We have investigated heavy-fermion behavior of the transition-metal oxides $A{\text{Cu}}_3{\text{Ru}}_4{\text{O}}_{12}$ ($A=\text{Na},\text{Ca},\text{La}$, and their mixtures). It has been known that ${\text{CaCu}}_3{\text{Ru}}_4{\text{O}}_{12}$ exhibits Kondo-like behavior attributable to ${\text{Cu}}^{2+}3d$ electrons, similar to that of some Ce-based heavy-fermion systems. However, we find striking suppression of the mass enhancement in ${\text{CaCu}}_3{\text{Ru}}_4{\text{O}}_{12}$, in which the Kondo-type effect is most pronounced. Such decrease in the density of states is reminiscent of the coherent-gap formation in Kondo lattice systems. Nevertheless, the behavior can not be interpreted within the conventional Kondo picture with localized moments because the Cu electrons are apparently itinerant. The present results indicate the importance of the duality of localized and itinerant nature, found also in some other $d$-electron systems which exhibit the Kondo-like behavior.

• Received 26 March 2009

DOI:https://doi.org/10.1103/PhysRevB.80.035113