Electronic surface states in cleaved transition metals

The local densities of states (LDS) on clean low-index surfaces of fcc Ni and bcc Fe are presented. They are calculated within the tight-binding approximation using a moment method and a continued fraction analysis. The d band degeneracy and the charge rearrangement occurring near the surface when cleaving the crystal are fully taken into account by computing exactly a large number of moments (22–26). The surface LDS are found to be strongly dependent on the cleavage plane. Indeed, whereas the LDS on dense planes are rather bulk like, sharp peaks arising from states localized near the surface are obtained near the atomic level of the metal on non-dense planes of cleavage. In addition, the bandwidth is broadened by surface states for Ni. On the contrary, for bcc Fe the surface potential is not strong enough to modify the bandwidth. A comparison is made with spectroscopic observations.The method lends itself to other interesting applications in the surface physics field and, as an example, we present the results of calculations of the spatial distribution of the electronic charge near the surface and its deviation from spherical symmetry. The charge asphericity is slightly increased when going from a bulk to a surface site as the difference between the occupations of two surface orbitals can reach 0.34 ē instead of 0.15 ?c for a bulk site. Diagrams are shown for (100) and (110) cleaved Ni to illustrate the results. The consequences upon preferred adsorption sites according to the electronic affinity and size of the adsorbate are discussed.