  Single cobalt atoms on the (111) surfaces of noble metals were for a long
time considered prototypical systems for the Kondo effect in scanning tunneling
microscopy experiments. Yet, recent first-principle calculations suggest that
the experimentally observed spectroscopic zero-bias anomaly (ZBA) should be
interpreted in terms of excitations of the Co atom's spin and the formation of
a novel quasiparticle, the spinaron, a magnetic polaron resulting from the
interaction of spin excitations with conduction electrons, rather than in terms
of a Kondo resonance. Here we present state-of-the-art spin-averaged and
spin-polarized scanning tunneling spectroscopy measurements on Co atoms on the
Cu(111) surface in magnetic fields of up to 12 T, that allow us to discriminate
between the different theoretical models and to invalidate the prevailing
Kondo-based interpretation of the ZBA. Employing extended ab-initio
calculations, we instead provide strong evidence for multiple spinaronic states
in the system. Our work opens a new avenue of research to explore the
characteristics and consequences of these intriguing hybrid many-body states as
well as their design in man-made nanostructures.
