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 electronic states near scattering centers. Since the charge density on Be(10 (1) over bar 0) is mainly provided by a surface state located near the edge of the first Brillouin zone, the wave functions are significantly different from plane waves. It is shown how this difference is reflected in the STM images. By numerical post-processing we find that the observed wave patterns exhibit a multiperiodic structure with individual periods differing by inverse lattice vectors. This result constitutes a direct experimental illustration of the Bloch theorem for electrons in a periodic potential.