なかのにっき

Variation of the local electronic structure at rutile TiO2(110) surfaces was studied by scanning tunneling spectroscopy (STS). Structural surface features such as step edges, (1×2) reconstructed strands, and their terminations were correlated to changes in tunneling spectra. In particular, band-gap states, associated with a reduced surface, showed characteristic variations. In addition, electronic variations due to extrinsic defects are discussed. Nanometer wide protrusions in constant current scanning tunneling microscopy images were identified in STS as local electronic alterations. These features are interpreted to be due to local band bending induced by individual, charged impurity atoms.

Detailed kinetic Monte Carlo-molecular dynamics (KMC-MD) simulations of hyperthermal energy (10-100 eV) copper homoepitaxy reveal a reentrant layer-by-layer growth mode at low temperatures (50 K) and reasonable fluxes (1 ML/s. where ML stands for monolayer). This growth mode is the result of atoms with hyperthermal kinetic energies becoming inserted into islands when the impact site is near a step edge. The yield for atomic insertion as calculated with molecular dynamics near (111) step edges reaches a maximum near 18 eV. KMC-MD simulations of growing films find a minimum in the rms roughness as a function of energy near 25 eV. We find that the rms roughness saturates just beyond 0.5 ML of coverage in films grown with energies greater than 25 eV due to the onset of adatom-vacancy formation near 20 eV. Adatom-vacancy pairs increase the island nuclei density and the step-edge density, which increase the number of sites available to insert atoms. Smoothest growth in this regime is achieved by maximizing island and step-edge densities, which consequently reverses the traditional roles of temperature and flux: low temperatures and high fluxes produce the smoothest surfaces in these films. Dramatic increases in island densities are found to persist at room temperature, where island densities increase an order of magnitude from 20 to 150 eV.

The magnetoconductive properties of ultrathin Pb films deposited on Si(111) are measured and compared with density-functional electronic band-structure calculations on two-dimensional, free-standing, 1 to 8 monolayers thick Pb(111) slabs. A description with free-standing slabs is possible because it turned out that the Hall coefficient is independent of the substrate and of the crystalline order in the film. We show that the oscillations in sign of the Hall coefficient observed as a function of film thickness can be explained directly from the thickness dependent variations of the electronic band-structure at the Fermi energy.

Angle-resolved photoemission data from Al(001) taken with photon energies up to several hundred electron volts show an unexpected surface sensitivity at high energies. The surface state at the center of the surface Brillouin zone can be clearly observed up to photon energies higher than 700 eV. The surface to bulk intensity ratio appears to increase with photon energy, despite the longer inelastic mean-free path of the electrons and the increased vibrational amplitude at the surface. We explain this surprising behavior by considering the effect of phonon excitation in the photoemission event.