Multi-orbital Kondo physics of Co in Cu hosts

TL;DR

This study combines advanced computational methods to analyze the multi-orbital Kondo effect of cobalt impurities in copper, revealing the importance of orbital fluctuations and accurately predicting the Kondo temperature.

Contribution

It introduces a comprehensive approach using density functional theory and quantum Monte Carlo to model multi-orbital Kondo physics in Co/Cu systems, emphasizing orbital fluctuations.

Findings

Low energy resonances in all five Co d-orbitals.

Formation of a Fermi liquid state at low temperatures.

Calculated Kondo temperatures agree with experiments.

Abstract

We investigate the electronic structure of cobalt atoms on a copper surface and in a copper host by combining density functional calculations with a numerically exact continuous-time quantum Monte Carlo treatment of the five-orbital impurity problem. In both cases we find low energy resonances in the density of states of all five Co $d$-orbitals. The corresponding self-energies indicate the formation of a Fermi liquid state at low temperatures. Our calculations yield the characteristic energy scale -- the Kondo temperature -- for both systems in good agreement with experiments. We quantify the charge fluctuations in both geometries and suggest that Co in Cu must be described by an Anderson impurity model rather than by a model assuming frozen impurity valency at low energies. We show that fluctuations of the orbital degrees of freedom are crucial for explaining the Kondo temperatures obtained in our calculations and measured in experiments.