  A low-temperature scanning tunneling microscope was employed to study the
differential conductance in an atomic junction formed by an adsorbed Co atom on
a Cu(100) surface and a copper-covered tip. A zero-bias anomaly (ZBA) reveals
spin scattering off the Co atom, which is assigned to a Kondo effect. The ZBA
exhibits a characteristic asymmetric lineshape when electrons tunnel between
tip and sample, while upon the tip-Co contact it symmetrizes and broadens.
Through density functional theory calculations and the non-equilibrium
non-crossing approximation we show that the lineshape broadening is mainly a
consequence of the additional coupling to the tip, while non-equilibrium
effects only modify the large-bias tails of the ZBA.
