  The interplay between the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction
and the Kondo effect is expected to provide the driving force for the emergence
of many phenomena in strongly correlated electron materials. Two magnetic
impurities in a metal are the smallest possible system containing all these
ingredients and define a bottom up approach towards a long term understanding
of concentrated / dense systems. Here we report on the experimental and
theoretical investigation of iron dimers buried below a Cu(100) surface by
means of low temperature scanning tunnelling spectroscopy (STS) combined with
density functional theory (DFT) and numerical renormalization group (NRG)
calculations. 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. It oscillates as function of dimer separation revealing
that it is related to the RKKY interaction mediated by the conduction
electrons. Simulations based on density functional theory support this concept
showing the same oscillation period and trends in the coupling strength as
found in the experiment.
