A Tunable Two-impurity Kondo system in an atomic point contact

TL;DR

This paper reports the experimental realization of a tunable two-impurity Kondo system using cobalt dimers between an STM tip and a gold surface, revealing a crossover from local Kondo screening to non-local singlet formation.

Contribution

First experimental implementation of a tunable two-impurity Kondo system with sub-picometer control over impurity interactions.

Findings

Observation of a crossover from Kondo screening to singlet formation.

Precise control of impurity interaction via tip-sample distance.

Rich physical behavior governed by magnetic coupling.

Abstract

Two magnetic atoms, one attached to the tip of a Scanning Tunneling Microscope (STM) and one adsorbed on a metal surface, each constituting a Kondo system, have been proposed as one of the simplest conceivable systems potentially exhibiting quantum critical behaviour. We have succeeded in implementing this concept experimentally for cobalt dimers clamped between an STM tip and a gold surface. Control of the tip-sample distance with sub-picometer resolution allows us to tune the interaction between the two cobalt atoms with unprecedented precision. Electronic transport measurements on this two-impurity Kondo system reveal a rich physical scenario which is governed by a crossover from local Kondo screening to non-local singlet formation due to antiferromagnetic coupling as a function of separation of the cobalt atoms.