Academic Affiliation: Georg‐August‐University of Göttingen

Talk: October 29, 2018

From a single Kondo atom to coupled systems

Many phenomena in strongly correlated electron physics, especially for heavy-fermion metals, like unconventional superconductivity or quantum critical behavior are attributed to the competition of Ruderman Kittel Kasuya Yosida (RKKY) interaction with Kondo physics. Two magnetic impurities in a metal are the smallest possible system containing all these ingredients and define a bottom approach towards a long-term understanding of larger systems.
In contrast to other groups, we have investigated bulk Kondo systems with a Low temperature scanning tunneling microscope (STM). Surprisingly, this surface sensitive method also gives access to sub surface systems in metal that were previously assumed to be “invisible” due to the effective screening in metals.
We started the discussion from single non-magnetic impurities like Ag and Ge showing a weak Friedel oscillation in the local density of states. This observation is explained in the framework of electron focusing. The Kondo effect induced by magnetic Fe or Co impurities strongly influences the energy-dependent scattering behavior. Although each impurity – magnetic as well as non-magnetic – shows a characteristic fingerprint in the spectroscopic data, the results allow discussing the universal behavior of the Kondo effect.
Finally, experimental and theoretical investigation of iron dimers buried below a Cu(100) surface by means of low temperature scanning tunneling spectroscopy (STS) combined with density functional theory (DFT) and numerical renormalization group (NRG) calculations are presented. The Kondo effect, in particular the width of the Abrikosov-Suhl resonance, is strongly altered or even suppressed due to magnetic coupling between the impurities. We observe a long-range interaction, which oscillates as function of dimer separation revealing that it is related to the RKKY interaction mediated by the conduction electrons.