203 research outputs found
Asperity contacts at the nanoscale: comparison of Ru and Au
We develop and validate an interatomic potential for ruthenium based on the
embedded atom method framework with the Finnis/Sinclair representation. We
confirm that the new potential yields a stable hcp lattice with reasonable
lattice and elastic constants and surface and stacking fault energies. We
employ molecular dynamics simulations to bring two surfaces together; one flat
and the other with a single asperity. We compare the process of asperity
contact formation and breaking in Au and Ru, two materials currently in use in
micro electro mechanical system switches. While Au is very ductile at 150 and
300 K, Ru shows considerably less plasticity at 300 and 600 K (approximately
the same homologous temperature). In Au, the asperity necks down to a single
atom thick bridge at separation. While similar necking occurs in Ru at 600 K,
it is much more limited than in Au. On the other hand, at 300 K, Ru breaks by a
much more brittle process of fracture/decohesion with limited plastic
deformation.Comment: 10 pages, 13 figure
Discrete molecular dynamics studies of the folding of a protein-like model
Background: Many attempts have been made to resolve in time the folding of
model proteins in computer simulations. Different computational approaches have
emerged. Some of these approaches suffer from the insensitivity to the
geometrical properties of the proteins (lattice models), while others are
computationally heavy (traditional MD).
Results: We use a recently-proposed approach of Zhou and Karplus to study the
folding of the protein model based on the discrete time molecular dynamics
algorithm. We show that this algorithm resolves with respect to time the
folding --- unfolding transition. In addition, we demonstrate the ability to
study the coreof the model protein.
Conclusion: The algorithm along with the model of inter-residue interactions
can serve as a tool to study the thermodynamics and kinetics of protein models.Comment: 15 pages including 20 figures (Folding & Design in press
Energy landscape of a simple model for strong liquids
We calculate the statistical properties of the energy landscape of a minimal
model for strong network-forming liquids. Dynamics and thermodynamic properties
of this model can be computed with arbitrary precision even at low
temperatures. A degenerate disordered ground state and logarithmic statistics
for the energy distribution are the landscape signatures of strong liquid
behavior. Differences from fragile liquid properties are attributed to the
presence of a discrete energy scale, provided by the particle bonds, and to the
intrinsic degeneracy of topologically disordered networks.Comment: Revised versio
Non-Gaussian energy landscape of a simple model for strong network-forming liquids: accurate evaluation of the configurational entropy
We present a numerical study of the statistical properties of the potential
energy landscape of a simple model for strong network-forming liquids. The
model is a system of spherical particles interacting through a square well
potential, with an additional constraint that limits the maximum number of
bonds, , per particle. Extensive simulations have been carried out
as a function of temperature, packing fraction, and . The dynamics
of this model are characterized by Arrhenius temperature dependence of the
transport coefficients and by nearly exponential relaxation of dynamic
correlators, i.e. features defining strong glass-forming liquids. This model
has two important features: (i) landscape basins can be associated with bonding
patterns; (ii) the configurational volume of the basin can be evaluated in a
formally exact way, and numerically with arbitrary precision. These features
allow us to evaluate the number of different topologies the bonding pattern can
adopt. We find that the number of fully bonded configurations, i.e.
configurations in which all particles are bonded to neighbors, is
extensive, suggesting that the configurational entropy of the low temperature
fluid is finite. We also evaluate the energy dependence of the configurational
entropy close to the fully bonded state, and show that it follows a logarithmic
functional form, differently from the quadratic dependence characterizing
fragile liquids. We suggest that the presence of a discrete energy scale,
provided by the particle bonds, and the intrinsic degeneracy of fully bonded
disordered networks differentiates strong from fragile behavior.Comment: Final version. Journal of Chemical Physics 124, 204509 (2006
Distributed Generation and Resilience in Power Grids
We study the effects of the allocation of distributed generation on the
resilience of power grids. We find that an unconstrained allocation and growth
of the distributed generation can drive a power grid beyond its design
parameters. In order to overcome such a problem, we propose a topological
algorithm derived from the field of Complex Networks to allocate distributed
generation sources in an existing power grid.Comment: proceedings of Critis 2012 http://critis12.hig.no
Critical field-exponents for secure message-passing in modular networks
We study secure message-passing in the presence of multiple adversaries in modular networks. We assume a dominant fraction of nodes in each module have the same vulnerability, i.e., the same entity spying on them. We find both analytically and via simulations that the links between the modules (interlinks) have effects analogous to a magnetic field in a spin-system in that for any amount of interlinks the system no longer undergoes a phase transition. We then define the exponents δ, which relates the order parameter (the size of the giant secure component) at the critical point to the field strength (average number of interlinks per node), and γ, which describes the susceptibility near criticality. These are found to be δ = 2 and γ = 1 (with the scaling of the order parameter near the critical point given by β = 1). When two or more vulnerabilities are equally present in a module we find δ = 1 and γ = 0 (with β ≥ 2). Apart from defining a previously unidentified universality class, these exponents show that increasing connections between modules is more beneficial for security than increasing connections within modules. We also measure the correlation critical exponent ν, and the upper critical dimension d c, finding that as for ordinary percolation, suggesting that for secure message-passing d c = 6. These results provide an interesting analogy between secure message-passing in modular networks and the physics of magnetic spin-systems
Mode-coupling theory predictions for a limited valency attractive square-well model
Recently we have studied, using numerical simulations, a limited valency
model, i.e. an attractive square well model with a constraint on the maximum
number of bonded neighbors. Studying a large region of temperatures and
packing fractions , we have estimated the location of the liquid-gas
phase separation spinodal and the loci of dynamic arrest, where the system is
trapped in a disordered non-ergodic state. Two distinct arrest lines for the
system are present in the system: a {\it (repulsive) glass} line at high
packing fraction, and a {\it gel} line at low and . The former is
essentially vertical (-controlled), while the latter is rather horizontal
(-controlled) in the plane. We here complement the molecular
dynamics results with mode coupling theory calculations, using the numerical
structure factors as input. We find that the theory predicts a repulsive glass
line -- in satisfactory agreement with the simulation results -- and an
attractive glass line which appears to be unrelated to the gel line.Comment: 12 pages, 6 figures. To appear in J. Phys. Condens. Matter, special
issue: "Topics in Application of Scattering Methods for Investigation of
Structure and Dynamics of Soft Condensed Matter", Fiesole, November 200
Sandpiles on multiplex networks
We introduce the sandpile model on multiplex networks with more than one type
of edge and investigate its scaling and dynamical behaviors. We find that the
introduction of multiplexity does not alter the scaling behavior of avalanche
dynamics; the system is critical with an asymptotic power-law avalanche size
distribution with an exponent on duplex random networks. The
detailed cascade dynamics, however, is affected by the multiplex coupling. For
example, higher-degree nodes such as hubs in scale-free networks fail more
often in the multiplex dynamics than in the simplex network counterpart in
which different types of edges are simply aggregated. Our results suggest that
multiplex modeling would be necessary in order to gain a better understanding
of cascading failure phenomena of real-world multiplex complex systems, such as
the global economic crisis.Comment: 7 pages, 7 figure
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