657 research outputs found
Efficiency at maximum power: An analytically solvable model for stochastic heat engines
We study a class of cyclic Brownian heat engines in the framework of
finite-time thermodynamics. For infinitely long cycle times, the engine works
at the Carnot efficiency limit producing, however, zero power. For the
efficiency at maximum power, we find a universal expression, different from the
endoreversible Curzon-Ahlborn efficiency. Our results are illustrated with a
simple one-dimensional engine working in and with a time-dependent harmonic
potential.Comment: 6 pages, 3 figure
Enteroxins of F4+ Escherichia coli induce IL17 in the pig small intestine and enhance colonization with F18ac+ verotoxigenic E. coli
Warped space-time for phonons moving in a perfect nonrelativistic fluid
We construct a kinematical analogue of superluminal travel in the ``warped''
space-times curved by gravitation, in the form of ``super-phononic'' travel in
the effective space-times of perfect nonrelativistic fluids. These warp-field
space-times are most easily generated by considering a solid object that is
placed as an obstruction in an otherwise uniform flow. No violation of any
condition on the positivity of energy is necessary, because the effective
curved space-times for the phonons are ruled by the Euler and continuity
equations, and not by the Einstein field equations.Comment: 7 pages, 1 figure. Version as published; references update
Influence of the Barrier Shape on Resonant Activation
The escape of a Brownian particle over a dichotomously fluctuating barrier is
investigated for various shapes of the barrier. The problem of resonant
activation is revisited with the attention on the effect of the barrier shape
on optimal value of the mean escape time in the system. The characteristic
features of resonant behavior are analyzed for barriers switching either
between different heights, or "on" and "off" positions. PACS number(s):
05.10-a, 02.50.-r, 82.20.-wj.Comment: 7 pages, 8 figures, RevTex4. Manuscript has been revised and
enhanced. Pictures have been made more clear and some of them have been
cancelled. Additional references have been added. The paper has been
submitted to Phys. Rev.
Virgo detector characterization and data quality: tools
Detector characterization and data quality studies—collectively referred to as DetChar activities in this article—are paramount to the scientific exploitation of the joint dataset collected by the LIGO-Virgo-KAGRA global network of ground-based gravitational-wave (GW) detectors. They take place during each phase of the operation of the instruments (upgrade, tuning and optimization, data taking), are required at all steps of the dataflow (from data acquisition to the final list of GW events) and operate at various latencies (from near real-time to vet the public alerts to offline analyses). This work requires a wide set of tools which have been developed over the years to fulfill the requirements of the various DetChar studies: data access and bookkeeping; global monitoring of the instruments and of the different steps of the data processing; studies of the global properties of the noise at the detector outputs; identification and follow-up of noise peculiar features (whether they be transient or continuously present in the data); quick processing of the public alerts. The present article reviews all the tools used by the Virgo DetChar group during the third LIGO-Virgo Observation Run (O3, from April 2019 to March 2020), mainly to analyze the Virgo data acquired at EGO. Concurrently, a companion article focuses on the results achieved by the DetChar group during the O3 run using these tools
Bounds and phase diagram of efficiency at maximum power for tight-coupling molecular motors
The efficiency at maximum power (EMP) for tight-coupling molecular motors is
investigated within the framework of irreversible thermodynamics. It is found
that the EMP depends merely on the constitutive relation between the
thermodynamic current and force. The motors are classified into four generic
types (linear, superlinear, sublinear, and mixed types) according to the
characteristics of the constitutive relation, and then the corresponding ranges
of the EMP for these four types of molecular motors are obtained. The exact
bounds of the EMP are derived and expressed as the explicit functions of the
free energy released by the fuel in each motor step. A phase diagram is
constructed which clearly shows how the region where the parameters (the load
distribution factor and the free energy released by the fuel in each motor
step) are located can determine whether the value of the EMP is larger or
smaller than 1/2. This phase diagram reveals that motors using ATP as fuel
under physiological conditions can work at maximum power with higher efficiency
() for a small load distribution factor ().Comment: 5 pages, 4 figure
Virgo detector characterization and data quality: results from the O3 run
