  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.
