6,938 research outputs found
Energy and entropy of relativistic diffusing particles
We discuss energy-momentum tensor and the second law of thermodynamics for a
system of relativistic diffusing particles. We calculate the energy and entropy
flow in this system. We obtain an exact time dependence of energy, entropy and
free energy of a beam of photons in a reservoir of a fixed temperature.Comment: 14 pages,some formulas correcte
Thermoelectric efficiency at maximum power in a quantum dot
We identify the operational conditions for maximum power of a
nanothermoelectric engine consisting of a single quantum level embedded between
two leads at different temperatures and chemical potentials. The corresponding
thermodynamic efficiency agrees with the Curzon-Ahlborn expression up to
quadratic terms in the gradients, supporting the thesis of universality beyond
linear response.Comment: 4 pages, 3 figure
Correlated evolution of nucleotide substitution rates and allelic variation in Mhc-DRB lineages of primates
<p>Abstract</p> <p>Background</p> <p>The major histocompatibility complex (MHC) is a key model of genetic polymorphism. Selection pressure by pathogens or other microevolutionary forces may result in a high rate of non-synonymous substitutions at the codons specifying the contact residues of the antigen binding sites (ABS), and the maintenance of extreme MHC allelic variation at the population/species level. Therefore, selection forces favouring MHC variability for any reason should cause a correlated evolution between substitution rates and allelic polymorphism. To investigate this prediction, we characterised nucleotide substitution rates and allelic polymorphism (i.e. the number of alleles detected in relation to the number of animals screened) of several <it>Mhc </it>class II <it>DRB </it>lineages in 46 primate species, and tested for a correlation between them.</p> <p>Results</p> <p>First, we demonstrate that species-specific and lineage-specific evolutionary constraints favour species- and lineage-dependent substitution rate at the codons specifying the ABS contact residues (i.e. certain species and lineages can be characterised by high substitution rate, while others have low rate). Second, we show that although the degree of the non-synonymous substitution rate at the ABS contact residues was systematically higher than the degree of the synonymous substitution rate, these estimates were strongly correlated when we controlled for species-specific and lineage-specific effects, and also for the fact that different studies relied on different sample size. Such relationships between substitution rates of different types could even be extended to the non-contact residues of the molecule. Third, we provide statistical evidence that increased substitution rate along a MHC gene may lead to allelic variation, as a high substitution rate can be observed in those lineages in which many alleles are maintained. Fourth, we show that the detected patterns were independent of phylogenetic constraints. When we used phylogenetic models that control for similarity between species, due to common descent, and focused on variations within a single lineage (<it>DRB1*03</it>), the positive relationship between different substitution rates and allelic polymorphisms was still robust. Finally, we found the same effects to emerge in the analyses that eliminated within-species variation in MHC traits by using strictly single population-level studies. However, in a set of contrasting analyses, in which we focused on the non-functional <it>DRB6 </it>locus, the correlation between substitution rates and allelic variation was not prevalent.</p> <p>Conclusion</p> <p>Our results indicate that positive selection for the generation of allelic polymorphism acting on the functional part of the protein has consequences for the nucleotide substitution rate along the whole exon 2 sequence of the <it>Mhc-DRB </it>gene. Additionally, we proved that an increased substitution rate can promote allelic variation within lineages. Consequently, the evolution of different characteristics of genetic polymorphism is not independent.</p
Heat exchange between two interacting nanoparticles beyond the fluctuation-dissipation regime
We show that the observed non-monotonic behavior of the thermal conductance
between two nanoparticles when they are brought into contact is originated by
an intricate phase space dynamics. Here it is assumed that this dynamics
results from the thermally activated jumping through a rough energy landscape.
