Observing the scattering phase shift of isolated Kondo impurities at surfaces

The spin-flip scattering at Co adatoms on Ag(111) leads to the formation of a Kondo resonance. We determined the energy-dependent scattering phase shift of electrons scattering off single magnetic adatoms. A scanning tunneling microscope operating at 6 K was used to determine the amplitude of the Friedel oscillations created in the local density of states of the Ag(111) surface state electrons by the Co adatoms. We find that the standing-wave pattern around Co adsorbates exhibits a minimum in amplitude near E-F of width 2k(B)T(K), where T-K=92 K is the Kondo temperature of the system. The wave pattern amplitude as a function of energy can be explained by assuming an energy-dependent phase shift for the scattering process.