Suppression of the Mass Enhancement in CaCu3Ru4O12

We have investigated heavy-fermion behavior of the transition-metal oxides ACu3Ru4O12 (A = Na, Ca, La, and their mixtures). It has been known that CaCu3Ru4O12 exhibits Kondo-like behavior attributable to Cu2+ 3d electrons, similar to that of some Ce-based heavy-fermion systems. However, we find striking "suppression" of the mass enhancement in CaCu3Ru4O12, in which the Kondo-like effect is most pronounced. Such decrease of 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.