225 research outputs found
Descending from infinity: Convergence of tailed distributions
We investigate the relaxation of long-tailed distributions under stochastic
dynamics that do not support such tails. Linear relaxation is found to be a
borderline case in which long tails are exponentially suppressed in time but
not eliminated. Relaxation stronger than linear suppresses long tails
immediately, but may lead to strong transient peaks in the probability
distribution. A delta function initial distribution under stronger than linear
decay displays not one but two different regimes of diffusive spreading
Quantum-dot Carnot engine at maximum power
We evaluate the efficiency at maximum power of a quantum-dot Carnot heat
engine. The universal value of the coefficients at the linear and quadratic
order in the temperature gradient are reproduced. Curzon-Ahlborn efficiency is
recovered in the limit of weak dissipation.Comment: 6 pages, 1 figur
Universality of efficiency at maximum power
We investigate the efficiency of power generation by thermo-chemical engines.
For strong coupling between the particle and heat flows and in the presence of
a left-right symmetry in the system, we demonstrate that the efficiency at
maximum power displays universality up to quadratic order in the deviation from
equilibrium. A maser model is presented to illustrate our argument.Comment: 4 pages, 2 figure
Stochastically driven single level quantum dot: a nano-scale finite-time thermodynamic machine and its various operational modes
We describe a single-level quantum dot in contact with two leads as a
nanoscale finite-time thermodynamic machine. The dot is driven by an external
stochastic force that switches its energy between two values. In the isothermal
regime, it can operate as a rechargeable battery by generating an electric
current against the applied bias in response to the stochastic driving, and
re-delivering work in the reverse cycle. This behavior is reminiscent of the
Parrondo paradox. If there is a thermal gradient the device can function as a
work-generating thermal engine, or as a refrigerator that extracts heat from
the cold reservoir via the work input of the stochastic driving. The efficiency
of the machine at maximum power output is investigated for each mode of
operation, and universal features are identified.Comment: 4 pages, 3 figures, V2: typos corrected around eq.(12
Extracting chemical energy by growing disorder: Efficiency at maximum power
We consider the efficiency of chemical energy extraction from the environment
by the growth of a copolymer made of two constituent units in the
entropy-driven regime. We show that the thermodynamic nonlinearity associated
with the information processing aspect is responsible for a branching of the
system properties such as power, speed of growth, entropy production, and
efficiency, with varying affinity. The standard linear thermodynamics argument
which predicts an efficiency of 1/2 at maximum power is inappropriate because
the regime of maximum power is located either outside of the linear regime or
on a separate bifurcated branch, and because the usual thermodynamic force is
not the natural variable for this optimization.Comment: 6 pages, 4 figure
A continuous-time persistent random walk model for flocking
Random walkers characterized by random positions and random velocities lead
to normal diffusion. A random walk was originally proposed by Einstein to model
Brownian motion and to demonstrate the existence of atoms and molecules. Such a
walker represents an inanimate particle driven by environmental fluctuations.
On the other hand, there are many examples of so-called "persistent random
walkers", including self-propelled particles that are able to move with almost
constant speed while randomly changing their direction of motion. Examples
include living entities (ranging from flagellated unicellular organisms to
complex animals such as birds and fish), as well as synthetic materials. Here
we discuss such persistent non-interacting random walkers as a model for active
particles. We also present a model that includes interactions among particles,
leading to a transition to flocking, that is, to a net flux where the majority
of the particles move in the same direction. Moreover, the model exhibits
secondary transitions that lead to clustering and more complex spatially
structured states of flocking. We analyze all these transitions in terms of
bifurcations using a number of mean field strategies (all to all interaction
and advection-reaction equations for the spatially structured states), and
compare these results with direct numerical simulations of ensembles of these
interacting active particles
Suitability of Permanent Probe Implants For the Measurement of Intramedullary Perfusion and Temperature Near the Bone Cortex: A Pilot Study Using a Rabbit Model
This study was conducted to test the suitability of permanent probe implants for the measurement of intramedullary perfusion by laser Doppler flowmetry and for the measurement of temperature near the bone cortex. Measurements were carried out on the conscious animal in order to rule out the influence of anaesthesia on intramedullary perfusion and temperature. During the first experimental animal trials, some of the probes made of polysulphon broke and/or gave false temperature measurements, so the original probe design was modified. The probes were reinforced with metal, and the temperature sensors were made less permeable to moisture. These modified probes were found to be suitable for permanent measurement of intramedullary perfusion and of temperature near the cortex in the conscious rabbit.
Efficiency at maximum power of low dissipation Carnot engines
We study the efficiency at maximum power, , of engines performing
finite-time Carnot cycles between a hot and a cold reservoir at temperatures
and , respectively. For engines reaching Carnot efficiency
in the reversible limit (long cycle time, zero dissipation),
we find in the limit of low dissipation that is bounded from above by
and from below by . These bounds are reached when
the ratio of the dissipation during the cold and hot isothermal phases tend
respectively to zero or infinity. For symmetric dissipation (ratio one) the
Curzon-Ahlborn efficiency is recovered.Comment: 4 pages, 1 figure, 1 tabl
Automated search for galactic star clusters in large multiband surveys: I. Discovery of 15 new open clusters in the Galactic anticenter region
Aims: According to some estimations, there are as many as 100000 open
clusters in the Galaxy, but less than 2000 of them have been discovered,
measured, and cataloged. We plan to undertake data mining of multiwavelength
surveys to find new star clusters. Methods: We have developed a new method to
search automatically for star clusters in very large stellar catalogs, which is
based on convolution with density functions. We have applied this method to a
subset of the Two Micron All Sky Survey catalog toward the Galactic anticenter.
We also developed a method to verify whether detected stellar groups are real
star clusters, which tests whether the stars that form the spatial density peak
also fall onto a single isochrone in the color-magnitude diagram. By fitting an
isochrone to the data, we estimate at the same time the main physical
parameters of a cluster: age, distance, color excess. Results: For the present
paper, we carried out a detailed analysis of 88 overdensity peaks detected in a
field of degrees near the Galactic anticenter. From this analysis,
15 overdensities were confirmed to be new open clusters and the physical and
structural parameters were determined for 12 of them; 10 of them were
previously suspected to be open clusters by Kronberger (2006) and Froebrich
(2007). The properties were also determined for 13 yet-unstudied known open
clusters, thus almost tripling the sample of open clusters with studied
parameters in the anticenter. The parameters determined with this method showed
a good agreement with published data for a set of well-known clusters.Comment: accepted to A&
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