University of Potsdam

About: University of Potsdam is a education organization based out in Potsdam, Germany. It is known for research contribution in the topics: Population & Computer science. The organization has 9629 authors who have published 26740 publications receiving 759745 citations. The organization is also known as: Universität Potsdam.

Metastability in Reversible Diffusion Processes I: Sharp Asymptotics for Capacities and Exit Times

Abstract: We develop a potential theoretic approach to the problem of metastability for reversible diffusion processes with generators of the form 1 +rF(·)r on R d or subsets of R d , whereF is a smooth function with finitely many local minima. In analogy to previous work on discrete Markov chains, we show that metastable exit times from the attractive domains of the minima of F can be related, up to multiplicative errors that tend to one as # 0, to the capacities of suitably constructed sets. We show that these capacities can be computed, again up to multiplicative errors that tend to one, in terms of local characteristics of F at the starting minimum and the relevant saddle points. As a result, we are able to give the first rigorous proof of the classical Eyring-Kramers formulain dimension larger than 1. The estimates on capacities make use of their variational representation and monotonicity properties of Dirichlet forms. The methods developed here are extensions of our earlier work on discrete Markov chains to continuous diffusion processes.

Reprogrammable recovery and actuation behaviour of shape-memory polymers

Abstract: Shape memory is the capability of a material to be deformed and fixed into a temporary shape. Recovery of the original shape can then be triggered only by an external stimulus. Shape-memory polymers are highly deformable materials that can be programmed to recover a memorized shape in response to a variety of environmental and spatially localized stimuli as a one-way effect. The shape-memory function can also be generated as a reversible effect enabling actuation behaviour through macroscale deformation and processing, specifically by dictating the macromolecular orientation of actuation units and of the skeleton structure of geometry-determining units in the polymers. Shape-memory polymers can be programmed and reprogrammed into arbitrary shapes. Both recovery and actuation behaviour are reprogrammable. In this Review, we outline the common basis and key differences between the two shape-memory behaviours of polymers in terms of mechanism, fabrication schemes and characterization methods. We discuss which combination of macromolecular architecture and macroscale processing is necessary for coordinated, decentralized and responsive physical behaviour. The extraction of relevant thermomechanical information is described, and design criteria are shown for microscale and macroscale morphologies to gain high levels of recovered or actuation strains as well as on-demand 2D-to-3D shape transformations. Finally, real-world applications and key future challenges are highlighted. Shape-memory materials can generate programmable movements triggered by an external stimulus, such as an environmental change. In this Review, the authors discuss mechanisms, fabrication schemes, characterization methods and applications of the one-way shape-memory effect enabling shape recovery and of reversible shape-memory effects exhibiting actuation behaviour.

Amplifying the Pacific Climate System Response to a Small 11-Year Solar Cycle Forcing

Abstract: One of the mysteries regarding Earth's climate system response to variations in solar output is how the relatively small fluctuations of the 11-year solar cycle can produce the magnitude of the observed climate signals in the tropical Pacific associated with such solar variability. Two mechanisms, the top-down stratospheric response of ozone to fluctuations of shortwave solar forcing and the bottom-up coupled ocean-atmosphere surface response, are included in versions of three global climate models, with either mechanism acting alone or both acting together. We show that the two mechanisms act together to enhance the climatological off-equatorial tropical precipitation maxima in the Pacific, lower the eastern equatorial Pacific sea surface temperatures during peaks in the 11-year solar cycle, and reduce low-latitude clouds to amplify the solar forcing at the surface.

Diamonds sampled by plumes from the core–mantle boundary

Abstract: Diamond formation occurs in high-pressure conditions more than 150 kilometres deep in the Earth's mantle. The diamonds make it to the surface in vertical pipe-like structures made up of volcanic rocks called kimberlites. Several thousand such kimberlite pipes have been mapped so far, but research has focused on very old cratons, the areas of oldest continental crust, as this is where most economically viable diamonds are found. Trond Torsvik and colleagues use a plate-tectonic reconstruction for the past 540 million years to locate the positions of these cratons relative to the deep mantle at times when kimberlites were erupted. The kimberlites are shown to have been associated with the edges of large-scale heterogeneities in the deepest mantle, which the authors infer were zones at the core–mantle boundary where magma upwelling generated the mantle plumes that led to the formation of the kimberlites. These plumes may have controlled the distribution of almost all kimberlites that have erupted in the past 540 million years. Diamonds are formed under high pressure more than 150 kilometres deep in the Earth's mantle, and are brought to the surface mainly by volcanic rocks called kimberlites. Here, plate reconstructions and tomographic images have been used to show that the edges of the largest heterogeneities in the deepest mantle, stable for at least 200 million years and possibly for 540 million years, seem to have controlled the eruption of most Phanerozoic kimberlites. This has implications for future exploration for kimberlites. Diamonds are formed under high pressure more than 150 kilometres deep in the Earth’s mantle and are brought to the surface mainly by volcanic rocks called kimberlites. Several thousand kimberlites have been mapped on various scales1,2,3,4, but it is the distribution of kimberlites in the very old cratons (stable areas of the continental lithosphere that are more than 2.5 billion years old and 300 kilometres thick or more5) that have generated the most interest, because kimberlites from those areas are the major carriers of economically viable diamond resources. Kimberlites, which are themselves derived from depths of more than 150 kilometres, provide invaluable information on the composition of the deep subcontinental mantle lithosphere, and on melting and metasomatic processes at or near the interface with the underlying flowing mantle. Here we use plate reconstructions and tomographic images to show that the edges of the largest heterogeneities in the deepest mantle, stable for at least 200 million years and possibly for 540 million years, seem to have controlled the eruption of most Phanerozoic kimberlites. We infer that future exploration for kimberlites and their included diamonds should therefore be concentrated in continents with old cratons that once overlay these plume-generation zones at the core–mantle boundary.