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

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.

Accepted for publication in Phys. Rev. X