2,726 research outputs found
Towards More Accurate Molecular Dynamics Calculation of Thermal Conductivity. Case Study: GaN Bulk Crystals
Significant differences exist among literature for thermal conductivity of
various systems computed using molecular dynamics simulation. In some cases,
unphysical results, for example, negative thermal conductivity, have been
found. Using GaN as an example case and the direct non-equilibrium method,
extensive molecular dynamics simulations and Monte Carlo analysis of the
results have been carried out to quantify the uncertainty level of the
molecular dynamics methods and to identify the conditions that can yield
sufficiently accurate calculations of thermal conductivity. We found that the
errors of the calculations are mainly due to the statistical thermal
fluctuations. Extrapolating results to the limit of an infinite-size system
tend to magnify the errors and occasionally lead to unphysical results. The
error in bulk estimates can be reduced by performing longer time averages using
properly selected systems over a range of sample lengths. If the errors in the
conductivity estimates associated with each of the sample lengths are kept
below a certain threshold, the likelihood of obtaining unphysical bulk values
becomes insignificant. Using a Monte-Carlo approach developed here, we have
determined the probability distributions for the bulk thermal conductivities
obtained using the direct method. We also have observed a nonlinear effect that
can become a source of significant errors. For the extremely accurate results
presented here, we predict a [0001] GaN thermal conductivity of 185 at 300 K, 102 at 500 K, and 74
at 800 K. Using the insights obtained in the work, we have achieved a
corresponding error level (standard deviation) for the bulk (infinite sample
length) GaN thermal conductivity of less than 10 , 5 , and 15 at 300 K, 500 K, and 800 K respectively
Kapitza conductance and phonon scattering at grain boundaries by simulation
We use a nonequilibrium molecular-dynamics method to compute the Kapitza resistance of three twist grain boundaries in silicon, which we find to increase significantly with increasing grain boundary energy, i.e., with increasing structural disorder at the grain boundary. The origin of this Kapitza resistance is analyzed directly by studying the scattering of packets of lattice vibrations of well-defined polarization and frequency from the grain boundaries. We find that scattering depends strongly on the wavelength of the incident wave packet. In the case of a high-energy grain boundary, the scattering approaches the prediction of the diffuse mismatch theory at high frequencies, i.e., as the wavelength becomes comparable to the lattice parameter of the bulk crystal. We discuss the implications of our results in terms of developing a general model of scattering probabilities that can be applied to mesoscale models of heat transport in polycrystalline systems
Enhanced Anandamide Plasma Levels in Patients with Complex Regional Pain Syndrome following Traumatic Injury: A Preliminary Report
The complex regional pain syndrome (CRPS) is a disabling neuropathic pain condition that may develop following injuries of the extremities. The pathogenesis of this syndrome is not clear; however, it includes complex interactions between the nervous and the immune system resulting in chronic inflammation, pain and trophic changes. This interaction may be mediated by chronic stress which is thought to activate the endogenous cannabinoid (endocannabinoid) system (ECS). We conducted an open, prospective, comparative clinical study to determine plasma level of the endocannabinoid anandamide by high-performance liquid chromatography and a tandem mass spectrometry system in 10 patients with CRPS type I versus 10 age- and sex-matched healthy controls. As compared to healthy controls, CRPS patients showed significantly higher plasma concentrations of anandamide. These results indicate that the peripheral ECS is activated in CRPS. Further studies are warranted to evaluate the role of the ECS in the limitation of inflammation and pain. Copyright (C) 2009 S. Karger AG, Base
Effective Free Energy for Individual Dynamics
Physics and economics are two disciplines that share the common challenge of
linking microscopic and macroscopic behaviors. However, while physics is based
on collective dynamics, economics is based on individual choices. This
conceptual difference is one of the main obstacles one has to overcome in order
to characterize analytically economic models. In this paper, we build both on
statistical mechanics and the game theory notion of Potential Function to
introduce a rigorous generalization of the physicist's free energy, which
includes individual dynamics. Our approach paves the way to analytical
treatments of a wide range of socio-economic models and might bring new
insights into them. As first examples, we derive solutions for a congestion
model and a residential segregation model.Comment: 8 pages, 2 figures, presented at the ECCS'10 conferenc
Phase Behaviour of Binary Hard-Sphere Mixtures: Free Volume Theory Including Reservoir Hard-Core Interactions
Comprehensive calculations were performed to predict the phase behaviour of
large spherical colloids mixed with small spherical colloids that act as
depletant. To this end, the free volume theory (FVT) of Lekkerkerker et al.