A hierarchy of relaxation times plays the key role in the description of this
complex phase space behaviour. Our theory enables us to analyze the heat
transfer just before and at the moment of contact.Comment: 4 pages, 1 figure, approved for publication in Physical Review
Letter
Extraction of shear viscosity in stationary states of relativistic particle systems
Starting from a classical picture of shear viscosity we construct a
stationary velocity gradient in a microscopic parton cascade. Employing the
Navier-Stokes ansatz we extract the shear viscosity coefficient . For
elastic isotropic scatterings we find an excellent agreement with the analytic
values. This confirms the applicability of this method. Furthermore for both
elastic and inelastic scatterings with pQCD based cross sections we extract the
shear viscosity coefficient for a pure gluonic system and find a good
agreement with already published calculations.Comment: 17 pages, 7 figure
Calculation of shear viscosity using Green-Kubo relations within a parton cascade
The shear viscosity of a gluon gas is calculated using the Green-Kubo
relation. Time correlations of the energy-momentum tensor in thermal
equilibrium are extracted from microscopic simulations using a parton cascade
solving various Boltzmann collision processes. We find that the pQCD based
gluon bremsstrahlung described by Gunion-Bertsch processes significantly lowers
the shear viscosity by a factor of 3-8 compared to elastic scatterings. The
shear viscosity scales with the coupling as 1/(alpha_s^2\log(1/alpha_s)). For a
constant coupling constant the shear viscosity to entropy density ratio has no
dependence on temperature. Replacing the pQCD-based collision angle
distribution of binary scatterings by an isotropic form decreases the shear
viscosity by a factor of 3.Comment: 17 pages, 5 figure
Dynamical description of vesicle growth and shape change
We systematize and extend the description of vesicle growth and shape change
using linear nonequilibrium thermodynamics. By restricting the study to shape
changes from spheres to axisymmetric ellipsoids, we are able to give a
consistent formulation which includes the lateral tension of the vesicle
membrane. This allows us to generalize and correct a previous calculation. Our
present calculations suggest that, for small growing vesicles, a prolate
ellipsoidal shape should be favored over oblate ellipsoids, whereas for large
growing vesicles oblates should be favored over prolates. The validity of this
prediction is examined in the light of the various assumptions made in its
derivation.Comment: 6 page
Electron transport through single Mn12 molecular magnets
We report transport measurements through a single-molecule magnet, the Mn12
derivative [Mn12O12(O2C-C6H4-SAc)16(H2O)4], in a single-molecule transistor
geometry. Thiol groups connect the molecule to gold electrodes that are
fabricated by electromigration. Striking observations are regions of complete
current suppression and excitations of negative differential conductance on the
energy scale of the anisotropy barrier of the molecule. Transport calculations,
taking into account the high-spin ground state and magnetic excitations of the
molecule, reveal a blocking mechanism of the current involving non-degenerate
spin multiplets.Comment: Accepted for Phys. Rev. Lett., 5 pages, 4 figure
The Kondo Effect in the Presence of Magnetic Impurities
We measure transport through gold grain quantum dots fabricated using
electromigration, with magnetic impurities in the leads. A Kondo interaction is
observed between dot and leads, but the presence of magnetic impurities results
in a gate-dependent zero-bias conductance peak that is split due to an RKKY
interaction between the spin of the dot and the static spins of the impurities.
A magnetic field restores the single Kondo peak in the case of an
antiferromagnetic RKKY interaction. This system provides a new platform to
study Kondo and RKKY interactions in metals at the level of a single spin.Comment: 5 pages, 4 figure
Efficiency at maximum power of minimally nonlinear irreversible heat engines
We propose the minimally nonlinear irreversible heat engine as a new general
theoretical model to study the efficiency at the maximum power of heat
engines operating between the hot heat reservoir at the temperature and
the cold one at (). Our model is based on the extended
Onsager relations with a new nonlinear term meaning the power dissipation. In
this model, we show that is bounded from the upper side by a function
of the Carnot efficiency as . We demonstrate the validity of our theory by showing that
the low-dissipation Carnot engine can easily be described by our theory.Comment: 6 pages, 1 figur
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