  The coupling of a spin to an underlying substrate is the basis for a plethora
of phenomena. In the case of a metallic substrate, Kondo screening of the
adatom magnetic moment can occur. As the substrate turns superconducting, an
intriguing situation emerges where the pair breaking due to the adatom spins
leads to Yu-Shiba-Rusinov bound states, but also intertwines with Kondo
phenomena. Through scanning tunneling spectroscopy, we analyze the
interdependence of Kondo screening and superconductivity. Our data obtained on
single Fe adatoms on Nb(110) show that the coupling and the resulting YSR
states are strongly adsorption site-dependent and reveal a quantum phase
transition at a Kondo temperature comparable to the superconducting gap. The
experimental signatures are rationalized by combined density functional theory
and continuous-time quantum Monte-Carlo calculations to rigorously treat
magnetic and hybridization effects on equal footing.
