Lonely adatoms in space

There is a close relation between the problems of second layer nucleation in epitaxial crystal growth and chemical surface reactions, such as hydrogen recombination, on interstellar dust grains. In both cases standard rate equation analysis has been found to fail because the process takes place in a confined geometry. Using scaling arguments developed in the context of second layer nucleation, I present a simple derivation of the hydrogen recombination rate for small and large grains. I clarify the reasons for the failure of rate equations for small grains, and point out a logarithmic correction to the reaction rate when the reaction is limited by the desorption of hydrogen atoms (the second order reaction regime)

Scaling properties of step bunches induced by sublimation and related mechanisms: A unified perspective

This work provides a ground for a quantitative interpretation of experiments on step bunching during sublimation of crystals with a pronounced Ehrlich-Schwoebel (ES) barrier in the regime of weak desorption. A strong step bunching instability takes place when the kinetic length is larger than the average distance between the steps on the vicinal surface. In the opposite limit the instability is weak and step bunching can occur only when the magnitude of step-step repulsion is small. The central result are power law relations of the between the width, the height, and the minimum interstep distance of a bunch. These relations are obtained from a continuum evolution equation for the surface profile, which is derived from the discrete step dynamical equations for. The analysis of the continuum equation reveals the existence of two types of stationary bunch profiles with different scaling properties. Through a mathematical equivalence on the level of the discrete step equations as well as on the continuum level, our results carry over to the problems of step bunching induced by growth with a strong inverse ES effect, and by electromigration in the attachment/detachment limited regime. Thus our work provides support for the existence of universality classes of step bunching instabilities [A. Pimpinelli et al., Phys. Rev. Lett. 88, 206103 (2002)], but some aspects of the universality scenario need to be revised.Comment: 21 pages, 8 figure

Asymptotic step profiles from a nonlinear growth equation for vicinal surfaces

We study a recently proposed nonlinear evolution equation describing the collective step meander on a vicinal surface subject to the Bales-Zangwill growth instability [O. Pierre-Louis et al., Phys. Rev. Lett. (80), 4221 (1998)]. A careful numerical analysis shows that the dynamically selected step profile consists of sloped segments, given by an inverse error function and steepening as sqrt(t), which are matched to pieces of a stationary (time-independent) solution describing the maxima and minima. The effect of smoothening by step edge diffusion is included heuristically, and a one-parameter family of evolution equations is introduced which contains relaxation by step edge diffusion and by attachment-detachment as special cases. The question of the persistence of an initially imposed meander wavelength is investigated in relation to recent experiments.Comment: 4 pages, 5 included figures. Typo in Eq.(5) corrected, section headlines added and Ref.[12] update

Advancing the Right to Health: The Vital Role of Law

Effective laws and an enabling legal environment are essential to a healthy society. Most public health challenges – from infectious and non-communicable diseases to injuries, from mental illness to universal health coverage – have a legal component. At global, national and local levels, law is a powerful tool for advancing the right to health. This tool is, however, often underutilized. This report aims to raise awareness about the role that public health laws can play in advancing the right to health and in creating the conditions for all people to live healthy lives. The report provides guidance about issues and requirements to be addressed during the process of developing or reforming public health laws, with case studies drawn from countries around the world to illustrate effective practices and critical features of effective public health legislation. Advancing the right to health: the vital role of law is the result of a collaboration between the World Health Organisation, the International Development Law Organisation (IDLO), the O’Neill Institute for National and Global Health Law, Washington D.C., USA, and Sydney Law School, University of Sydney. The Project Directors were: Professor Lawrence O. Gostin, Linda D. and Timothy J. O’Neill Professor of Global Health Law and University Professor, Georgetown University; Faculty Director, O’Neill Institute for National and Global Health Law, Georgetown University; Mr David Patterson, Senior Legal Expert – Health; Department of Research & Learning, International Development Law Organization; Professor Roger Magnusson, Professor of Health Law & Governance, Sydney Law School, University of Sydney; Mr Oscar Cabrera, Executive Director, O’Neill Institute for National and Global Health Law, Georgetown University Law Center; Ms Helena Nygren-Krug (2011–2013), Senior Advisor, Human Rights & Law, UNAIDS. The content and structure of the report reflect the consensus reached at the second of two international consultations in public health law that preceded the preparation of the report, hosted by WHO and IDLO in Cairo, Egypt, 26-28 April 2010. Part 1 introduces the human right to health and its role in guiding and evaluating law reform efforts, including efforts to achieve the goal of universal health coverage. Part 2 discusses the process of public health law reform. The law reform process refers to the practical steps involved in advancing the political goal of law reform, and the kinds of issues and obstacles that may be encountered along the way. Part 2 identifies some of the actors who may initiate or lead the public health law reform process, discusses principles of good governance during that process, and ways of building a consensus around the need for public health law reform. Part 3 turns from the process of reforming public health laws to the substance or content of those laws. It identifies a number of core areas of public health practice where regulation is essential in order to ensure that governments (at different levels) discharge their basic public health functions. Traditionally, these core areas of public health practice have included: the provision of clean water and sanitation, monitoring and surveillance of public health threats, the management of communicable diseases, and emergency powers. Building on these core public health functions, Part 3 goes on to consider a range of other public health priorities where law has a critical role to play. These priorities include tobacco control, access to essential medicines, the migration of health care workers, nutrition, maternal, reproductive and child health, and the role of law in advancing universal access to quality health services for all members of the population. The report includes many examples that illustrate the ways in which different countries have used law to protect the health of their populations in ways that are consistent with their human rights obligations. Countries vary widely in terms of their constitutional structure, size, history and political culture. For these reasons, the examples given are not intended to be prescriptive, but to provide useful comparisons for countries involved in the process of legislative review

