Quantum Criticality in Transition-Metal Oxides

We report on experiments of the bulk susceptibility χ ( T ), heat capacity C ( T )/ T , resistivity ρ ( T ) and nuclear resonances (NMR and NQR) in order to review evidence of quantum critical behaviour in some metallic transition-metal oxides. In analogy to the conventional 4 f - and 5 f -electron based heavy-fermion compounds, the prerequisites of quantum criticality, i.e. a magnetic phase transition at T =0 accompanied by non-Fermi liquid behaviour, are observed in LiV 2 O 4 which was the first transition-metal oxide showing heavy-fermion properties. Furthermore, we investigate the large group of copper-ruthenates of the composition A Cu 3 Ru x Ti 4− x O 12 which also demonstrate heavy-fermion and non-Fermi liquid properties. For LaCu 3 Ru x Ti 4− x O 12 we establish a magnetic phase diagram where the substitution of titanium by ruthenium cations suppresses long-range magnetic order in favour of a magnetic quantum critical point, which comes along with a metal-to-insulator transition. Our experiments elucidate open questions concerning the nature of the heavy-fermion properties, intermediate valence, and the origin of the metal-to-insulator transition. Specifically, the role of the ruthenium and copper d -electrons is addressed: the metal-to-insulator transition turns out to correlate with the increasing itinerancy of local moments at the Cu site in our doping experiments.