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Top 20 Most Read Articles
February 2009
The 20 articles with the most full-text downloads during the month, in descending order.
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Molecular simulation methods for soft matter AIP Conf. Proc. 1091, pp. 1-43; doi:http://dx.doi.org/10.1063/1.3082281 (43 pages) Online Publication Date: 30 January 2009
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Molecular simulation methods that are relevant to the study of soft condensed matter are reviewed. A short discussion of molecular interaction potentials is followed by an discussion of molecular dynamics algorithms, stochastic dynamics, and thermostarting methods. The standard Metropolis Monte Carlo algorithm is described, and a short introduction given to weighted and biased sampling methods. Throughout, examples are chosen from the field of soft condensed matter, including colloidal systems, liquid crystals, and biopolymers. |
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Abstracts of selected contributions AIP Conf. Proc. 1091, pp. 267-297; doi:http://dx.doi.org/10.1063/1.3082299 (31 pages) Online Publication Date: 30 January 2009
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Introduction to Monte Carlo simulations of polymers AIP Conf. Proc. 1091, pp. 55-78; doi:http://dx.doi.org/10.1063/1.3082336 (24 pages) Online Publication Date: 30 January 2009
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In this lecture the basic aspects of Monte Carlo simulations are introduced, choosing models of polymers as examples. First the distinction between simple sampling and the Metropolis importance sampling algorithm is discussed, and the main limitations of the latter are discussed: lack of information on the partition function; “dynamical” correlation of “observations” and the resulting interpretation in terms of master equations, which also is the basis for applications to simulate the dynamics of fluctuations, diffusion processes and relaxation processes far from equilibrium; finite size effects, and their analysis in the context of simulations of second‐order and first‐order phase transitions. Examples discussed will include the dynamics of polymer melts, dynamics of translocation of polymers through membranes, and mixing of symmetrical binary mixtures, as well as asymmetrical polymer solutions. Also an introduction to path integral quantum Monte Carlo will be given, mentioning the application to crystalline orthorhombic polyethylene as an example. |
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From micro to macro scales using simulation: examples from hydrodynamics, elasticity and plasticity AIP Conf. Proc. 1091, pp. 79-94; doi:http://dx.doi.org/10.1063/1.3082337 (16 pages) Online Publication Date: 30 January 2009
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Molecular dynamics and Monte‐Carlo simulations, based on the knowledge of molecular interactions, have long been used to obtain information of simple bulk properties of materials, such as equation of state, elastic constants or transport coefficients. In this chapter, I will review several examples in which simulations are used to investigate intermediate length scales associated e.g. with interfaces, or with nanoscale heterogeneous behavior of materials. I will show how the resulting information can be used to infer macroscale properties of materials or interfaces. |
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Lectures on Scattering Amplitudes via AdS∕CFT AIP Conf. Proc. 1031, pp. 43-60; doi:http://dx.doi.org/10.1063/1.2972015 (18 pages) Online Publication Date: 31 July 2008
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We review recent progress in computing scattering amplitudes of planar N = 4 super Yang‐Mills at strong coupling by using the AdS∕CFT duality. We do explicit computations by using both dimensional regularization and a cut‐off in the radial direction. Up to an additive constant, independent of the kinematics, the finite piece of the amplitude is the same in both regularizations. The latter scheme is particularly appropriate for understanding the conformal properties of the amplitudes. |
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Static and dynamic properties of a reversible gel AIP Conf. Proc. 1091, pp. 166-178; doi:http://dx.doi.org/10.1063/1.3082276 (13 pages) Online Publication Date: 30 January 2009
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We study a microscopically realistic model of a physical gel and use computer simulations to investigate its static and dynamic properties at thermal equilibrium. The phase diagram comprises a sol phase, a coexistence region ending at a critical point, a gelation line, and an equilibrium gel phase unrelated to phase separation. The global structure of the gel is homogeneous, but the stress is supported by a fractal network. Gelation results in a dramatic slowing down of the dynamics, which can be used to locate the transition, which otherwise shows no structural signatures. Moreover, the equilibrium gel dynamics is highly heterogeneous as a result of the presence of particle families with different mobilities. An analysis of gel dynamics in terms of mobile and arrested particles allows us to elucidate several differences between the dynamics of equilibrium gels and that of glass‐formers. |
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On the relaxation dynamics of glass‐forming systems: Insights from computer simulations AIP Conf. Proc. 1091, pp. 95-108; doi:http://dx.doi.org/10.1063/1.3082338 (14 pages) Online Publication Date: 30 January 2009