[Europhys. Lett. 20 (1992) 559] is used as a basis and is extended to
explicitly include the hard-sphere character of colloidal depletants into the
expression for the free volume fraction. Taking the excluded volume of the
depletants into account in both the system and the reservoir provides a
relation between the depletant concentration in the reservoir and in the system
that accurately matches with computer simulation results of Dijkstra et al.
[Phys. Rev. E 59 (1999) 5744]. Moreover, the phase diagrams for highly
asymmetric mixtures with size ratios q . 0:2 obtained by using this new
approach corroborates simulation results significantly better than earlier FVT
applications to binary hard-sphere mixtures. The phase diagram of a binary
hard-sphere mixture with a size ratio of q = 0:4, where a binary interstitial
solid solution is formed at high densities, is investigated using a numerical
free volume approach. At this size ratio, the obtained phase diagram is
qualitatively different from previous FVT approaches for hard-sphere and
penetrable depletants, but again compares well with simulation predictions.Comment: The following article has been accepted by The Journal of Chemical
Physics. After it is published, it will be found at
https://doi.org/10.1063/5.003796
Dynamic scaling regimes of collective decision making
We investigate a social system of agents faced with a binary choice. We
assume there is a correct, or beneficial, outcome of this choice. Furthermore,
we assume agents are influenced by others in making their decision, and that
the agents can obtain information that may guide them towards making a correct
decision. The dynamic model we propose is of nonequilibrium type, converging to
a final decision. We run it on random graphs and scale-free networks. On random
graphs, we find two distinct regions in terms of the "finalizing time" -- the
time until all agents have finalized their decisions. On scale-free networks on
the other hand, there does not seem to be any such distinct scaling regions
Quantum Games
In these lecture notes we investigate the implications of the identification
of strategies with quantum operations in game theory beyond the results
presented in [J. Eisert, M. Wilkens, and M. Lewenstein, Phys. Rev. Lett. 83,
3077 (1999)]. After introducing a general framework, we study quantum games
with a classical analogue in order to flesh out the peculiarities of game
theoretical settings in the quantum domain. Special emphasis is given to a
detailed investigation of different sets of quantum strategies.Comment: 13 pages (LaTeX), 3 figure
On Spatial Consensus Formation: Is the Sznajd Model Different from a Voter Model?
In this paper, we investigate the so-called ``Sznajd Model'' (SM) in one
dimension, which is a simple cellular automata approach to consensus formation
among two opposite opinions (described by spin up or down). To elucidate the SM
dynamics, we first provide results of computer simulations for the
spatio-temporal evolution of the opinion distribution , the evolution of
magnetization , the distribution of decision times and
relaxation times . In the main part of the paper, it is shown that the
SM can be completely reformulated in terms of a linear VM, where the transition
rates towards a given opinion are directly proportional to frequency of the
respective opinion of the second-nearest neighbors (no matter what the nearest
neighbors are). So, the SM dynamics can be reduced to one rule, ``Just follow
your second-nearest neighbor''. The equivalence is demonstrated by extensive
computer simulations that show the same behavior between SM and VM in terms of
, , , , and the final attractor statistics. The
reformulation of the SM in terms of a VM involves a new parameter , to
bias between anti- and ferromagnetic decisions in the case of frustration. We
show that plays a crucial role in explaining the phase transition
observed in SM. We further explore the role of synchronous versus asynchronous
update rules on the intermediate dynamics and the final attractors. Compared to
the original SM, we find three additional attractors, two of them related to an
asymmetric coexistence between the opposite opinions.Comment: 22 pages, 20 figures. For related publications see
http://www.ais.fraunhofer.de/~fran
Reinforced communication and social navigation generate groups in model networks
To investigate the role of information flow in group formation, we introduce
a model of communication and social navigation. We let agents gather
information in an idealized network society, and demonstrate that heterogeneous
groups can evolve without presuming that individuals have different interests.
In our scenario, individuals' access to global information is constrained by
local communication with the nearest neighbors on a dynamic network. The result
is reinforced interests among like-minded agents in modular networks; the flow
of information works as a glue that keeps individuals together. The model
explains group formation in terms of limited information access and highlights
global broadcasting of information as a way to counterbalance this
fragmentation. To illustrate how the information constraints imposed by the
communication structure affects future development of real-world systems, we
extrapolate dynamics from the topology of four social networks.Comment: 7 pages, 3 figure
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