487 research outputs found
A Fluid Dynamics Calculation of Sputtering from a Cylindrical Thermal Spike
The sputtering yield, Y, from a cylindrical thermal spike is calculated using
a two dimensional fluid dynamics model which includes the transport of energy,
momentum and mass. The results show that the high pressure built-up within the
spike causes the hot core to perform a rapid expansion both laterally and
upwards. This expansion appears to play a significant role in the sputtering
process. It is responsible for the ejection of mass from the surface and causes
fast cooling of the cascade. The competition between these effects accounts for
the nearly linear dependence of with the deposited energy per unit depth
that was observed in recent Molecular Dynamics simulations. Based on this we
describe the conditions for attaining a linear yield at high excitation
densities and give a simple model for this yield.Comment: 10 pages, 9 pages (including 9 figures), submitted to PR
Does strong heterogeneity promote cooperation by group interactions?
Previous research has highlighted the importance of strong heterogeneity for
the successful evolution of cooperation in games governed by pairwise
interactions. Here we determine to what extent this is true for games governed
by group interactions. We therefore study the evolution of cooperation in the
public goods game on the square lattice, the triangular lattice and the random
regular graph, whereby the payoffs are distributed either uniformly or
exponentially amongst the players by assigning to them individual scaling
factors that determine the share of the public good they will receive. We find
that uniformly distributed public goods are more successful in maintaining high
levels of cooperation than exponentially distributed public goods. This is not
in agreement with previous results on games governed by pairwise interactions,
indicating that group interactions may be less susceptible to the promotion of
cooperation by means of strong heterogeneity as originally assumed, and that
the role of strongly heterogeneous states should be reexamined for other types
of games.Comment: 12 pages, 4 figures; accepted for publication in New Journal of
Physics [related work available at http://arxiv.org/abs/0708.1746 and
http://www.matjazperc.com/
Renormalization Group Analysis of a Noisy Kuramoto-Sivashinsky Equation
We have analyzed the Kuramoto-Sivashinsky equation with a stochastic noise
term through a dynamic renormalization group calculation. For a system in which
the lattice spacing is smaller than the typical wavelength of the linear
instability occurring in the system, the large-distance and long-time behavior
of this equation is the same as for the Kardar-Parisi-Zhang equation in one and
two spatial dimensions. For the case the agreement is only qualitative.
On the other hand, when coarse-graining on larger scales the asymptotic flow
depends on the initial values of the parameters.Comment: 8 pages, 5 figures, revte
Noisy Kuramoto-Sivashinsky equation for an erosion model
We derive the continuum equation for a discrete model for ion sputtering. We
follow an approach based on the master equation, and discuss how it can be
truncated to a Fokker-Planck equation and mapped to a discrete Langevin
equation. By taking the continuum limit, we arrive at the Kuramoto-Sivashinsky
equation with a stochastic noise term.Comment: latex (w/ multicol.sty), 4 pages; to appear in Physical Review E (Oct
1996
Facilitators on networks reveal optimal interplay between information exchange and reciprocity
Reciprocity is firmly established as an important mechanism that promotes cooperation. An efficient
information exchange is likewise important, especially on structured populations, where interactions between
players are limited. Motivated by these two facts, we explore the role of facilitators in social dilemmas on networks.
Facilitators are here mirrors to their neighbors—they cooperate with cooperators and defect with defectors—but
they do not participate in the exchange of strategies. As such, in addition to introducing direct reciprocity, they
also obstruct information exchange. In well-mixed populations, facilitators favor the replacement and invasion
of defection by cooperation as long as their number exceeds a critical value. In structured populations, on the
other hand, there exists a delicate balance between the benefits of reciprocity and the deterioration of information
exchange. Extensive Monte Carlo simulations of social dilemmas on various interaction networks reveal that
there exists an optimal interplay between reciprocity and information exchange, which sets in only when a small
number of facilitators occupy the main hubs of the scale-free network. The drawbacks of missing cooperative
hubs are more than compensated for by reciprocity and, at the same time, the compromised information exchange
is routed via the auxiliary hubs with only marginal losses in effectivity. These results indicate that it is not always
optimal for the main hubs to become leaders of the masses, but rather to exploit their highly connected state to
promote tit-for-tat-like behavior
Coulomb Explosion and Thermal Spikes
A fast ion penetrating a solid creates a track of excitations. This can
produce displacements seen as an etched track, a process initially used to
detect energetic particles but now used to alter materials. From the seminal
papers by Fleischer et al. [Phys. Rev. 156, 353 (1967)] to the present [C.