Spiral Growth and Step Edge Barriers

The growth of spiral mounds containing a screw dislocation is compared to the growth of wedding cakes by two-dimensional nucleation. Using phase field simulations and homoepitaxial growth experiments on the Pt(111) surface we show that both structures attain the same characteristic large scale shape when a significant step edge barrier suppresses interlayer transport. The higher vertical growth rate observed for the spiral mounds on Pt(111) reflects the different incorporation mechanisms for atoms in the top region and can be formally represented by an enhanced apparent step edge barrier.Comment: 11 pages, 4 figures, partly in colo

Linear theory of unstable growth on rough surfaces

Unstable homoepitaxy on rough substrates is treated within a linear continuum theory. The time dependence of the surface width $W(t)$ is governed by three length scales: The characteristic scale $l_0$ of the substrate roughness, the terrace size $l_D$ and the Ehrlich-Schwoebel length $l_{ES}$. If $l_{ES} \ll l_D$ (weak step edge barriers) and $l_0 \ll l_m \sim l_D \sqrt{l_D/l_{ES}}$, then $W(t)$ displays a minimum at a coverage $\theta_{\rm min} \sim (l_D/l_{ES})^2$, where the initial surface width is reduced by a factor $l_0/l_m$. The r\^{o}le of deposition and diffusion noise is analyzed. The results are applied to recent experiments on the growth of InAs buffer layers [M.F. Gyure {\em et al.}, Phys. Rev. Lett. {\bf 81}, 4931 (1998)]. The overall features of the observed roughness evolution are captured by the linear theory, but the detailed time dependence shows distinct deviations which suggest a significant influence of nonlinearities

A universal ionization threshold for strongly driven Rydberg states

We observe a universal ionization threshold for microwave driven one-electron Rydberg states of H, Li, Na, and Rb, in an {\em ab initio} numerical treatment without adjustable parameters. This sheds new light on old experimental data, and widens the scene for Anderson localization in light matter interaction.Comment: 4 pages, 1 figur

Accurate rate coefficients for models of interstellar gas-grain chemistry

The methodology for modeling grain-surface chemistry has been greatly improved by taking into account the grain size and fluctuation effects. However, the reaction rate coefficients currently used in all practical models of gas-grain chemistry are inaccurate by a significant amount. We provide expressions for these crucial rate coefficients that are both accurate and easy to incorporate into gas-grain models. We use exact results obtained in earlier work, where the reaction rate coefficient was defined by a first-passage problem, which was solved using random walk theory. The approximate reaction rate coefficient presented here is easy to include in all models of interstellar gas-grain chemistry. In contrast to the commonly used expression, the results that it provides are in perfect agreement with detailed kinetic Monte Carlo simulations. We also show the rate coefficient for reactions involving multiple species.Comment: 4 pages, 2 figure

Driven Lattice Gases with Quenched Disorder: Exact Results and Different Macroscopic Regimes

We study the effect of quenched spatial disorder on the steady states of driven systems of interacting particles. Two sorts of models are studied: disordered drop-push processes and their generalizations, and the disordered asymmetric simple exclusion process. We write down the exact steady-state measure, and consequently a number of physical quantities explicitly, for the drop-push dynamics in any dimensions for arbitrary disorder. We find that three qualitatively different regimes of behaviour are possible in 1-$d$ disordered driven systems. In the Vanishing-Current regime, the steady-state current approaches zero in the thermodynamic limit. A system with a non-zero current can either be in the Homogeneous regime, chracterized by a single macroscopic density, or the Segregated-Density regime, with macroscopic regions of different densities. We comment on certain important constraints to be taken care of in any field theory of disordered systems.Comment: RevTex, 17pages, 18 figures included using psfig.st

Level Crossing Analysis of Growing surfaces

We investigate the average frequency of positive slope $\nu_{\alpha}^{+}$, crossing the height $\alpha = h- \bar h$ in the surface growing processes. The exact level crossing analysis of the random deposition model and the Kardar-Parisi-Zhang equation in the strong coupling limit before creation of singularities are given.Comment: 5 pages, two column, latex, three figure