron systems.