Non-Fermi-liquid behaviour in La₄Ru₆O₁₉

Understanding the complexities of electronic and magnetic ground states in solids is one of the main goals of solid-state physics. Transition-metal oxides have proved to be particularly fruitful in this regard, especially for those materials with the perovskite structure, where the special characteristics of transition-metal-oxygen orbital hybridization determine their properties. Ruthenates have recently emerged as an important family of perovskites because of the unexpected evolution from high-temperature ferromagnetism in SrRuO₃ to low-temperature superconductivity in Sr₂RuO₄ (refs 1, 2). Here we show that a ruthenate in a different structural family, La₄Ru₆O₁₉, displays a number of highly unusual properties, most notably non-Fermi-liquid behaviour. The properties of La₄Ru₆O₁₉ have no analogy among the thousands of previously characterized transitionmetal oxides. Instead, they resemble those of CeCu_6-xAu_x -5 a widely studied f-electron-based heavy fermion intermetallic compound that is often considered as providing the best example of non-Fermi-liquid behaviour. In the ruthenate, non-Fermi-liquid behaviour appears to arise from just the right balance between the interactions of localized electronic states derived from Ru-Ru bonding and delocalized states derived from Ru-O hybridization.