126 research outputs found
Relationship between clustering and algorithmic phase transitions in the random k-XORSAT model and its NP-complete extensions
We study the performances of stochastic heuristic search algorithms on
Uniquely Extendible Constraint Satisfaction Problems with random inputs. We
show that, for any heuristic preserving the Poissonian nature of the underlying
instance, the (heuristic-dependent) largest ratio of constraints per
variables for which a search algorithm is likely to find solutions is smaller
than the critical ratio above which solutions are clustered and
highly correlated. In addition we show that the clustering ratio can be reached
when the number k of variables per constraints goes to infinity by the
so-called Generalized Unit Clause heuristic.Comment: 15 pages, 4 figures, Proceedings of the International Workshop on
Statistical-Mechanical Informatics, September 16-19, 2007, Kyoto, Japan; some
imprecisions in the previous version have been correcte
Topological Signature of First Order Phase Transitions
We show that the presence and the location of first order phase transitions
in a thermodynamic system can be deduced by the study of the topology of the
potential energy function, V(q), without introducing any thermodynamic measure.
In particular, we present the thermodynamics of an analytically solvable
mean-field model with a k-body interaction which -depending on the value of k-
displays no transition (k=1), second order (k=2) or first order (k>2) phase
transition. This rich behavior is quantitatively retrieved by the investigation
of a topological invariant, the Euler characteristic, of some submanifolds of
the configuration space. Finally, we conjecture a direct link between the Euler
characteristic and the thermodynamic entropy.Comment: 6 pages, 2 figure
La fauna asociada a Tubularia crocea (Agassiz, 1862) (anthomedusae; tubulariidae) y la aplicación de un método de cartificación
A systematic list of the organisms found on the polyps of T. crocea is presented. Monthly samples from the coast of Mar del Plata (38°03'S ; 57°31'W) allowed the analysis of the dynamic of the associations, specially with picnogonids, amphipods, annelids and nematodes, which varied in composition and abundance along of year. We present a modification of a mapping model of Marfenin (1980) and its use to present the different stages of the polyps and the localization of the associated organisms.Se presenta un listado sistemático de los organismos hallados sobre los pólipos de T. creocea. Muestras mensuales obtenidas en la costa de Mar del Plata (38°O8'S ; 57°31'W) permitieron el análisis de la dinámica de las asociaciones, especialmente con picnogónidos, anfípodos, anélidos y nemátodos, las cuales variaron en composición y abundancia a través del año. Nosotros presentamos una modificación del modelo de cartificación de Marfenin (1980) y su uso para representar distintos estados de los pólipos y la localización de los organismos asociados
Can the jamming transition be described using equilibrium statistical mechanics?
When materials such as foams or emulsions are compressed, they display solid
behaviour above the so-called `jamming' transition. Because compression is done
out-of-equilibrium in the absence of thermal fluctuations, jamming appears as a
new kind of a nonequilibrium phase transition. In this proceeding paper, we
suggest that tools from equilibrium statistical mechanics can in fact be used
to describe many specific features of the jamming transition. Our strategy is
to introduce thermal fluctuations and use statistical mechanics to describe the
complex phase behaviour of systems of soft repulsive particles, before sending
temperature to zero at the end of the calculation. We show that currently
available implementations of standard tools such as integral equations,
mode-coupling theory, or replica calculations all break down at low temperature
and large density, but we suggest that new analytical schemes can be developed
to provide a fully microscopic, quantitative description of the jamming
transition.Comment: 8 pages, 6 figs. Talk presented at Statphys24 (July 2010, Cairns,
Australia
Critical scaling and heterogeneous superdiffusion across the jamming/rigidity transition of a granular glass
The dynamical properties of a dense horizontally vibrated bidisperse granular
monolayer are experimentally investigated. The quench protocol produces states
with a frozen structure of the assembly, but the remaining degrees of freedom
associated with contact dynamics control the appearance of macroscopic
rigidity. We provide decisive experimental evidence that this transition is a
critical phenomenon, with increasingly collective and heterogeneous
rearrangements occurring at length scales much smaller than the grains'
diameter, presumably reflecting the contact force network fluctuations.
Dynamical correlation time and length scales soar on both sides of the
transition, as the volume fraction varies over a remarkably tiny range (). We characterize the motion of individual grains,
which becomes super-diffusive at the jamming transition , signaling
long-ranged temporal correlations. Correspondingly, the system exhibits
long-ranged four-point dynamical correlations in space that obey critical
scaling at the transition density.Comment: 4 pages, 8 figure
Exact time-average distribution for a stationary non-Markovian massive Brownian particle coupled to two heat baths
Using a time-averaging technique we obtain exactly the probability
distribution for position and velocity of a Brownian particle under the
influence of two heat baths at different temperatures. These baths are
expressed by a white noise term, representing the fast dynamics, and a colored
noise term, representing the slow dynamics. Our exact solution scheme accounts
for inertial effects, that are not present in approaches that assume the
Brownian particle in the over-damped limit. We are also able to obtain the
contribution associated with the fast noise that are usually neglected by other
approaches.Comment: accepted for publication in Phys. Rev.
