SURFACE-STATES AS PROBES OF BURIED IMPURITIES

Shockley surface states at metal surfaces decay exponentially into the crystal, typically penetrating several interplanar spacings. Within t his distance defects such as impurity atoms will scatter the surface s tate electrons and give rise to characteristic standing wave patterns in the local density of states outside the surface, similar to those r ecently detected with the STM due to scattering by adatoms. A multiple -scattering theory is developed to model this situation and applied to impurities near the Cu(111) surface. The results are discussed within a two-dimensional scattering model and the prospects for observation with the STM considered.