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We discuss the relaxation dynamics of a simple lattice gas model for glass‐forming systems and show that with increasing density of particles this dynamics slows down very quickly. By monitoring the trajectory of tagged particles we find that their motion is very heterogeneous in space and time, leading to regions in space in which there is a fast dynamics and others in which it is slow. We determine how the geometric properties of these quickly relaxing regions depend on density and time. Motivated by this heterogeneous hopping dynamics, we use a simple model, a variant of a continuous time random walk, to characterize the relaxation dynamics. In particular we find from this model that for large displacements the self part of the van Hove function shows an exponential tail, in agreement with recent findings from experiments and simulations of glass‐forming systems. |
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Pattern formation in systems with competing interactions AIP Conf. Proc. 1091, pp. 44-54; doi:http://dx.doi.org/10.1063/1.3082335 (11 pages) Online Publication Date: 30 January 2009
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There is a growing interest, inspired by advances in technology, in the low temperature physics of thin films. These quasi‐2D systems show a wide range of ordering effects including formation of striped states, reorientation transitions, bubble formation in strong magnetic fields, etc. The origins of these phenomena are, in many cases, traced to competition between short ranged exchange ferromagnetic interactions, favoring a homogeneous ordered state, and the long ranged dipole‐dipole interaction, which opposes such ordering on the scale of the whole sample. The present theoretical understanding of these phenomena is based on a combination of variational methods and a variety of approximations, e.g., mean‐field and spin‐wave theory. The comparison between the predictions of these approximate methods and the results of MonteCarlo simulations are often difficult because of the slow relaxation dynamics associated with the long‐range nature of the dipole‐dipole interactions. In this note we will review recent work where we prove existence of periodic structures in some lattice and continuum model systems with competing interactions. The continuum models have also been used to describe micromagnets, diblock polymers, etc. |
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Molecular ordering at an interface by molecular dynamics AIP Conf. Proc. 1091, pp. 134-140; doi:http://dx.doi.org/10.1063/1.3082272 (7 pages) Online Publication Date: 30 January 2009
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In this paper, we present results obtained by Molecular Dynamics on the molecular ordering at a liquid‐vapor interface. Molecules are made of simple diatomics: while entering the interface from the gas phase, the prefered molecular orientations shifts from parallel to perpendicular to the plane of the liquid‐vapor interface. We mention possible extension of those properties in biological membranes. |
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Modeling and simulation of macrocapillarity AIP Conf. Proc. 1091, pp. 141-150; doi:http://dx.doi.org/10.1063/1.3082273 (10 pages) Online Publication Date: 30 January 2009
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Macroscopic capillarity, or macrocapillarity for short, refers to capillary phenomena occurring during twophase and multiphase flow in porous media. Wetting phenomena and hysteresis in porous media are at present poorly understood in the sense that neither in physics nor in engineering a fully predictive theory seems to exist, that can describe or predict all observations. This paper extends the consitutive assumptions of a recent approach based on the concept of hydraulic percolation of fluid phases. The theory proposed here allows prediction of residual saturations. It can describe displacement processes in which imbibition and drainage occur simultaneously. This contrasts with the established traditional theory where capillary forces are lumped into capillary pressure function and relative permeabilities, and these functions need to be specified for each displacement process as input. Contrary to the traditional theory the approach advanced here allows to predict capillary pressure saturation relations as output. |
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Climate and the Earth’s radiation budget AIP Conf. Proc. 247, pp. 55-77; doi:http://dx.doi.org/10.1063/1.41922 (23 pages) Online Publication Date: 29 May 2008
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Results obtained by a NASA multisatellite experiment on the balance of solar radiation and thermal radiation in the atmosphere are presented. The role of radiative—convective—dynanic interactions in producing climate change is discussed. The importance of measuring the feedback between clouds and climate is pointed out. (AIP) |
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Topic 1: Attracting Girls Into Physics AIP Conf. Proc. 628, pp. 9-12; doi:http://dx.doi.org/10.1063/1.1505270 (4 pages) Online Publication Date: 20 August 2002
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We discuss the problems in attracting girls into physics. We present some of the projects undertaken in various countries to ameliorate the problem, and we conclude with some follow‐up suggestions that can be implemented in all countries. © 2002 American Institute of Physics |
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Boundary‐induced heterogeneous absorbing states AIP Conf. Proc. 1091, pp. 204-211; doi:http://dx.doi.org/10.1063/1.3082283 (8 pages) Online Publication Date: 30 January 2009
