Heavy-Fermion like Correlations and Crystal-Field Excitations in Copper-Ruthenates
Among the large class of A-site ordered perovskites of stoichiometry AC3B4O12, the copper-ruthenates ACu3Ru4O12 (A = Na, Ca, Sr, La, Pr, Nd) are metallic compounds and reveal heavy-fermion (HF) behavior with a quadratic T-dependence of the low-temperature electrical resistivity and a significantly enhanced Sommerfeld-coefficient. Both the Kadowaki-Woods and the Wilson ratio localize them among conventional f-electron based HF systems. With the facts that electronic doping at the A-site does not weaken HF formation and that the Ru valence stays stable at 4+, one can conclude that a Kondo lattice is built up by Ru and screened by copper electrons. This places these compounds among the few d-electron based ones in which the HF behavior is caused by the Kondo effect and not by frustration. While the symmetry is conserved in all compounds between room temperature and 2K, an abrupt change in the lattice constant at 154K is observed in CaCu3Ru4O12 and together with susceptibility and specific heat data identified in terms of a valence transition. Furthermore, CaCu3Ru4O12 exhibits non-Fermi liquid properties below 2K. For A=Pr and Nd, the physics is dominated by the local moment of the rare earth ion including crystal-field effects. These are visible in susceptibility, heat capacity and inelastic neutron scattering data and imply relatively small splittings.