CaCu$_3$Ru$_4$O$_{12}$: a high Kondo-temperature transition metal oxide

TL;DR

This study combines spectroscopy and theoretical calculations to reveal that CaCu$_3$Ru$_4$O$_{12}$ is a high-temperature Kondo system with a Kondo temperature between 500-1000 K, involving correlated Cu and Ru ions.

Contribution

It provides a comprehensive experimental and theoretical analysis demonstrating CaCu$_3$Ru$_4$O$_{12}$ as a high Kondo-temperature transition metal oxide with detailed insights into its electronic correlations.

Findings

CaCu$_3$Ru$_4$O$_{12}$ exhibits magnetic Cu$^{2+}$ ions hybridized with itinerant electrons.

The Kondo temperature is estimated to be between 500 and 1000 K.

The material is classified as a Kondo system based on susceptibility calculations.

Abstract

We present a comprehensive study of CaCu$_3$Ru$_4$O$_{12}$ using bulk sensitive hard and soft x-ray spectroscopy combined with local-density approximation (LDA) + dynamical mean-field theory (DMFT) calculations. Correlation effects on both the Cu and Ru ions can be observed. From the Cu $2p$ core level spectra we deduce the presence of magnetic Cu$^{2+}$ ions hybridized with a reservoir of itinerant electrons. The strong photon energy dependence of the valence band allows us to disentangle the Ru, Cu, and O contributions and thus to optimize the DMFT calculations. The calculated spin and charge susceptibilities show that the transition metal oxide CaCu$_3$Ru$_4$O$_{12}$ must be classified as a Kondo system and that the Kondo temperature is in the range of 500-1000 K.