Fluctuation theorems for stochastic dynamics
Fluctuation theorems make use of time reversal to make predictions about
entropy production in many-body systems far from thermal equilibrium. Here we
review the wide variety of distinct, but interconnected, relations that have
been derived and investigated theoretically and experimentally. Significantly,
we demonstrate, in the context of Markovian stochastic dynamics, how these
different fluctuation theorems arise from a simple fundamental time-reversal
symmetry of a certain class of observables. Appealing to the notion of Gibbs
entropy allows for a microscopic definition of entropy production in terms of
these observables. We work with the master equation approach, which leads to a
mathematically straightforward proof and provides direct insight into the
probabilistic meaning of the quantities involved. Finally, we point to some
experiments that elucidate the practical significance of fluctuation relations.Comment: 48 pages, 2 figures. v2: minor changes for consistency with published
versio
Work and heat fluctuations in two-state systems: a trajectory thermodynamics formalism
Two-state models provide phenomenological descriptions of many different
systems, ranging from physics to chemistry and biology. We investigate work
fluctuations in an ensemble of two-state systems driven out of equilibrium
under the action of an external perturbation. We calculate the probability
density P(W) that a work equal to W is exerted upon the system along a given
non-equilibrium trajectory and introduce a trajectory thermodynamics formalism
to quantify work fluctuations in the large-size limit. We then define a
trajectory entropy S(W) that counts the number of non-equilibrium trajectories
P(W)=exp(S(W)/kT) with work equal to W. A trajectory free-energy F(W) can also
be defined, which has a minimum at a value of the work that has to be
efficiently sampled to quantitatively test the Jarzynski equality. Within this
formalism a Lagrange multiplier is also introduced, the inverse of which plays
the role of a trajectory temperature. Our solution for P(W) exactly satisfies
the fluctuation theorem by Crooks and allows us to investigate
heat-fluctuations for a protocol that is invariant under time reversal. The
heat distribution is then characterized by a Gaussian component (describing
small and frequent heat exchange events) and exponential tails (describing the
statistics of large deviations and rare events). For the latter, the width of
the exponential tails is related to the aforementioned trajectory temperature.
Finite-size effects to the large-N theory and the recovery of work
distributions for finite N are also discussed. Finally, we pay particular
attention to the case of magnetic nanoparticle systems under the action of a
magnetic field H where work and heat fluctuations are predicted to be
observable in ramping experiments in micro-SQUIDs.Comment: 28 pages, 14 figures (Latex
On the range of validity of the fluctuation theorem for stochastic Markovian dynamics
We consider the fluctuations of generalized currents in stochastic Markovian
dynamics. The large deviations of current fluctuations are shown to obey a
Gallavotti-Cohen (GC) type symmetry in systems with a finite state space.
However, this symmetry is not guaranteed to hold in systems with an infinite
state space. A simple example of such a case is the Zero-Range Process (ZRP).
Here we discuss in more detail the already reported breakdown of the GC
symmetry in the context of the ZRP with open boundaries and we give a physical
interpretation of the phases that appear. Furthermore, the earlier analytical
results for the single-site case are extended to cover multiple-site systems.
We also use our exact results to test an efficient numerical algorithm of
Giardina, Kurchan and Peliti, which was developed to measure the current large
deviation function directly. We find that this method breaks down in some
phases which we associate with the gapless spectrum of an effective
Hamiltonian.Comment: 37 pages, 10 figures. Minor alterations, fixed typos (as appeared in
JSTAT
Identification of Giardia lamblia DHHC Proteins and the Role of Protein S-palmitoylation in the Encystation Process
Protein S-palmitoylation, a hydrophobic post-translational modification, is performed by protein acyltransferases that have a common DHHC Cys-rich domain (DHHC proteins), and provides a regulatory switch for protein membrane association. In this work, we analyzed the presence of DHHC proteins in the protozoa parasite Giardia lamblia and the function of the reversible S-palmitoylation of proteins during parasite differentiation into cyst. Two specific events were observed: encysting cells displayed a larger amount of palmitoylated proteins, and parasites treated with palmitoylation inhibitors produced a reduced number of mature cysts. With bioinformatics tools, we found nine DHHC proteins, potential protein acyltransferases, in the Giardia proteome. These proteins displayed a conserved structure when compared to different organisms and are distributed in different monophyletic clades. Although all Giardia DHHC proteins were found to be present in trophozoites and encysting cells, these proteins showed a different intracellular localization in trophozoites and seemed to be differently involved in the encystation process when they were overexpressed. dhhc transgenic parasites showed a different pattern of cyst wall protein expression and yielded different amounts of mature cysts when they were induced to encyst. Our findings disclosed some important issues regarding the role of DHHC proteins and palmitoylation during Giardia encystation.Fil: Merino, Maria Cecilia. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Investigación Médica Mercedes y Martín Ferreyra. Universidad Nacional de Córdoba. Instituto de Investigación Médica Mercedes y Martín Ferreyra; ArgentinaFil: Zamponi, Nahuel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Investigación Médica Mercedes y Martín Ferreyra. Universidad Nacional de Córdoba. Instituto de Investigación Médica Mercedes y Martín Ferreyra; ArgentinaFil: Vranych, Cecilia Verónica. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Investigación Médica Mercedes y Martín Ferreyra. Universidad Nacional de Córdoba. Instituto de Investigación Médica Mercedes y Martín Ferreyra; ArgentinaFil: Touz, Maria Carolina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Investigación Médica Mercedes y Martín Ferreyra. Universidad Nacional de Córdoba. Instituto de Investigación Médica Mercedes y Martín Ferreyra; ArgentinaFil: Ropolo, Andrea Silvana. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Investigación Médica Mercedes y Martín Ferreyra. Universidad Nacional de Córdoba. Instituto de Investigación Médica Mercedes y Martín Ferreyra; Argentin
- …