Effective mass enhancement in B site Fe doping heavy fermion compound CaCu3Ru4O12

Polycrystalline samples of CaCu3Ru4−xFexO12 (x=0, 0.04, 0.1, 0.2) were synthesized via conventional solid state reactions. The specific heats of these four compounds have been investigated at temperature down to 2 K. Following the Fe doping, the linear electronic specific heat coefficient increased from 90 mJ/(f.u. mol K2) for x=0 to 271 mJ/(f.u. mol) for x=0.2, indicating an effective mass enhancement. We propose that such enhancement mainly comes from the additional Kondo coupling between the itinerant electrons in Ru 4d orbitals and the localized electrons in 3d orbitals of Fe dopants. Moreover, below a certain temperature, an upturn appears in the Cp/T-T2 curves for all the Fe doped samples CaCu3Ru4−xFexO12 with x≤0.2. The origin of this upturn may be a new magnetic order state which needs to be identified with further investigations.