Modification of electron density in surface states: Scanning tunnelling microscopy observation of standing waves on Pd overlayers

Modification of electron density in the surface states of Au(111) and Cu(111) surfaces by Pd deposition is demonstrated. Using a low-temperature scanning tunnelling microscope, electronic standing wave patterns are observed on a monolayer-high Pd layer whose wavelength is longer than that of the substrates. The observation indicates an upward shift of the energy level of the surface states and reduced electron density in the states on the overlayer. A standing wave pattern depending on magnetic field was observed around an impurity atom on the Au(111) surface.