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We study two different types of systems with many absorbing states (with and without a conservation law) and scrutinize the effect of walls∕boundaries (either absorbing or reflecting) into them. In some cases, non‐trivial structured absorbing configurations (characterized by a background field) develop around the wall. We study such structures using a mean‐field approach as well as computer simulations. The main results are: i) for systems in the directed percolation class, a very fast (exponential) convergence of the background to its bulk value is observed; ii) for systems with a conservation law, power‐law decaying landscapes are induced by both types of walls: while for absorbing walls this effect is already present in the mean‐field approximation, for reflecting walls the structured background is a noise‐induced effect. The landscapes are shown to converge to their asymptotic bulk values with an exponent equal to the inverse of the bulk correlation length exponent. Finally, the implications of these results in the context of self‐organizing systems are discussed. |
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Symbols: Weapons of Math Destruction AIP Conf. Proc. 951, pp. 200-203; doi:http://dx.doi.org/10.1063/1.2820933 (4 pages) Online Publication Date: 28 November 2007
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This paper is part of an ongoing investigation of how students use and understand mathematics in introductory physics. Our previous research [1] revealed that differences in score as large as 50% can be observed between numeric and symbolic versions of the same question. We have expanded our study of numeric and symbolic differences to include 10 pairs of questions on a calculus based introductory physics final exam. We find that not all physics problems exhibit such large differences and that in the cases where a large difference is observed that the largest difference occurs for the poorest students. With these 10 questions we have been able to develop phenomenological categories to characterize the properties of each of the questions. We will discuss what question properties are necessary to observe differences in score on the numeric and symbolic versions. We will also discuss what insights these categories give us about how students think about and use symbols in physics. |
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AIP Conf. Proc. 1020, pp. 1195-1202; doi:http://dx.doi.org/10.1063/1.2963740 (8 pages) Online Publication Date: 8 July 2008
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In the recent years, scientists developed the seismic rehabilitation of structures and their view points were changed from sufficient strength to the performance of structures (Performance Base Design) to prepare a safe design. Nonlinear Static Procedure analysis (NSP) or pushover analysis is a new method that is chosen for its speed and simplicity in calculations. “Seismic Rehabilitation Code for Existing Buildings” and FEMA 356 considered this method. Result of this analysis is a target displacement that is the base of the performance and rehabilitation procedure of the structures. Exact recognition of that displacement could develop the workability of pushover analysis. In these days, Nonlinear Dynamic Analysis (NDP) is only method can exactly apply the seismic ground motions. In this case because it consumes time, costs very high and is more difficult than other methods, is not applicable as much as NSP. A coefficient used in NSP for determining the target displacement is C2 (Stiffness and Strength Degradations Coefficient) and is applicable for correcting the errors due to eliminating the stiffness and strength degradations in hysteretic loops. In this study it has been tried to analysis three concrete frames with shear walls by several accelerations that scaled according to FEMA 273 and FEMA 356. These structures were designed with Iranian 2800 standard (vers.3). Finally after the analyzing by pushover method and comparison results with dynamic analysis, calculated C2 was comprised with values in rehabilitation codes. |
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Flow Visualization within the Evaporator of Planar Loop Heat Pipe AIP Conf. Proc. 746, pp. 195-202; doi:http://dx.doi.org/10.1063/1.1867135 (8 pages) Online Publication Date: 16 March 2005
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A planar micro loop heat pipe (LHP) with coherent porous silicon (CPS) wick in the evaporator is a two‐phase heat transfer device that utilizes evaporation and condensation to transfer heat. This CPS wick has thousands of pores, which are 2 micrometer in diameter, contained over an area of one square centimeter. As heat is applied to the evaporator, liquid is vaporized and evaporator chamber’s pressure is increased. A meniscus formed at the liquid/vapor interface inside the pore of the CPS wick is supported by capillary forces even though pressure force pushes it down. Vapor flows through the vapor line to the condenser and condenses. Liquid is transported back to the evaporator due to pressure difference. The internal thermodynamics and fluid dynamics are poorly understood due to the difficulty of taking internal measurements and the complexity of two‐phase phenomena. To understand this thermal device, the clear evaporator machined from Pyrex glass was utilized to monitor the complex phenomena which occur in the evaporator. These phenomena include vapor formation, nucleate boiling, evaporation, depriming, and pressure oscillation. DI‐water was utilized as the working fluid. © 2005 American Institute of Physics |
