The impact ionization rate and its orientation dependence in k space is calculated for GaAs. The numerical results indicate a strong correlation to the band structure. The use of a q-dependent screening function for the Coulomb interaction between conduction and valence electrons is found to be essential. Comparison with recent results for Si, GaAs shows a harder threshold behavior. A simple fit formula is presented for easy calculation of the direction-dependent transition rate. With only the band structure as input the numerical results are closely reproduced in an arbitrary direction in the Brillouin zone.

We describe a method to perform self-consistent band structure calculations. This combination of the extended linear augmented plane wave ͑ELAPW͒-kp method with a plane-wave basis set offers a scheme to construct the crystal potential alternative to the well-known full-potential linear augmented plane wave ͑FLAPW͒ technique. We propose a representation of the crystal density that is free from unphysical computational parameters specific to the representation of the wave functions. The valence density is divided into two parts, one of which is expanded in a Fourier series and the other one is localized within small spheres surrounding the nuclei. It is shown that to a good approximation the latter part can be represented by its Y 00 component. The quality of the representation is controlled by the number of Fourier components of the density, and the computational effort can be balanced with the desired accuracy. By construction the density is smooth everywhere in the unit cell. The technique of constructing the potential, the augmented Fourier components method ͑AFC͒, is described. The properties of the method are demonstrated using the cubic semiconductors Si, SiC, GaAs, BaTiO 3 , KNbO 3 , KTaO 3 , and metallic 1T chalcogenides TiS 2 and TiSe 2 as examples. The self-consistent density-of-states curves are presented. With the AFC ELAPW-kp method optical properties of TiSe 2 are calculated; complex dielectric function and reflectivity are in good agreement with experimental results.

A combined experimental and theoretical study of verylow-energy electron diffraction at the (0001) surface of 2H-NbSe2 is presented. Electron transmission spectra have been measured for energies up to 50 eV above the Fermi level with k || varying along the ΓK line of the Brillouin zone. Ab initio calculations of the spectra have been performed with the extended linear augmented plane wave k·p method. The experimental spectra are interpreted in terms of three-dimensional k || -resolved one-electron band structure. Special attention is paid to the quasi-particle lifetimes: by comparing the broadening of the spectral structures in the experimental and calculated spectra the energy dependence of the optical potential −iVi is determined. A sharp increase of Vi at 20 eV is detected, which is associated with a plasmon peak in the Im[−1/ε] function. Furthermore, the electron energy loss spectrum and the reflectivity of NbSe2 are calculated ab initio and compared with optical experiments. The obtained information on the dispersions and lifetimes of the unoccupied states is important for photoemission studies of the 3D band structure of the valence band.

Mass transport processes on metal surfaces play a key role in epitaxial growth and coarsening processes. They are usually described in terms of independent, statistical diffusion and attachment/detachment of individual metal adatoms or vacancies. Here we present high-speed scanning tunnelling microscopy (video-STM) observations of the dynamic behaviour of five-atom-wide, hexagonally ordered strings of Au atoms embedded in the square lattice of the Au(100)-(1x1) surface that reveal quasi-collective lateral motion of these strings perpendicular to as well as along the string direction. The perpendicular motion can be ascribed to small atomic displacements in the strings induced by propagating kinks, which also provides a mechanism for the exchange of Au atoms between the two string ends, required for motion in string direction. In addition, quasi-one-dimensional transport of Au adatoms along the string boundaries may contribute to the latter phenomenon according to density functional calculations.

Angle-resolved photoelectron spectroscopy ͑ARPES͒ is commonly applied to map the shape of Fermi surfaces. Here we quantify the errors of simple criteria for extracting Fermi vectors by ARPES that are induced by strongly varying matrix elements. Sophisticated methods for determining the three-dimensional Fermi vector based on temperature and photon energy dependent photoemission are discussed with reference to data of the quasi-two-dimensional system 1T-TiTe 2 .