Trautmann, S. Klaumunzer and H. Trinkaus, Phys. Rev. Lett. 85, 3648 (2000)],
`Coulomb explosion' and thermal spike models are treated as conflicting models
for describing ion track effects. Here molecular dynamics simulations of
electronic-sputtering, a surface manifestation of ion track formation, show
that `Coulomb explosion' produces a `heat' spike so that these are early and
late aspects of the same process. Therefore, differences in scaling are due to
the use of incomplete spike models.Comment: Submitted to PRL. 4 pages, 3 figures. For related movies see:
http://dirac.ms.virginia.edu/~emb3t/coulomb/coulomb.html PACS added in new
versio
Security in the Age of Systemic Risk: Strategies, Tactics and Options for Dealing with Femtorisks and Beyond
The world today is increasingly confronted with systemic threats and challenges, in which femtorisks - small-scale dangers that are inherent to system structures and function and which pose asymmetrically catastrophic risks - can build in consequence, spreading uncontrollably like epidemics in both natural and social systems in such diverse areas as ecology, epidemiology, finance, the Internet, terrorism, and international relations. They have been successfully modeled in ecology in the context of complex adaptive systems: systems made up of individual agents, whose interactions have macroscopic consequences that feed back to influence individual behavior. While acknowledging challenges, this paper argues for the value of applying to societal systems the approaches that natural scientists have developed in quantifying and modeling biological interactions and ecosystems
Determinants of public cooperation in multiplex networks
Synergies between evolutionary game theory and statistical physics have
significantly improved our understanding of public cooperation in structured
populations. Multiplex networks, in particular, provide the theoretical
framework within network science that allows us to mathematically describe the
rich structure of interactions characterizing human societies. While research
has shown that multiplex networks may enhance the resilience of cooperation,
the interplay between the overlap in the structure of the layers and the
control parameters of the corresponding games has not yet been investigated.
With this aim, we consider here the public goods game on a multiplex network,
and we unveil the role of the number of layers and the overlap of links, as
well as the impact of different synergy factors in different layers, on the
onset of cooperation. We show that enhanced public cooperation emerges only
when a significant edge overlap is combined with at least one layer being able
to sustain some cooperation by means of a sufficiently high synergy factor. In
the absence of either of these conditions, the evolution of cooperation in
multiplex networks is determined by the bounds of traditional network
reciprocity with no enhanced resilience. These results caution against overly
optimistic predictions that the presence of multiple social domains may in
itself promote cooperation, and they help us better understand the complexity
behind prosocial behavior in layered social systems.Comment: 12 pages, 3 figures; accepted for publication in New Journal of
Physic
When agreement-accepting free-riders are a necessary evil for the evolution of cooperation
Agreements and commitments have provided a novel mechanism to promote cooperation in social dilemmas in both one-shot and repeated games. Individuals requesting others to commit to cooperate (proposers) incur a cost, while their co-players are not necessarily required to pay any, allowing them to free-ride on the proposal investment cost (acceptors). Although there is a clear complementarity in these behaviours, no dynamic evidence is currently available that proves that they coexist in different forms of commitment creation. Using a stochastic evolutionary model allowing for mixed population states, we identify non-trivial roles of acceptors as well as the importance of intention recognition in commitments. In the one-shot prisoner's dilemma, alliances between proposers and acceptors are necessary to isolate defectors when proposers do not know the acceptance intentions of the others. However, when the intentions are clear beforehand, the proposers can emerge by themselves. In repeated games with noise, the incapacity of proposers and acceptors to set up alliances makes the emergence of the first harder whenever the latter are present. As a result, acceptors will exploit proposers and take over the population when an apology-forgiveness mechanism with too low apology cost is introduced, and hence reduce the overall cooperation level.SCOPUS: ar.jinfo:eu-repo/semantics/publishe
Resolution of the stochastic strategy spatial prisoner's dilemma by means of particle swarm optimization
We study the evolution of cooperation among selfish individuals in the
stochastic strategy spatial prisoner's dilemma game. We equip players with the
particle swarm optimization technique, and find that it may lead to highly
cooperative states even if the temptations to defect are strong. The concept of
particle swarm optimization was originally introduced within a simple model of
social dynamics that can describe the formation of a swarm, i.e., analogous to
a swarm of bees searching for a food source. Essentially, particle swarm
optimization foresees changes in the velocity profile of each player, such that
the best locations are targeted and eventually occupied. In our case, each
player keeps track of the highest payoff attained within a local topological
neighborhood and its individual highest payoff. Thus, players make use of their
own memory that keeps score of the most profitable strategy in previous
actions, as well as use of the knowledge gained by the swarm as a whole, to
find the best available strategy for themselves and the society. Following
extensive simulations of this setup, we find a significant increase in the
level of cooperation for a wide range of parameters, and also a full resolution
of the prisoner's dilemma. We also demonstrate extreme efficiency of the
optimization algorithm when dealing with environments that strongly favor the
proliferation of defection, which in turn suggests that swarming could be an
important phenomenon by means of which cooperation can be sustained even under
highly unfavorable conditions. We thus present an alternative way of
understanding the evolution of cooperative behavior and its ubiquitous presence
in nature, and we hope that this study will be inspirational for future efforts
aimed in this direction.Comment: 12 pages, 4 figures; accepted for publication in PLoS ON
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