A team of physicists from the University of Cologne has solved a long-standing problem in condensed matter physics: They have directly observed the Kondo effect (the rearrangement of electrons in a metal caused by magnetic impurities) visible in a single artificial atom. This has not been done successfully in the past because the magnetic orbits of atoms are generally not directly observable with most measurement techniques.

However, Dr. from the Institute of Experimental Physics at the University of Cologne. An international research team led by Wouter Joly used a new technique to observe the Kondo effect on an artificial orbit inside a one-dimensional wire floating on top of a metallic graphene sheet. They report their discoveries in a recently published paper. Natural Physics.

Understanding the Kondo effect

When electrons moving inside a metal collide with a magnetic atom, they are affected by the spin of the atom – as the magnetic pole of the elementary particles tries to shield the effect of the atomic spin, the sea of ​​electrons clusters close to the atom, creating a new many-body situation called the Kondo resonance.

This collective behavior is known as the Kondo effect and is often used to describe the interaction of metals with magnetic atoms. However, other types of interactions can lead to very similar experimental signatures and question the role of the Kondo effect for individual magnetic atoms on surfaces.

Innovative experimental methods

Physicists used a new experimental approach to show that one-dimensional wires also undergo the Kondo effect: Electrons trapped inside the wires create standing waves that can be thought of as expanded atomic orbitals. This artificial orbit, its connection with the electron sea, and the resonance transitions between the orbit and the sea can be visualized with a scanning tunneling microscope. This experimental technique uses a sharp metal needle to measure electrons with atomic resolution. This allowed the team to measure the Kondo effect with unprecedented precision.

“Magnetic atoms on surfaces resemble the story of a man who had never seen an elephant before and tried to imagine its shape by touching it in a dark room. “If you just touch the trunk, you imagine a completely different animal than when you touch the side,” said Kamiel van Efferen, a PhD student who conducted the experiments.

“For a long time, only the Kondo resonance was measured. However, there may be other explanations for the signals observed in these measurements, just as the trunk of an elephant may be that of a snake.”

A research group at the Institute of Experimental Physics specializes in growing and studying two-dimensional materials (crystalline solids consisting of only a few layers of atoms) such as graphene and monolayer molybdenum disulfide (MoS2). They discovered that a metal wire of atoms forms at the interface of two MoS2 crystals, one of which is a mirror image of the other.

Using scanning tunneling microscopy, they were able to simultaneously measure magnetic states and the Kondo resonance at extremely low temperatures, such as -272.75 degrees C (0.4 Kelvin), where the Kondo effect occurs.

Correlation of theory with experimental data

“Although our measurements left no doubt that we were observing the Kondo effect, we did not yet know how well our unconventional approach compared to theoretical predictions,” Jolie said. he added. For this, the team consulted two theoretical physicists, Professor Dr. from the University of Cologne, who are world-renowned experts in the field of Kondo physics. Achim Roche and Dr. from Forschungszentrum Jülich. He enlisted the help of Theo Kosti.

After processing the experimental data on the Jülich supercomputer, it turned out that the Kondo resonance could be accurately predicted from the shape of the artificial trajectories in the magnetic wires; This confirmed a decades-old prediction by one of the founding fathers of condensed matter physics. , Philip W. Anderson.
Scientists now plan to use their magnetic cables to study more exotic phenomena.

“By placing our one-dimensional wires on a superconductor or quantum spin liquid, we can create many-body states arising from quasiparticles other than electrons,” explained Kamiel van Efferen. “The fascinating states of matter resulting from these interactions are now clearly visible, allowing us to understand them at a completely new level.”