Heavy-Fermion like Correlations and Crystal-Field Excitations in Copper-Ruthenates with Perovskite-related Structure

Among the large class of A-site ordered perovskites of stoichiometry AC3B4O12, this work focuses on detailed investigations of the copper-ruthenates ACu3Ru4O12 (A = Na, Ca, Sr, La, Pr, Nd). They were found to belong to the few examples of purely d-electron based transition-metal compounds showing heavy-fermion properties. It is well established that ruthenates reveal a rather complex and unpredictable behavior and during the last decade experiments on ruthenates have provided a wealth of interesting discoveries. The thesis is especially motivated as compounds within the vast class of heavy-fermion compounds exhibit outstanding properties like superconductivity, the proximity to a quantum critical point, and other exotic ground states. Experiments were carried out utilizing x-ray diffraction, neutron diffraction and spectroscopy, magnetic and resistivity measurements, as well as heat capacity and muon spectroscopy techniques. A broad range of external parameters is scanned and depending on the specific technique, a temperature range from 50mK to 400K is covered and external magnetic fields up to 14T are applied. All compounds investigated are metallic paramagnets with an enhanced Pauli-susceptibility, a quadratic temperature-dependence of the low-temperature electrical resistivity and a significantly enhanced Sommerfeld coefficient consistently manifesting a heavy-fermion state. Toward low temperatures, CaCu3Ru4O12 exhibits non-Fermi liquid properties visible in a logarithmic divergence of the heat capacity and a positive linear temperature-dependence of the resistivity. Moreover, an intermediate-valence transition in CaCu3Ru4O12 can be related to an unusual spontaneous structural contraction appearing at 154K on cooling without affecting the symmetry. It is visible as abrupt change in the lattice constant, a broad feature in the susceptibility, and as tiny peak in the specific heat. The inverse magnetic susceptibility of the Sr and the La compound exhibits an unusual linear decrease over a wide temperature range while the compounds with A = Pr and Nd are dominated by the local moment of the rare earth ions including CEF effects, also clearly visible as Schottky peaks in the heat capacity and as dispersionless inelastic intensities in the dynamic structure factor