GIANT FRIEDEL OSCILLATIONS ON THE BERYLLIUM(0001) SURFACE

Large-amplitude electron density oscillations were observed on a Be(00 01) surface by means of variable-temperature scanning tunneling micros copy. Fourier transforms of the images showed a ring of radius 2k(F), where k(F) is the Fermi wave vector of the Be(0001) surface state, Thi s wavelength was expected from Friedel oscillations caused by electron ic screening of surface defects, but the amplitude of the waves for en ergies near the Fermi energy was anomalously large and inconsistent wi th the Friedel concept of screening. The enhanced amplitude of the wav es must be a many-body effect, either in the electron gas (possibly an incipient charge density wave) or in the response of the lattice (ele ctron-phonon coupling).