Evidence for a Ru$^{4+}$ Kondo Lattice in LaCu$_3$Ru$_4$O$_{12}$

TL;DR

This study demonstrates heavy-fermion behavior in LaCu3Ru4O12 due to a Kondo lattice of Ru 4d electrons, which is gradually diluted by Ti substitution, leading to a breakdown of heavy-fermion properties near the metal-insulator transition.

Contribution

It provides experimental evidence for a Ru4+ Kondo lattice in a 3d transition metal oxide and explores its evolution with Ti substitution.

Findings

Heavy-fermion behavior observed in LaCu3Ru4O12.

Kondo lattice is diluted with Ti substitution.

Heavy-fermion properties break down near the metal-insulator transition.

Abstract

Rare $d$-electron derived heavy-fermion properties of the solid-solution series LaCu$_3$Ru$_x$Ti$_{4-x}$O$_{12}$ were studied for $1 \leq x \leq 4$ by resistivity, susceptibility, specific-heat measurements, and magnetic-resonance techniques. The pure ruthenate ($x = 4$) is a heavy-fermion metal characterized by a resistivity proportional to $T^2$ at low temperatures $T$. The coherent Kondo lattice formed by the localized Ru 4$d$ electrons is screened by the conduction electrons leading to strongly enhanced effective electron masses. By increasing titanium substitution the Kondo lattice becomes diluted resulting in single-ion Kondo properties like in the paradigm $4f$-based heavy-fermion compound Ce$_x$La$_{1-x}$Cu$_{2.05}$Si$_2$ [M. Ocko {\em et al.}, Phys. Rev. B \textbf{64}, 195106 (2001)]. In LaCu$_3$Ru$_x$Ti$_{4-x}$O$_{12}$ the heavy-fermion behavior finally breaks down on crossing the metal-to-insulator transition close to $x = 2$.