The mystery of the Kondo effect has finally been solved

The mystery of the Kondo effect has finally been solved

A team of physicists at the University of Cologne has solved a long-standing problem in condensed matter physics: they have directly observed the Kondo effect (recombination of electrons in a metal due to magnetic impurities) visible in a single artificial atom. This has not been done successfully in the past, as the magnetic orbits of atoms cannot usually be directly observed using most measurement techniques. However, an international research team led by Dr. Wouter Joly at the Institute for Experimental Physics at the University of Cologne used a new technique to observe the Kondo effect in an artificial orbit inside a one-dimensional wire floating on top of a graphene metal sheet. They report their discovery in the article “Modified Kondo screening along the double boundary of a magnetic mirror in monolayer MoS2” in Nature physics.
When electrons moving through a metal collide with a magnetic atom, they are affected by the atomic spin – the magnetic pole of elementary particles. In an attempt to examine the effect of atomic spin, the sea of ​​electrons crowd together near the atom, forming a new many-body state called 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, calling into question the role of the Kondo effect for single magnetic atoms on surfaces.
Physicists used a new experimental approach to show that their one-dimensional wires also undergo the Kondo effect: electrons trapped in the wires form standing waves, which can be thought of as extended atomic orbitals. This artificial orbit, its coupling to the electron sea, as well as the resonant transitions between the orbit and the sea can be imaged using a scanning tunneling microscope. This experimental technique uses a sharp metal needle to measure electrons with atomic precision. This allowed the team to measure the Kondo effect with unparalleled precision.
“With magnetic atoms on surfaces, it’s like the story of a person who has never seen an elephant before and tries to imagine what it looks like by touching it once in a dark room. If you just feel the trunk,” said Camille van Everen, the doctoral student who conducted the experiments. You imagine a completely different animal than if you were touching its side.” “For a long time, only Kondo resonance was measured. But there could be other explanations for the signals observed in these measurements, just as an elephant’s trunk could also be a snake.
The research group at the Institute of Experimental Physics specializes in the development and exploration of 2D materials – crystalline solids composed of only a few layers of atoms – such as graphene and single-layer molybdenum disulfide (MoS2). They found that at the interface of two MoS2 crystals, one of which is a mirror image of the other, a metallic wire of atoms is formed. Using a scanning tunneling microscope, they were able to simultaneously measure magnetic states and Kondo resonance, at an astonishingly low temperature of -272.75 °C (0.4 K), where the Kondo effect appears.
“While our measurements left no doubt that we observed the Kondo effect, we do not yet know how well our unconventional approach compares to theoretical predictions,” Jolie added. For this reason, the team enlisted the help of two theoretical physicists, Professor Dr. Achim Rösch from the University of Cologne and Dr. Theo Costi from Forschungszentrum Jülich, both world-renowned experts in the field of Kondo physics. After processing the experimental data in the Jülich supercomputer, it turns out that Kondo resonance can be accurately predicted from the shape of artificial orbitals in magnetic wires, validating a decades-old prediction from one of the founding fathers of condensed matter physics. , Philip W. Anderson.
Scientists now plan to use their magnetic wires to study more strange phenomena. “By placing our one-dimensional wires on a superconductor or on a spin quantum fluid, we can create many-body states that emerge from quasiparticles other than electrons,” Camille van Everen explained. “The fascinating states of matter that arise from these interactions can now be clearly seen, which will allow us to understand them on a whole new level.”


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