Los Angeles, November 15


group of scientists has solved a long-standing condensed matter physics puzzle by directly observing the Kondo effect (the re-grouping of electrons in a metal produced by magnetic impurities) in a single artificial atom.

This has not been done successfully in the past since most measuring techniques do not allow for direct observation of atom magnetic orbitals.

The worldwide research team lead by Dr Wouter Jolie from the University of Cologne's Institute for Experimental Physics, on the other hand, employed a novel technique to witness the Kondo effect in an artificial orbital inside a one-dimensional wire floating above a metallic sheet of graphene.

They describe their discovery in the article "Modulated Kondo screening along magnetic mirror twin boundaries in monolayer MoS2" published in Nature Physics.

When electrons passing through a metal come into contact with a magnetic atom, they are influenced by the atom's spin, which is the magnetic pole of elementary particles. In order to screen out the impact of atomic spin, the electron sea gathers close to the atom, generating a new many-body state known as the Kondo resonance.

The Kondo effect refers to this collective behaviour and is frequently used to describe metals interacting with magnetic atoms. However, alternative sorts of interactions can produce very similar experimental signs, calling the Kondo effect for single magnetic atoms on surfaces into question.

The researchers demonstrated that their one-dimensional wires are similarly subject to the Kondo effect by trapping electrons in standing waves, which can be thought of as extended atomic orbitals.

The scanning tunnelling microscope can image this manufactured orbital, its coupling to the electron sea, and the resonant transitions between orbital and sea. A fine metallic needle is used in this experiment to measure electrons with atomic precision. This has enabled scientists to measure the Kondo effect with unprecedented accuracy.

"With magnetic atoms on surfaces, it is like the story about a person who has never seen an elephant and tries to imagine its shape by touching it once in a dark room. If you only feel the trunk, you imagine a completely different animal than if you are touching the side," said Camiel van Efferen, the doctoral student who conducted the experiments.

"For a long time, only the Kondo resonance was measured. But there could be other explanations for the signals observed in these measurements, just like the elephant's trunk could also be a snake."

The research group at the Institute of Experimental Physics specializes in the growth and exploration of 2D materials - crystalline solids consisting of just a few layers of atoms - such as graphene and monolayer molybdenum disulfide (MoS2).

They found that at the interface of two MoS2 crystals, one of which is the mirror image of the other, a metallic wire of atoms forms. With their scanning tunnelling microscope, they could simultaneously measure the magnetic states and the Kondo resonance, at an astonishingly low temperature of -272.75 degrees C (0.4 Kelvin), at which the Kondo effect emerges.

"While our measurement left no doubts that we observed the Kondo effect, we did not yet know how well our unconventional approach could be compared to theoretical predictions," Jolie added.

For that, the team enlisted the help of two theoretical physicists, Professor Dr Achim Rosch from the University of Cologne and Dr Theo Costi from Forschungszentrum Julich, both world-renowned experts in the field of Kondo physics.

After crunching the experimental data in the supercomputer in Julich, it turned out that the Kondo resonance could be exactly predicted from the shape of the artificial orbitals in the magnetic wires, validating a decades-old prediction from one of the founding fathers of condensed matter physics, Philip W. Anderson

The scientists are now planning to use their magnetic wires to investigate even more exotic phenomena. "Placing our 1D wires on a superconductor or on a quantum spin-liquid, we could create many-body states emerging from other quasiparticles than electrons," explained Camiel van Efferen.

"The fascinating states of matter that arise from these interactions can now be seen clearly, which will allow us to understand them on a completely new level."

Scientists solve physics riddle by observing Kondo effect

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