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We theoretically demonstrate that screw dislocation (SD), a 1D topological defect widely present in semiconductors, exhibits ubiquitously a new form of spin-orbit coupling (SOC) effect. Differing from the widely known conventional 2D Rashba-Dresselhaus (RD) SOC effect that typically exists at surfaces or interfaces, the deep-level nature of SD-SOC states in semiconductors readily makes it an ideal SOC. Remarkably, the spin texture of 1D SD-SOC, pertaining to the inherent symmetry of SD, exhibits a significantly higher degree of spin coherency than the 2D RD-SOC. Moreover, the 1D SD-SOC can be tuned by ionicity in compound semiconductors to ideally suppress spin relaxation, as demonstrated by comparative first-principles calculations of SDs in Si/Ge, GaAs, and SiC. Our findings therefore open a new door to manipulating spin transport in semiconductors by taking advantage of an otherwise detrimental topological defect.

The defect formation in epitaxial NaCl films (up to 5.6 nm thick) during low-energy (50--1500 eV) electron bombardment has been studied by a combination of low-energy electron diffraction and energy loss spectroscopy. At temperatures below 200 K two types of color center are generated at the surface, showing distinct electronic transition energies and different kinetics of formation. A rate equation analysis provides evidence for a random distribution of stable, immobile $F$ and $M$ centers on the surface. Their saturation density increases with the film thickness, indicating an indirect generation mechanism involving the whole film, presumably via self-trapping of excitons. The angular distribution of the inelastic electron scattering of excitons reveals their surface polariton character, providing the energy transport from film bulk to surface and within the surface. At room temperature the aggregation of Na clusters is observed. Patterns of high and low surface $F$- and $M$-center densities are potentially invaluable as templates for nanostructure devices on ionic crystal surfaces like NaCl(001), because of the chemical reactivity of surface color centers.

Low index surfaces are in most cases stable even after adsorption, whereas vicinal surfaces may be only stable without or even with adsorbates. Since these phenomena are rather frequently reported, some carefully studied examples are discussed here, to show the importance of the effect and possible reasons for the sometimes dramatic morphology changes. Metals on semiconductors and metals are well studied and therefore the best examples for the present purpose. Since vacancies at the surface may act as point defects in a similar way, f-centers on NaCl are included into the discussion.

An energy loss spectroscopy-low energy electron diffraction (ELS-LEED) study of surface plasmon dispersion on Ag(111) and Ag(110) is presented. This new technique allows us to improve the resolution in k -space, which constitutes the major limiting factor for studying surface plasmon dispersion. The uncertainty in the determination of the dispersion coefficients is thus improved by more than a factor of two with respect to conventional HREELS. In accord with previous literature we find for both Ag faces a positive dispersion, which is anisotropic with respect to the crystal face and, for Ag(110), to the crystal azimuth. The quadratic term, on the contrary, is isotropic.

A LEED system which provides high resolution in energy and momentum simultaneously has been used to record electron energy loss spectra of thin NaCl films epitaxially grown on Ge(100). While measurements in specular geometry show energetically equidistant dipole losses due to single and multiple excitations of the surface phonon polariton, off-specular spectra show multiple loss features which evidently are the result of a combination of one impact and several dipole scattering processes with individually resolved energy and momentum.

Electrical transport in ultrathin Cs layers on Si(001) has been studied combining macroscopic conductivity measurements with low-energy electron diffraction, energy loss spectroscopy, and measurements of the work function. At temperatures around 150K, growth of the first three atomic layers proceeds layer-by-layer. The completion of each layer correlates with stepwise increases of the surface sheet conductance with coverage. Calibrating the Cs coverage by combined conductivity and work function measurements, the areal density of a single atomic layer is determined as 0.5 monolayers (3.39×1014cm-2). Electron spectroscopy reveals a semiconductor-metal transition of the surface upon completion of the first atomic layer, which correlates with the onset of a macroscopically measured sheet conductance in the 10-5Ω-1 range. While the conductance can be ascribed to electrical transport within surface states, its dependence on temperature indicates an activation barrier, which, most likely, is due to domain boundaries. At coverages of one monolayer and beyond, the Cs Si(001) surface exhibits a high metal-like conductance in the 10-3Ω-1 range. © 2005 The American Physical Society.

A low-energy electron microscopy study of two-dimensional Si(001) island shapes near thermal equilibrium on $10\ifmmode\times\else\texttimes\fi{}15 \ensuremath{\mu}{\mathrm{m}}^{2}$ large single-domain terraces reveals a continuous increase of island aspect ratio and a shape transition from elliptical to ``American-football''-like with increasing island size. The size-dependent island shapes are driven by elastic relaxation caused by the intrinsic surface stress anisotropy present on Si(001). Analysis of the measured elliptical island shapes based on an elastic-model calculation allows a quantitative determination of step energies and of the surface stress anisotropy as a function of temperature.