221 research outputs found
Evidence of ion diffusion at room temperature in microcrystals of the Bi2Sr2CaCu2O8+delta superconductor
We have studied Bi-2212 microcrystals aged at ambient conditions for 40 days.
Combined x-ray absorption near edge structure and x-ray fluorescence
measurements with micrometer space resolution show both an increase of Cu
with respect to Cu and an enrichment in Cu vs Bi and Sr cation content
near the sample edges in the b-axis direction. A parallel study on an
electrically contacted sample has indirectly detected the O loss, observing
both a resistivity increase and an increase in sample thickness near the edges.
We conclude that the O out-diffusion along the b-axis is accompanied by Cu
cation migration in the same direction.Comment: RevTeX 4, 10 pages, 3 figure
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Teaching varies with task complexity in wild chimpanzees
Understanding social influences on how apes acquire tool behaviors can help us model the evolution of culture and technology in humans. Humans scaffold novice tool skills with diverse strategies, including the transfer of tools between individuals. Chimpanzees transfer tools, and this behavior meets criteria for teaching. However, it is unclear how task complexity relates to this form of helping. Here, we find differences between 2 wild chimpanzee populations in rate, probability, and types of tool transfer during termite gathering. Chimpanzees showed greater helping in the population where termite gathering is a more complex tool task. In wild chimpanzees, as in humans, regular and active provisioning of learning opportunities may be essential to the cultural transmission of complex skills.Cumulative culture is a transformative force in human evolution, but the social underpinnings of this capacity are debated. Identifying social influences on how chimpanzees acquire tool tasks of differing complexity may help illuminate the evolutionary origins of technology in our own lineage. Humans routinely transfer tools to novices to scaffold their skill development. While tool transfers occur in wild chimpanzees and fulfill criteria for teaching, it is unknown whether this form of helping varies between populations and across tasks. Applying standardized methods, we compared tool transfers during termite gathering by chimpanzees in the Goualougo Triangle, Republic of Congo, and in Gombe, Tanzania. At Goualougo, chimpanzees use multiple, different tool types sequentially, choose specific raw materials, and perform modifications that improve tool efficiency, which could make it challenging for novices to manufacture suitable tools. Termite gathering at Gombe involves a single tool type, fishing probes, which can be manufactured from various materials. Multiple measures indicated population differences in tool-transfer behavior. The rate of transfers and probability of transfer upon request were significantly higher at Goualougo, while resistance to transfers was significantly higher at Gombe. Active transfers of tools in which possessors moved to facilitate possession change upon request occurred only at Goualougo, where they were the most common transfer type. At Gombe, tool requests were typically refused. We suggest that these population differences in tool-transfer behavior may relate to task complexity and that active helping plays an enhanced role in the cultural transmission of complex technology in wild apes
On the accuracy of the melting curves drawn from modelling a solid as an elastic medium
An ongoing problem in the study of a classical many-body system is the
characterization of its equilibrium behaviour by theory or numerical
simulation. For purely repulsive particles, locating the melting line in the
pressure-temperature plane can be especially hard if the interparticle
potential has a softened core or contains some adjustable parameters. A method
is hereby presented that yields reliable melting-curve topologies with
negligible computational effort. It is obtained by combining the Lindemann
melting criterion with a description of the solid phase as an elastic
continuum. A number of examples are given in order to illustrate the scope of
the method and possible shortcomings. For a two-body repulsion of Gaussian
shape, the outcome of the present approach compares favourably with the more
accurate but also more computationally demanding self-consistent harmonic
approximation.Comment: 25 pages, 7 figure
Simulation of adsorbate-induced faceting on curved surfaces
A simple solid-on-solid model, proposed earlier to describe overlayer-induced
faceting of bcc(111) surface, is applied to faceting of spherical surfaces
covered by adsorbate monolayer. Monte Carlo simulation results show that
morphology of faceted surface depends on annealing temperature. At initial
stage surface around the [111] pole consists of 3-sided pyramids and step-like
facets, then step-like facets dominate and their number decreases with
temperature, finally a single big pyramid is formed. It is shown that there is
reversible phase transition at which faceted surface transforms to almost
spherical one. It is found that temperature of this phase transition is an
increasing function of surface curvature. Simulation results show that
measurements of high temperature properties performed directly and after fast
cooling to low temperature lead to different results.Comment: 8 pages, 10 figure
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