No abstract

We have determined the diffusion coefficient of a Cu adatom on Cn(100) by using helium-atombeam scattering as a probe. Our approach relies on measuring in situ and in real time the separation between islands nucleated on individual terraces during a submonolayer deposition of Cu. The method can be applied~ith other surface-sensitive de'raction techniques as~e11.

The growth of Cu on a vicinal Cu surface is investigated using variable temperature scanning tunneling microscopy. A meandering instability caused by the step edge barrier for diffusion leads to a lateral patterning of the surface with a temperature-dependent, distinctive wavelength. This length scale is set by nucleation of one-dimensional islands at step edges. In the temperature range covered by our experiment (230 to 400 K) a specific slope of the growth features within the plane of the surface is selected. This may point to a pronounced spatial anisotropy of the step edge barrier. 68.35.Ct, 81.15.Hi Molecular beam epitaxy (MBE) and its analogs offer the possibility to fabricate solid state materials and artificial superlattices, structures with built-in periodicities at nearly any desired scale along the growth direction. A lateral structuring of materials with additional in-plane periodicities is the natural next step towards a functional modification of physical properties in a controlled manner. In general, current lithography schemes and other patterning techniques do not have sufficient spatial resolution to reach in-plane geometries at the nanometerscale, where, for example, quantum phenomena are expected to take place even at room temperature [1]. The use of inherent instabilities [2] in growth processes is currently being explored as a promising pathway for lateral nanostructuring. Such instabilities may be purely kinetic in origin (as evidenced on the basis of investigations of growth processes in homoepitaxial systems), associated with a larger energy barrier [the Ehrlich-Schwoebel (ES) barrier [3] ] for diffusion of adatoms at and in the vicinity of steps with respect to diffusion on terraces.On vicinal surfaces, which ideally are made up of terraces separated by regularly spaced steps of monoatomic height, growth proceeds not by nucleation of deposited atoms on terraces, but through incorporation on preexisting steps, which thereby advance. The ES barrier gives rise to an asymmetry in the adatom attachment to step edges favoring attachment from the lower side. While playing a stabilizing role during growth in the step train direction (TD) [3], the ES effect may induce a morphological instability along the step edge (step direction, SD), resulting in a transverse in-phase meandering of steps with a characteristic wavelength [4], which introduces thus an additional in-plane periodicity along SD. Since the pioneering work by Bales and Zangwill [4], numerous theoretical investigations have been devoted to this topic using continuum approaches [5,6] and Monte Carlo simulations [7]. Experimentally, very few studies address the predicted Bales-Zangwill (BZ) instability [8,9] and, in particular, no information on the meandering characteristics and its controlling parameters is available so far.

High-resolution helium-atom-scattering measurements of angular distributions and time-of-flight spectra are reported for W(001) in the temperature range from 200 to 1900 K. Angular variations in the total intensity show a reconstruction related superstructure peak whose angular position and intensity are temperature dependent in the range from 400 to 220 K. At 220 K the peak intensity is maximum and its position corresponds to that expected for a reconstructed (&2X&2) 845' low-temperature phase. Time-of-flight spectra show that the behavior of the superstructure peak at higher temperatures is attributable to the elastic component and must therefore be due to a surface structural feature. Of the measured phonon inelastic peaks in the time-of-flight spectra, the one assigned to a longitudinal mode shows the typical strong temperature dependence of a soft mode in the temperature range 450~T~220 K. For T~220 K two new phonon modes appear, associated with the Brillouin-zone boundary corresponding to the c(2X2) phase. Both observations suggest that the overall phase transition on W(001) is of the "displacive" and not the "order-disorder" type, as inferred from several recent structural studies.All experimental findings are consistent with a charge-density-wave mechanism driving the reconstruction in agreement with the recent theory of Wang and Weber [Phys. Rev. Lett. 58, 1452.