Surface-state bands on silicon as electron systems in reduced dimensions at atomic scales

When foreign atoms to a depth of around one atomic layer adsorb on silicon crystal surfaces, the adsorbates rearrange themselves by involving the substrate Si atoms; this results in peculiar periodic atomic arrangements, surface superstructures, in just the topmost surface layers. Then, characteristic electronic states are created there, which are sometimes quite different from the bulk electronic states in the interior of the crystal, leading to novel properties only at the surfaces. Here, surface superstructures are introduced that have two-dimensional or quasi-one-dimensional metallic electronic states on silicon surfaces. Sophisticated surface science techniques, e.g., scanning tunnelling microscopy, photoemission spectroscopy, electron-energy-loss spectroscopy, and microscopic four-point-probes reveal characteristic phenomena such as phase transitions accompanying symmetry breakdown, electron standing waves, charge-density waves, sheet plasmons, and surface electronic transport, in which surface-state bands play main roles. These results show that surface superstructures on silicon provide fruitful platforms on which to investigate the physics of atomic-scale low-dimensional electron systems.