Heavy fermions, metal-to-insulator transition, and quantum criticality in La y Cu3Ru x Ti4−x O12

Abstract

In this work we investigate the solid-solution series La y Cu3Ru x Ti4−x O12. The titanate La2/3Cu3Ti4O12 (x = 0) is an antiferromagnetic insulator exhibiting colossal dielectric constants, while the ruthenate LaCu3Ru4O12 (x = 4) is known as a rare d-electron derived heavy-fermion compound. Detailed structural investigations, AC- and DC-magnetization measurements, resistivity, specific-heat, and magnetic-resonance investigations have been performed for all polycrystalline compounds prepared by solid-state synthesis. These experiments have been accompanied by band-structure calculations. Close to the Ru concentration x = 2 we identify a quantum-critical point coinciding with a metal-to-insulator transition. The quantum-critical point separates an insulating spin glass from a paramagnetic metal. Interestingly, there is no evidence for a divergence of the effective mass upon reaching the quantum-critical point from the metallic side. In the paramagnetic metal, Ru behaves like a canonical Kondo ion. While the Ru oxidation state remains stable at + 4 for the whole concentration regime, the Cu valence seems to decrease from + 2 in the insulating antiferromagnet with localized copper spins to a significantly lower value in the metallic heavy-fermion compounds.