Exploring the phase diagram of the two-impurity Kondo problem

TL;DR

This study investigates the phase diagram of two coupled Kondo impurities using Co atoms on Cu2N/Cu(100), revealing a new phase where both impurities are simultaneously screened under strong magnetic fields, enabled by low Kondo temperatures.

Contribution

It demonstrates the ability to explore the two-impurity Kondo phase diagram experimentally using low-temperature atomic systems and magnetic fields, revealing a previously inaccessible screening phase.

Findings

Identification of a new phase with simultaneous impurity screening

Successful experimental access to different phase regions via magnetic fields

Quantitative analysis of Kondo screening using differential conductance and modeling

Abstract

A system of two exchange-coupled Kondo impurities in a magnetic field gives rise to a rich phase space hosting a multitude of correlated phenomena. Magnetic atoms on surfaces probed through scanning tunnelling microscopy provide an excellent platform to investigate coupled impurities, but typical high Kondo temperatures prevent field-dependent studies from being performed, rendering large parts of the phase space inaccessible. We present an integral study of pairs of Co atoms on insulating Cu2N/Cu(100), which each have a Kondo temperature of only 2.6 K. In order to cover the different regions of the phase space, the pairs are designed to have interaction strengths similar to the Kondo temperature. By applying a sufficiently strong magnetic field, we are able to access a new phase in which the two coupled impurities are simultaneously screened. Comparison of differential conductance spectra taken on the atoms to simulated curves, calculated using a third order transport model, allows us to independently determine the degree of Kondo screening in each phase.