Giant Friedel Oscillations on the

Large-amplitude electron density oscillations were observed on a Be(0001) surface by means of variable-temperature scanning tunnelingmicroscopy. Fourier transforms of the images showed a ring of radius 2kF, where kF is the Fermi wave vector of the Be(0001) surface state. This wavelength was expected from Friedel oscillations caused by elec-tronic screening of surface defects, but the amplitude of the waves for energies near the Fermi energywas anomalously large and inconsistent with the Friedel concept of screen-ing. The enhanced amplitude of the waves must be a many-body effect, either in the electron gas (possibly an incipient charge density wave) or in the response of the lattice (electron-phonon coupling). One of the fundamental concepts of solid-state physics is screening, that is, the ability of a material to rearrange its electron sea in order to minimize the disturbance of an impurity or defect. Friedel showed that screening of a point-charged impurity in th