Kondo-type behavior of theRu4+lattice inLaCu3Ru4O12

Rare $d$-electron-derived heavy-fermion properties of the solid-solution series ${\mathrm{LaCu}}_{3}{\mathrm{Ru}}_{x}{\mathrm{Ti}}_{4\ensuremath{-}x}{\mathrm{O}}_{12}$ were studied for $1\ensuremath{\le}x\ensuremath{\le}4$ by resistivity, susceptibility, specific-heat measurements, and magnetic-resonance techniques. Our results suggest the existence of a coherent Kondo lattice formed by localized Ru $4d$ electrons leading to strongly enhanced effective electron masses. Pure ruthenate $(x=4)$ is a heavy-fermion metal characterized by a resistivity proportional to ${T}^{2}$ at low temperatures $T$. By increasing titanium substitution the coherent Fermi-liquid state is disturbed, yielding single-ion Kondo-type properties as in the paradigm $4f$-based heavy-fermion compound ${\mathrm{Ce}}_{x}{\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Cu}}_{2.05}{\mathrm{Si}}_{2}$ [M. Ocko et al., Phys. Rev. B 64, 195106 (2001)]. In ${\mathrm{LaCu}}_{3}{\mathrm{Ru}}_{x}{\mathrm{Ti}}_{4\ensuremath{-}x}{\mathrm{O}}_{12}$ the heavy-fermion behavior finally breaks down upon crossing the metal-to-insulator transition close to $x=2$.