Self-diffusion of adatoms, dimers, and vacancies on Cu(100)

We use ab initio static relaxation methods and semi-empirical molecular-dynamics simulations to investigate the energetics and dynamics of the diffusion of adatoms, dimers, and vacancies on Cu(100). It is found that the dynamical energy barriers for diffusion are well approximated by the static, 0 K barriers and that prefactors do not depend sensitively on the species undergoing diffusion. The ab initio barriers are observed to be significantly lower when calculated within the generalized-gradient approximation (GGA) rather than in the local-density approximation (LDA). Our calculations predict that surface diffusion should proceed primarily via the diffusion of vacancies. Adatoms are found to migrate most easily via a jump mechanism. This is the case, also, of dimers, even though the corresponding barrier is slightly larger than it is for adatoms. We observe, further, that dimers diffuse more readily than they can dissociate. Our results are discussed in the context of recent submonolayer growth experiments of Cu(100).Comment: Submitted to the Physical Review B; 15 pages including postscript figures; see also http://www.centrcn.umontreal.ca/~lewi

Widespread exploitation of the honeybee by early Neolithic farmers.

This is the author's version of an article subsequently published in Nature. The definitive version is available from the publisher via: doi: 10.1038/nature15757.Copyright © 2015, Rights Managed by Nature Publishing GroupThe pressures on honeybee (Apis mellifera) populations, resulting from threats by modern pesticides, parasites, predators and diseases, have raised awareness of the economic importance and critical role this insect plays in agricultural societies across the globe. However, the association of humans with A. mellifera predates post-industrial-revolution agriculture, as evidenced by the widespread presence of ancient Egyptian bee iconography dating to the Old Kingdom (approximately 2400 BC). There are also indications of Stone Age people harvesting bee products; for example, honey hunting is interpreted from rock art in a prehistoric Holocene context and a beeswax find in a pre-agriculturalist site. However, when and where the regular association of A. mellifera with agriculturalists emerged is unknown. One of the major products of A. mellifera is beeswax, which is composed of a complex suite of lipids including n-alkanes, n-alkanoic acids and fatty acyl wax esters. The composition is highly constant as it is determined genetically through the insect's biochemistry. Thus, the chemical 'fingerprint' of beeswax provides a reliable basis for detecting this commodity in organic residues preserved at archaeological sites, which we now use to trace the exploitation by humans of A. mellifera temporally and spatially. Here we present secure identifications of beeswax in lipid residues preserved in pottery vessels of Neolithic Old World farmers. The geographical range of bee product exploitation is traced in Neolithic Europe, the Near East and North Africa, providing the palaeoecological range of honeybees during prehistory. Temporally, we demonstrate that bee products were exploited continuously, and probably extensively in some regions, at least from the seventh millennium cal BC, likely fulfilling a variety of technological and cultural functions. The close association of A. mellifera with Neolithic farming communities dates to the early onset of agriculture and may provide evidence for the beginnings of a domestication process.Natural Environment Research Council (NERC)English HeritageEuropean Research Council (ERC)Leverhulme TrustMinistère de la Culture et de la CommunicationMinistère de l’Enseignement Supérieur et de la RechercheRoyal SocietyWellcome Trus

Association of protein fractions and lipids from human erythrocyte membranes. I. Studies on a strongly bound protein fraction

Half of the protein content of human erythrocyte membranes was removed from the membrane by 10% acetic acid. The remaining lipoprotein complex was depolymerized by 90%acetic acid. Protein and lipid were separated by gel filtration, and the protein fraction was transferred into aqueous buffers. Association in aqueous buffers could be observed between the protein fraction and sonicated total lipids from human erythrocyte membranes. Lipoprotein formation was possible over a wide range of conditions with respect to pH and ionic strength, including physiological conditions, and to protein and lipid concentrations. After equilibrium centrifugation in a density gradient, the lipoproteins formed clearly visible, narrow bands which contained up to 70% of the proteins and the lipids applied to the gradient. The density of the bands did not depend significantly on pH, ionic strength or the kind of ions present during recombination. It was, however, strongly influenced by the weight ratio of protein and lipid in the recombination mixture. In the electron microscope, the lipoprotein aggregates showed the typical trilaminar structure of membranes. Most of the aggregates formed closed vesicles. The same appearance was exhibited by the protein-free lipids used for the recombination experiment

Portugal: Die Rolle der Regionen

SIGLEBibliothek Weltwirtschaft Kiel C 167860 / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman

On minimizing the maximum flow time in the online dial-a-ride problem

In the online dial-a-ride problem (OlDarp), objects must be transported by a server between points in a metric space. Transportation requests ( rides ) arrive online, specifying the objects to be transported and the corresponding source and destination. We investigate the OlDarp for the objective of minimizing the maximum flow time. It has been well known that there can be no strictly competitive online algorithm for this objective and no competitive algorithm at all on unbounded metric spaces. However, the question whether on metric spaces with bounded diameter there are competitive algorithms if one allows an additive constant in the definition competitive ratio, had been open for quite a while. We provide a negative answer to this question already on the uniform metric space with three points. Our negative result is complemented by a strictly 2-competitive algorithm for the Online Traveling Salesman Problem on the uniform metric space, a special case of the problem

On minimizing the maximum flow time in the online dial-a-ride problem

In the online dial-a-ride problem (OlDarp), objects must be transported by a server between points in a metric space. Transportation requests ( rides ) arrive online, specifying the objects to be transported and the corresponding source and destination. We investigate the OlDarp for the objective of minimizing the maximum flow time. It has been well known that there can be no strictly competitive online algorithm for this objective and no competitive algorithm at all on unbounded metric spaces. However, the question whether on metric spaces with bounded diameter there are competitive algorithms if one allows an additive constant in the definition competitive ratio, had been open for quite a while. We provide a negative answer to this question already on the uniform metric space with three points. Our negative result is complemented by a strictly 2-competitive algorithm for the Online Traveling Salesman Problem on the uniform metric space, a special case of the problem