Showing papers by "Ronan McGrath published in 2002"

Bulk termination of the quasicrystalline fivefold surface of Al70Pd21Mn9

Abstract: The structure of the ${\mathrm{Al}}_{70}{\mathrm{Pd}}_{21}{\mathrm{Mn}}_{9}$ surface has been investigated using high-resolution scanning tunneling microscopy. From two large fivefold terraces on the surface in a short decorated Fibonacci sequence, atomically resolved surface images have been obtained. One of these terraces carries a rare local configuration in the form of a ring. The location of the corresponding sequence of terminations in the bulk model $\mathcal{M}$ of icosahedral i-AlPdMn based on the three-dimensional tiling ${\mathcal{T}}^{*(2F)}$ of an F phase has been estimated using this ring configuration and the requirement from low-energy electron diffraction work of Gierer et al. that the average atomic density of the terminations is 0.136 atoms per ${\mathrm{\AA{}}}^{2}.$ A termination contains two atomic plane layers separated by a vertical distance of 0.48 \AA{}. The position of the bulk terminations is fixed within the layers of Bergman polytopes in the model $\mathcal{M}:$ they are 4.08 \AA{} in the direction of the bulk from a surface of the most dense Bergman layers. From the coding windows of the top planes in terminations in $\mathcal{M}$ we conclude that a Penrose (P1) tiling is possible on almost all fivefold terraces. The shortest edge of the tiling P1 is either 4.8 or 7.8 \AA{}. The experimentally derived tiling of the surface with the ring configuration has an edge length of $8.0\ifmmode\pm\else\textpm\fi{}0.3\mathrm{\AA{}}$ and hence matches the minimal edge length expected from the model.

Quasicrystal surfaces: structure and potential as templates

Abstract: We present a review of recent progress in determining the surface structure of quasicrystals, with emphasis on their connections to mathematical tiling models. The review focusses in particular on the five-fold surface of icosahedral Al-Pd-Mn and the ten-fold surface of decagonal Al-Ni-Co. We also assess their potential as templates for the formation of two-dimensional quasicrystalline overlayers with reference to recent investigations of atomic and molecular adsorption.

Tilings and Coverings of Quasicrystal Surfaces

Abstract: The tiling and covering of images derived from the experimental technique of high-resolution transmission electron microscopy (HRTEM) has been a fruitful avenue in studies of bulk quasicrystal structures. In particular, the quasi-unit-cell model has been successfully applied to the 2-dimensional decagonal AlNiCo (d-AlNiCo) quasicrystal (see Sect. 8.3.1). However, HRTEM is a bulk technique and can only be used to observe the average structure over the thickness of the sample. On the other hand, scanning tunneling microscopy (STM) is a purely surface technique. Hence, if high-quality images of quasicrystal surfaces can be obtained, then the possibility arises of applying tiling and covering methodologies to these surfaces, with a view to elucidating their structure and determining whether or not they are terminations of the bulk. A corollary of this approach is that such studies would provide independent verification of current bulk tiling and covering models. In this chapter, we describe our attempts to apply tilings and coverings to images of surfaces of quasicrystals obtained using STM [1, 2, 3, 4, 5, 6, 7].