A Novel Heavy-Fermion State in CaCu₃Ru₄O₁₂

We have measured susceptibility, specific heat, resistivity, and thermopower of CaCu₃Ti₄₋ₓRuₓO₁₂ and CaCu3-yMnyRu₄O₁₂, and have found that CaCu₃Ru₄O₁₂ can be regarded as a heavy-fermion oxide in d-electron systems. The Kondo temperature is near 200 K, and the susceptibility (1.4×10⁻³ emu/Cu mol) and the electron specific heat coefficient (28 mJ/Cu molK²) are moderately enhanced. The resistivity is proportional to T² at low temperatures, and satisfies the Kadowaki-Woods relation. The heavy-fermion state comes from the interaction between the localized moment of Cu 3d and the conduction electron of Ru 4d. An insulator-metal transition occurs between $x=1.5$ and 4 in CaCu₃Ti₄₋ₓRuₓO₁₂, which can be regarded as a transition from magnetic insulator to heavy-fermion metal.