Kondo Scattering Observed at a Single Magnetic Impurity

Electron scattering of isolated Ce adatoms on Ag(111) surfaces was studied with a low-temperature scanning tunneling microscope. Tunneling spectra obtained on and near Ce reveal a characteristic dip around the Fermi energy which is absent for nonmagnetic Ag adatoms. This feature is detected over a few atomic diameters around Ce atoms at the surface. The transition matrix element from the localized $f$ electron to the tunnel-current carrying continuum states bears a strong resemblance to discrete autoionized states. We interpret the dip spectrum as a Fano interference for the limit where the $f$ orbital has a very small matrix element.