Eur. Phys. J. Special Topics 224, 1061–1086 (2015)
https://doi.org/10.1140/epjst/e2015-02446-3

Review

Heavy fermions, metal-to-insulator transition, and quantum criticality in La_yCu₃Ru_xTi_4−xO₁₂

¹ Experimental Physics V, Center for Electronic Correlations and Magnetism, University of Augsburg, 86159 Augsburg, Germany
² Solid State Chemistry, Martin-Luther-University Halle-Wittenberg, 06099 Halle, Germany
³ Resource Strategy, University of Augsburg, 86159 Augsburg, Germany

^a e-mail: alois.loidl@physik.uni-augsburg.de

Received: 14 November 2014
Revised: 15 May 2015
Published online: 22 July 2015

Abstract

In this work we investigate the solid-solution series La_yCu₃Ru_xTi_4−xO₁₂. The titanate La_2/3Cu₃Ti₄O₁₂ (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.