Surface instabilities in granular matter and ion-sputtered surfaces

We apply a theoretical approach, originally introduced to describe aeolian ripples formation in sandy deserts, to the study of surface instability in ion sputtered surfaces. The two phenomena are distinct by several orders of magnitudes and by several physical mechanisms, but they obey to similar geometrical constraints and therefore they can be described by means of the same approach. This opens a novel conceptual framework for the study of the dynamical surface roughening and ripple formation on crystal and amorphous surfaces during ion sputtering.Comment: 14 pages, 3 figure

How ripples turn into dots: modeling ion-beam erosion under oblique incidence

Pattern formation on semiconductor surfaces induced by low energetic ion-beam erosion under normal and oblique incidence is theoretically investigated using a continuum model in form of a stochastic, nonlocal, anisotropic Kuramoto-Sivashinsky equation. Depending on the size of the parameters this model exhibits hexagonally ordered dot, ripple, less regular and even rather smooth patterns. We investigate the transitional behavior between such states and suggest how transitions can be experimentally detected.Comment: 11 pages, 4 figures, submitted for publication, revised versio

Replicating Nanostructures on Silicon by Low Energy Ion Beams

We report on a nanoscale patterning method on Si substrates using self-assembled metal islands and low-energy ion-beam irradiation. The Si nanostructures produced on the Si substrate have a one-to-one correspondence with the self-assembled metal (Ag, Au, Pt) nanoislands initially grown on the substrate. The surface morphology and the structure of the irradiated surface were studied by high-resolution transmission electron microscopy (HRTEM). TEM images of ion-beam irradiated samples show the formation of sawtooth-like structures on Si. Removing metal islands and the ion-beam induced amorphous Si by etching, we obtain a crystalline nanostructure of Si. The smallest structures emit red light when exposed to a UV light. The size of the nanostructures on Si is governed by the size of the self-assembled metal nanoparticles grown on the substrate for this replica nanopatterning. The method can easily be extended for tuning the size of the Si nanostructures by the proper choice of the metal nanoparticles and the ion energy in ion-irradiation. It is suggested that off-normal irradiation can also be used for tuning the size of the nanostructures.Comment: 12 pages, 7 figures, regular paper submitted to Nanotechnolog

Growth modes of Fe(110) revisited: a contribution of self-assembly to magnetic materials

We have revisited the epitaxial growth modes of Fe on W(110) and Mo(110), and propose an overview or our contribution to the field. We show that the Stranski-Krastanov growth mode, recognized for a long time in these systems, is in fact characterized by a bimodal distribution of islands for growth temperature in the range 250-700°C. We observe firstly compact islands whose shape is determined by Wulff-Kaischev's theorem, secondly thin and flat islands that display a preferred height, ie independant from nominal thickness and deposition procedure (1.4nm for Mo, and 5.5nm for W on the average). We used this effect to fabricate self-organized arrays of nanometers-thick stripes by step decoration. Self-assembled nano-ties are also obtained for nucleation of the flat islands on Mo at fairly high temperature, ie 800°C. Finally, using interfacial layers and solid solutions we separate two effects on the preferred height, first that of the interfacial energy, second that of the continuously-varying lattice parameter of the growth surface.Comment: 49 pages. Invited topical review for J. Phys.: Condens. Matte

Initial Penetration of an Elastic Axially Symmetric Indenter into a Rigid-Perfectly-Plastic Half-Space

This paper is concerned with the axially symmetric plastic flow of a rigid perfectly-plastic nonhardening halfspace. The initial penetration of the elastic indenter is studied based on Haar and von Karman hypothesis. The analytical distribution of contact stress and the approximate penetration depth of the indenter are obtained

Slowing and cooling molecules and neutral atoms by time-varying electric field gradients

A method of slowing, accelerating, cooling, and bunching molecules and neutral atoms using time-varying electric field gradients is demonstrated with cesium atoms in a fountain. The effects are measured and found to be in agreement with calculation. Time-varying electric field gradient slowing and cooling is applicable to atoms that have large dipole polarizabilities, including atoms that are not amenable to laser slowing and cooling, to Rydberg atoms, and to molecules, especially polar molecules with large electric dipole moments. The possible applications of this method include slowing and cooling thermal beams of atoms and molecules, launching cold atoms from a trap into a fountain, and measuring atomic dipole polarizabilities.Comment: 13 pages, 10 figures. Scheduled for publication in Nov. 1 Phys. Rev.

HREELS study of 0, molecular chemisorption on Ag( 001)

Abstract O2 adsorption on Ag(OO1) at 100 K has been investigated by HREELS. Contrary to previous reports we could resolve 4 oxygen related peaks, 3 of which are due to the internal stretching vibration as confirmed by isotope labelling. The lowest frequency mode at 63 meV is assigned to molecular oxygen chemisorbed at defect sites. The further two features are very close to each other in frequency, at 79 and 84 meV. We assign them to molecules chemisorbed in a peroxidic state in two different adsorption sites. Initially both sites are Mled simultaneously while at higher coverage adsorption in the higher frequency site is favoured