A Novel Heavy-Fermion State in CaCu3Ru4O12

We have measured the susceptibility, specific heat, resistivity, and thermopower of CaCu 3 Ti 4- x Ru x O 12 and CaCu 3- y Mn y Ru 4 O 12 , and have found that CaCu 3 Ru 4 O 12 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 mol K 2 )] are moderately enhanced. The resistivity is proportional to T 2 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 CaCu 3 Ti 4- x Ru x O 12 , which can be regarded as a transition from a magnetic insulator to a heavy-fermion metal.