A team of physicists from the University of Cologne has solved a long-standing puzzle in condensed matter physics: they have directly observed the visible Kondo effect (the reclustering of electrons in metals caused by magnetic impurities) in an artificial atom. This has been unsuccessful in the past because most measurement techniques often do not directly observe the magnetic orbits of atoms.
However, an international research team led by Dr. Wouter Jolie from the Institute of Experimental Physics at the University of Cologne used a new technique to observe the Kondo effect on an artificial track within a one-dimensional wire floating above a graphene metal sheet. They reported their findings in a paper recently published in Nature Physics.
When electrons moving in metal encounter magnetic atoms, they will be affected by the atomic spin-the atomic spin is the magnetic pole of the elementary particle. In order to shield the influence of the atomic spin, the sea of electrons gathers together close to the atoms, forming a new many-body state, which is called Kondo resonance. Often used to describe the interaction of metals with magnetic atoms. However, other types of interactions lead to very similar experimental features, raising questions about the role of the Kondo effect on individual magnetic atoms at the surface.
The physicists used a new experimental method to show that their one-dimensional wire is also affected by the Kondo effect: the electrons in the wire form standing waves, which can be thought of as extended atomic orbitals. This artificial orbit, its coupling to the electron sea, and the resonant switching between the orbit and the electron sea can all be imaged with scanning tunneling microscopy. This experimental technique uses a sharp metal needle to measure electrons at atomic resolution. This allowed the research team to measure the Kondo effect with unparalleled precision.
"For magnetic atoms on the surface, it's like a story: A person who has never seen an elephant tries to imagine its shape by touching it once in a dark room. If you only touch the trunk, the animal you imagine is completely different from the side if you touch it," said Camiel van Efferen, a doctoral student who conducted the experiment. "For a long time, only Kondo resonances were measured. But the signals observed in these measurements could have other explanations, just as the elephant's trunk could also be that of a snake."
The research group at the Institute of Experimental Physics specializes in the growth and exploration of two-dimensional materials (crystalline solids consisting 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, filaments of metal atoms form.
Using a scanning tunneling microscope, they were able to simultaneously measure the magnetic state and the Kondo resonance at the astonishingly low temperature of -272.75 degrees Celsius (0.4 Kelvin), the temperature at which the Kondo effect occurs.
Correlation between theory and experimental data
"While our measurements leave no doubt that we are observing the Kondo effect, we don't yet know how our unconventional approach compares with theoretical predictions," adds Jolly. To this end, the team enlisted the help of two theoretical physicists, Professor Achim Rosch from the University of Cologne and Dr. Theo Costi from the Jülich Research Center, both world-renowned experts in Kondo physics.
Analysis of the experimental data in Jülich's supercomputer revealed that the Kondo resonance can be accurately predicted based on the shape of artificial orbits in magnetic field lines, thus verifying the prediction made decades ago by Philip W. Anderson, one of the founders of condensed matter physics.
The scientists are now planning to use their magnetic field lines to study even more exotic phenomena. "By placing our one-dimensional wires on a superconductor or a quantum spin liquid, we can create many-body states produced by quasiparticles other than electrons. It is now possible to clearly see the fascinating states of matter resulting from these interactions, which will allow us to understand them on a completely new level," explains Kamil van Efren.