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Organic Semiconductors for Low—Cost Solar Cells AIP Conf. Proc. 1044, pp. 322-330; doi:http://dx.doi.org/10.1063/1.2993730 (9 pages) Online Publication Date: 22 September 2008
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The current cost of solar electricity derived from silicon photovoltaics is about 30 to 40 cents per kilowatt—hour. This cost is similar to peak—power charges in California during the height of summer, thus establishing a partial path to economic viability. However, this competitiveness is not viable in other seasons and many other locations. This paper will discuss the basic theory and progress of a new class of photovoltaic semiconductors derived from organic polymer materials. These materials have obtained promising results with 5% conversion efficiency. In addition, these materials can be manufactured relatively easily by using printing technologies and roll‐to‐roll coating machines, similar to those used to make photographic film or newspapers. Solar cells made this way would not only be cheaper, but could also be incorporated into roofing materials to reduce installation costs. Organic semiconductors can be dissolved in common solvents and sprayed or printed onto substrates, so they are very promising candidates for the solar production of electricity. |
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Global fluctuations in dissipative systems AIP Conf. Proc. 1091, pp. 201-203; doi:http://dx.doi.org/10.1063/1.3082282 (3 pages) Online Publication Date: 30 January 2009
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Our goal here is to investigate the influence of velocity correlations on the fluctuations of several global quantities for two different systems. In the first case, we study the fluctuations of the total number of particles, momentum and energy in a probabilistic ballistic annihilation model. For this model, when there is a binary encounter, the particles annihilate with certain probability. In the second case, we analyze the fluctuations of the total energy in a granular system which is driven by a stochastic thermostat. The theoretical analysis is based on the study of the equation for the two‐particle distribution function. The predictions are in very good agreement with simulations results. |
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AIP Conf. Proc. 1044, pp. 309-321; doi:http://dx.doi.org/10.1063/1.2993729 (13 pages) Online Publication Date: 22 September 2008
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If solar energy is to become a practical alternative to fossil fuels, we must have efficient ways to convert photons into electricity, fuel, and heat. The need for better conversion technologies is a driving force behind many recent developments in biology, materials, and especially nanoscience. |
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STEEL SHEAR WALLS, BEHAVIOR, MODELING AND DESIGN AIP Conf. Proc. 1020, pp. 5-18; doi:http://dx.doi.org/10.1063/1.2963889 (14 pages) Online Publication Date: 8 July 2008
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In recent years steel shear walls have become one of the more efficient lateral load resisting systems in tall buildings. The basic steel shear wall system consists of a steel plate welded to boundary steel columns and boundary steel beams. In some cases the boundary columns have been concrete‐filled steel tubes. Seismic behavior of steel shear wall systems during actual earthquakes and based on laboratory cyclic tests indicates that the systems are quite ductile and can be designed in an economical way to have sufficient stiffness, strength, ductility and energy dissipation capacity to resist seismic effects of strong earthquakes. This paper, after summarizing the past research, presents the results of two tests of an innovative steel shear wall system where the boundary elements are concrete‐filled tubes. Then, a review of currently available analytical models of steel shear walls is provided with a discussion of capabilities and limitations of each model. We have observed that the tension only “strip model”, forming the basis of the current AISC seismic design provisions for steel shear walls, is not capable of predicting the behavior of steel shear walls with length‐to‐thickness ratio less than about 600 which is the range most common in buildings. The main reasons for such shortcomings of the AISC seismic design provisions for steel shear walls is that it ignores the compression field in the shear walls, which can be significant in typical shear walls. The AISC method also is not capable of incorporating stresses in the shear wall due to overturning moments. A more rational seismic design procedure for design of shear walls proposed in 2000 by the author is summarized in the paper. The design method, based on procedures used for design of steel plate girders, takes into account both tension and compression stress fields and is applicable to all values of length‐to‐thickness ratios of steel shear walls. The method is also capable of including the effect of overturning moments and any normal forces that might act on the steel shear wall. |
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GBR - Devon
Chair and Lecturer (2 posts)
Sandia National Laboratories
US - NM - Albuquerque
Post Doc – Nuclear/ Nanoparticle Materials
Syracuse University
US - NY - Syracuse
Postdoc in Experimental CM Physics
University of Rochester
US - NY - Rochester
Biomedical Optics
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