A study team from the University of Cologne has, for the first time, directly measured the Kondo effect, which determines the behavior of magnetic atoms surrounded by a sea of electrons.
The Kondo effect refers to the re-grouping of electrons in a metal produced by magnetic impurities in a single artificial atom. It has previously not been observed successfully as most measuring techniques do not allow for direct observation of atom magnetic orbitals.
Hoping to solve this, an international team of researchers headed by Dr. Wouter Jolie of the Institute for Experimental Physics at the University of Cologne used a novel method to witness the Kondo effect in an artificial orbital inside a one-dimensional wire that was floating above a metallic graphene sheet. In the Nature Physics article “Modulated Kondo screening along magnetic mirror twin boundaries in monolayer MoS2,” they described their findings.
The spin of a magnetic atom, or the magnetic pole of elementary particles, affects electrons traveling through a metal. The electron sea groups together around the atom in an attempt to block the impact of the atomic spin, creating a new many-body state known as the Kondo resonance.
The term “Kondo effect” refers to this collective behavior, frequently used to explain how metals interact with magnetic atoms. Nonetheless, alternative forms of contact can result in strikingly similar experimental signs, raising doubts about the significance of the Kondo effect for individual magnetic atoms on surfaces.
The physicists employed a novel experimental approach to demonstrate that their one-dimensional wires are also sensitive to the Kondo effect: electrons trapped in the wires create standing waves, which can be thought of as extended atomic orbitals.
The scanning tunneling microscope can image this manufactured orbital, its coupling to the electron sea, and the resonant transitions between orbital and sea. This experiment used a fine metallic needle to measure electrons with atomic precision. This has enabled scientists to measure the Kondo effect with unprecedented precision.
With magnetic atoms on surfaces, it is like with the story about the 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. 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.
Camiel van Efferen, Doctoral Student, University of Cologne
Graphene and monolayer molybdenum disulfide (MoS2) are examples of 2D materials, which are crystalline solids made up of only a few layers of atoms. The research group at the Institute of Experimental Physics is focused on the development and study of these materials.
The team discovered that a metallic wire of atoms formed at the interface between two MoS2 crystals, one of which is the mirror image of the other. They were able to measure both the magnetic states and the Kondo resonance at the Kondo effect’s startlingly low temperature of -272.75 degrees Celsius (0.4 Kelvin) concurrently with their scanning tunneling microscope.
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.
Dr. Wouter Jolie, Professor, Institute for Experimental Physics, University of Cologne
The team sought assistance from two internationally recognized experts in the field of Kondo physics, Professor Dr. Achim Rosch from the University of Cologne and Dr. Theo Costi from Forschungszentrum Jülich, for this purpose.
The investigation found that Kondo resonance could be precisely predicted from the shape of the artificial orbitals in the magnetic wires after the experimental data was crunched in the Jülich supercomputer. This validated a prediction made decades earlier by Philip W. Anderson, one of the pioneers of condensed matter physics.
Now, the researchers want to explore even more unusual events using their magnetic wires.
van Efferen concluded, “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. 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.”
van Efferen, C., et al. (2023) Modulated Kondo screening along magnetic mirror twin boundaries in monolayer MoS2. Nature Physics. doi:10.1038/s41567-023-02250-w.