Showing papers by "Ronan McGrath published in 2004"

Pseudomorphic Growth of a Single Element Quasiperiodic Ultrathin Film on a Quasicrystal Substrate

Abstract: An ultrathin film with a periodic interlayer spacing was grown by the deposition of Cu atoms on the fivefold surface of the icosahedral Al70Pd21Mn9 quasicrystal. For coverages from 5 to 25 monolayers, a distinctive quasiperiodic low-energy electron diffraction pattern is observed. Scanning tunneling microscopy images show that the in-plane structure comprises rows having separations of S=4.5+/-0.2 A and L=7.3+/-0.3 A, whose ratio equals tau=1.618... within experimental error. The sequences of such row separations form segments of terms of the Fibonacci sequence, indicative of the formation of a pseudomorphic Cu film.

Structure of the tenfold d-Al-Ni-Co quasicrystal surface

Abstract: The structure of the tenfold surface of decagonal Al-Ni-Co was studied using low-energy electron diffraction (LEED) and scanning tunneling microscopy (STM). The surface region is a relaxed truncated bulk structure, having the same composition as the bulk. The outermost layer spacing is contracted by 10% relative to the bulk interlayer spacing, while the next layer spacing is expanded by 5%. A small degree of intralayer rumpling was observed within each layer. There is a one-to-one correspondence between protrusions observed in the STM images and a subset of atoms in the model structure, indicating that in-plane reconstruction is minimal.

Maximum density rule for bulk terminations of quasicrystals

Abstract: Bravais' rule, of wide validity for crystals, states that their surfaces correspond to the densest planes of atoms in the bulk. Comparing a theoretical model of icosahedral Al-Pd-Mn with experimental results on sputter-annealed surfaces, we find that this correspondence breaks down, i.e., the surfaces parallel to the densest planes in the model are not necessarily the most stable bulk terminations. The correspondence is restored by recognizing that there is a contribution to the surface not just from a single geometrical plane but from a layer of stacked atoms, possibly containing more than one plane. We find that not only does the stability of high-symmetry surfaces match the density of the corresponding layerlike bulk terminations but the exact spacings between surface terraces can be determined and the typical area of the terraces can be estimated by a simple analysis of the density of layers predicted by the bulk geometric model.

Low-energy electron diffraction study of potassium adsorbed on single-crystal graphite and highly oriented pyrolytic graphite

Abstract: Potassium adsorption on graphite has been a model system for the understanding of the interaction of alkali metals with surfaces. The geometries of the $(2\ifmmode\times\else\texttimes\fi{}2)$ structure of potassium on both single-crystal graphite (SCG) and highly oriented pyrolytic graphite (HOPG) were investigated for various preparation conditions for graphite temperatures between 55 and 140 K. In all cases, the geometry was found to consist of $\mathrm{K}$ atoms in the hollow sites on top of the surface. The $\mathrm{K}$-graphite average perpendicular spacing is $2.79\ifmmode\pm\else\textpm\fi{}0.03\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$, corresponding to an average $\mathrm{C}\text{\ensuremath{-}}\mathrm{K}$ distance of $3.13\ifmmode\pm\else\textpm\fi{}0.03\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$, and the spacing between graphite planes is consistent with the bulk spacing of $3.35\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$. No evidence was observed for a sublayer of potassium. The results of dynamical LEED studies for the clean SCG and HOPG surfaces indicate that the surface structures of both are consistent with the truncated bulk structure of graphite.

The forbidden beauty of quasicrystals

Abstract: TWENTY YEARS ago last month Danny Shechtman of the Technion Institute in Israel announced the discovery of a new metallic alloy. At the time Shechtman had been on sabbatical leave at the National Bureau of Standards in Washington DC, investigating the properties of mixtures of metals that had been melted together and rapidly cooled. He found that one of these alloys – aluminium manganese – displayed a diffraction pattern with 10-fold rotational symmetry. However, such symmetries were supposed to be forbidden by the laws of crystallography.

Quasicrystal surfaces as templates for artificial aperiodic systems: from nanoclusters to epilayers

Abstract: This paper presents a brief overview of recent progress in the investigation of the potential of clean quasicrystal surfaces for use as templates for the formation of ‘artificial’ aperiodic systems. The clean fivefold surfaces of i-Al–Pd–Mn and i-Al–Cu–Fe and the 10-fold surface of d-Al–Ni–Co are briefly discussed, and their use as templates for aperiodic ‘0’-dimensional (nanostructure) systems, of 1-d (quantum wire) systems, of 2-d (monolayer) systems, and of 3-d (epilayer) structures is reviewed.