The Advanced Virgo detector has contributed with its data to the rapid growth of the number of detected GW signals in the past few years, alongside the two Advanced LIGO instruments. First during the last month of the Observation Run 2 (O2) in August 2017 (with, most notably, the compact binary mergers GW170814 and GW170817), and then during the full Observation Run 3 (O3): an 11 months data taking period, between April 2019 and March 2020, that led to the addition of 79 events to the catalog of transient GW sources maintained by LIGO, Virgo and now KAGRA. These discoveries and the manifold exploitation of the detected waveforms benefit from an accurate characterization of the quality of the data, such as continuous study and monitoring of the detector noise sources. These activities, collectively named detector characterization and data quality or DetChar, span the whole workflow of the Virgo data, from the instrument front-end hardware to the final analyses. They are described in detail in the following article, with a focus on the results achieved by the Virgo DetChar group during the O3 run. Concurrently, a companion article describes the tools that have been used by the Virgo DetChar group to perform this work
A linear nonequilibrium thermodynamics approach to optimization of thermoelectric devices
Improvement of thermoelectric systems in terms of performance and range of
applications relies on progress in materials science and optimization of device
operation. In this chapter, we focuse on optimization by taking into account
the interaction of the system with its environment. For this purpose, we
consider the illustrative case of a thermoelectric generator coupled to two
temperature baths via heat exchangers characterized by a thermal resistance,
and we analyze its working conditions. Our main message is that both electrical
and thermal impedance matching conditions must be met for optimal device
performance. Our analysis is fundamentally based on linear nonequilibrium
thermodynamics using the force-flux formalism. An outlook on mesoscopic systems
is also given.Comment: Chapter 14 in "Thermoelectric Nanomaterials", Editors Kunihito
Koumoto and Takao Mori, Springer Series in Materials Science Volume 182
(2013
Stochastic processes and conformal invariance
We discuss a one-dimensional model of a fluctuating interface with a dynamic
exponent . The events that occur are adsorption, which is local, and
desorption which is non-local and may take place over regions of the order of
the system size. In the thermodynamic limit, the time dependence of the system
is given by characters of the conformal field theory of percolation. This
implies in a rigorous way a connection between CFT and stochastic processes.
The finite-size scaling behavior of the average height, interface width and
other observables are obtained. The avalanches produced during desorption are
analyzed and we show that the probability distribution of the avalanche sizes
obeys finite-size scaling with new critical exponents.Comment: 4 pages, 6 figures, revtex4. v2: change of title and minor
correction
Effect on longitudinal growth and anemia of zinc or multiple micronutrients added to vitamin A: a randomized controlled trial in children aged 6-24 months
<p>Abstract</p> <p>Background</p> <p>The benefits of zinc or multiple micronutrient supplementations in African children are uncertain. African children may differ from other populations of children in developing countries because of differences in the prevalence of zinc deficiency, low birth weight and preterm delivery, recurrent or chronic infections such as HIV, or the quality of complementary diets and genetic polymorphisms affecting iron metabolism.</p> <p>The aim of this study was to ascertain whether adding zinc or multiple micronutrients to vitamin A supplementation improves longitudinal growth or reduces prevalence of anemia in children aged 6-24 months.</p> <p>Methods</p> <p>Randomized, controlled double-blinded trial of prophylactic micronutrient supplementation to children aged 6-24 months. Children in three cohorts - 32 HIV-infected children, 154 HIV-uninfected children born to HIV-infected mothers, and 187 uninfected children born to HIV-uninfected mothers - were separately randomly assigned to receive daily vitamin A (VA) [n = 124], vitamin A plus zinc (VAZ) [n = 123], or multiple micronutrients that included vitamin A and zinc (MM) [n = 126].</p> <p>Results</p> <p>Among all children there were no significant differences between intervention arms in length-for-age Z scores (LAZ) changes over 18 months. Among stunted children (LAZ below -2) [n = 62], those receiving MM had a 0.7 Z-score improvement in LAZ versus declines of 0.3 in VAZ and 0.2 in VA (P = 0.029 when comparing effects of treatment over time). In the 154 HIV-uninfected children, MM ameliorated the effect of repeated diarrhea on growth. Among those experiencing more than six episodes, those receiving MM had no decline in LAZ compared to 0.5 and 0.6 Z-score declines in children receiving VAZ and VA respectively (P = 0.06 for treatment by time interaction). After 12 months, there was 24% reduction in proportion of children with anemia (hemoglobin below 11 g/dL) in MM arm (P = 0.001), 11% in VAZ (P = 0.131) and 18% in VA (P = 0.019). Although the within arm changes were significant; the between-group differences were not significant.</p> <p>Conclusions</p> <p>Daily multiple micronutrient supplementation combined with vitamin A was beneficial in improving growth among children with stunting, compared to vitamin A alone or to vitamin A plus zinc. Effects on anemia require further study.</p> <p>Trial registration</p> <p>This study is registered with ClinicalTrials.gov, number .NCT00156832.</p
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