Anisotropic Two-Dimensional Friedel Oscillations

Scanning tunneling microscopy at 4 K shows highly anisotropic screening charge density oscillations on $\mathrm{Be}\left(10\overline{1}0\right)$ in the vicinity of surface defects and steps. The forms of these oscillations, which derive from the Friedel oscillations, can be directly related to the two-dimensional band structure of the surface (two electron pockets at the boundary of the surface Brillouin zone). Simple calculations show that the Friedel oscillations in such a case will contain major contributions from wavelengths which do not correspond to any Fermi wave vector.