Formation of a quasicrystalline Pb monolayer on the ten-fold surface of the decagonal Al-Ni-Co quasicrystal

Lead has been deposited on the ten-fold surface of decagonal Al72Ni11Co17 to form an epitaxial quasicrystalline single-element monolayer. The overlayer grows through nucleation of nanometer-sized irregular islands and the coverage saturates at 1 ML. The overlayer is well-ordered quasiperiodically as evidenced by LEED and Fourier transforms of STM images. Annealing the film to 600 K improves the structural quality, but causes the evaporation of some material such that the film develops pores. Electronic structure measurements using X-ray photoemission spectroscopy indicate that the chemical interaction of the Pb atoms with the substrate is weak.Comment: 12 pages, 5 figure

Chewing gum modifies state-anxiety and alertness under conditions of social stress

Objectives: The finding that chewing gum can moderate state-anxiety under conditions of acute stress¹ has proved difficult to replicate.2,4 The present study examines the extent to which chewing gum can moderate state-anxiety under conditions of acute social stress. Method: In a between-participants design, 36 participants completed a task comprising a mock job interview (a variation on the Trier Social Stress Task3, which included a mental arithmetic component) whilst either chewing gum or without gum. Self-rated measures of mood and anxiety were taken at baseline, after a 10-minute presentation preparation stage, after the 10-minute presentation, and following a 5-minute recovery stage. Results: Post-presentation measures reflected increased state-anxiety and decrease self-rated calmness and contentedness. Chewing gum attenuated the rise in state-anxiety whilst increasing self-rated alertness. Chewing gum did not affect contentedness or calmness. Conclusions: The findings indicate that chewing gum can act to reduce anxiety under conditions of acute social stress: a finding consistent with Scholey et al.1 Furthermore, the data add to the growing body of literature demonstrating that chewing gum can increase alertness.1,2,4,

Formalizing Mathematical Knowledge as a Biform Theory Graph: A Case Study

A biform theory is a combination of an axiomatic theory and an algorithmic theory that supports the integration of reasoning and computation. These are ideal for formalizing algorithms that manipulate mathematical expressions. A theory graph is a network of theories connected by meaning-preserving theory morphisms that map the formulas of one theory to the formulas of another theory. Theory graphs are in turn well suited for formalizing mathematical knowledge at the most convenient level of abstraction using the most convenient vocabulary. We are interested in the problem of whether a body of mathematical knowledge can be effectively formalized as a theory graph of biform theories. As a test case, we look at the graph of theories encoding natural number arithmetic. We used two different formalisms to do this, which we describe and compare. The first is realized in ${\rm CTT}_{\rm uqe}$, a version of Church's type theory with quotation and evaluation, and the second is realized in Agda, a dependently typed programming language.Comment: 43 pages; published without appendices in: H. Geuvers et al., eds, Intelligent Computer Mathematics (CICM 2017), Lecture Notes in Computer Science, Vol. 10383, pp. 9-24, Springer, 201

Adatom capture by arrays of two-dimensional Ag islands on Ag(100)

We examine the capture of diffusing Ag adatoms by arrays of two-dimensional Ag islands subsequent to deposition on Ag(100) at room temperature. This is achieved by a combination of scanning tunneling microscopy experiments, kinetic Monte Carlo simulations, and diffusion equation analyses. The dependence of the capture rates on Ag-island size is shown to reflect larger island-free regions surrounding the larger islands, i.e., a strong correlation between island sizes and separations. This feature, and the influence of the local environment of the islands on capture, are elucidated by introducing suitable tessellations of the surface into “capture zones” for each island. We show that a Voronoi-type tessellation based on the distance from the island edges accurately reflects adatom capture. However, a tessellation exactly describing adatom capture is only obtained from a solution of the steady-state equation describing adatom deposition, diffusion, and capture by an array of islands distributed as in experiment. The stochastic nature of adatom capture is also quantified by analysis of the dependence on the deposition location of the probability for diffusing adatoms to be captured by a specific island. The experimental island size dependence of adatom capture is found to be entirely consistent with that obtained from a “canonical” model for the irreversible nucleation and growth of square islands

Evolution of far-from-equilibrium nanostructures on Ag(100) surfaces: Protrusions and indentations at extended step edges

Scanning tunneling microscopy is used to monitor the formation and relaxation of nanoprotrusions and nanoindentations at extended step edges following submonolayer deposition of Ag on Ag(100). Deposition of up to about 1/4 ML Ag produces isolated two-dimensional (2D) Ag clusters, which subsequently diffuse, collide, and coalesce with extended step edges, thus forming protrusions. Deposition of larger submonolayer amounts of Ag causes existing step edges to advance across terraces, incorporating 2D islands. The resulting irregular step structure rapidly straightens after terminating deposition, except for a few larger indentations. Relaxation of these far-from-equilibrium step-edge nanoconfigurations is monitored to determine rates for restructuring versus local geometry and feature size. This behavior is analyzed utilizing kinetic Monte Carlo simulations of an atomistic lattice-gas model for relaxation of step-edge nanostructures. In this model, mass transport is mediated by diffusion along the step edge (i.e., “periphery diffusion”). The model consistently fits observed behavior, and allows a detailed characterization of the relaxation process, including assessment of key activation energies