OBSERVATION OF INTERFERING BLOCH WAVES

We have observed anisotropic standing waves on the Be(10 (1) over bar 0) surface using a low-temperature scanning tunnelling microscope (STM ). Such charge density oscillations result from interference of electr onic states near scattering centers. Since the charge density on Be(10 (1) over bar 0) is mainly provided by a surface state located near th e edge of the first Brillouin zone, the wave functions are significant ly different from plane waves. It is shown how this difference is refl ected in the STM images. By numerical post-processing we find that the observed wave patterns exhibit a multiperiodic structure with individ ual periods differing by inverse lattice vectors. This result constitu tes a direct experimental illustration of the Bloch theorem for electr ons in a periodic potential.