Temperature and magnetic field dependence of a Kondo system in the weak coupling regime

The Kondo effect arises when a localized spin interacts with the electrons of the surrounding host. Scanning tunneling spectroscopy on individual magnetic impurities have renewed the interest in Kondo physics, however, a quantitative comparison with theoretical predictions remained challenging. Here we studied the zero-bias anomaly on an organic radical weakly coupled to a Au(111) surface which we where able to describe with astonishing agreement by perturbation theory as originally developed by Kondo 60 years ago. Our results demonstrate that Kondo physics can only be fully conceived by studying both temperature and magnetic field dependence of the resonance.