2,435 research outputs found
Critical Enhancement of the In-medium Nucleon-Nucleon Cross Section at low Temperatures
The in-medium nucleon-nucleon cross section is calculated starting from the
thermodynamic T-matrix at finite temperatures. The corresponding
Bethe-Salpeter-equation is solved using a separable representation of the Paris
nucleon-nucleon-potential. The energy-dependent in-medium N-N cross section at
a given density shows a strong temperature dependence. Especially at low
temperatures and low total momenta, the in-medium cross section is strongly
modified by in-medium effects. In particular, with decreasing temperature an
enhancement near the Fermi energy is observed. This enhancement can be
discussed as a precursor of the superfluid phase transition in nuclear matter.Comment: 10 pages with 4 figures (available on request from the authors),
MPG-VT-UR 34/94 accepted for publication in Phys. Rev.
Empires and Percolation: Stochastic Merging of Adjacent Regions
We introduce a stochastic model in which adjacent planar regions merge
stochastically at some rate , and observe analogies with the
well-studied topics of mean-field coagulation and of bond percolation. Do
infinite regions appear in finite time? We give a simple condition on
for this {\em hegemony} property to hold, and another simple condition for it
to not hold, but there is a large gap between these conditions, which includes
the case . For this case, a non-rigorous analytic
argument and simulations suggest hegemony.Comment: 13 page
Four-particle condensate in strongly coupled fermion systems
Four-particle correlations in fermion systems at finite temperatures are
investigated with special attention to the formation of a condensate. Instead
of the instability of the normal state with respect to the onset of pairing
described by the Gorkov equation, a new equation is obtained which describes
the onset of quartetting. Within a model calculation for symmetric nuclear
matter, we find that below a critical density, the four-particle condensation
(alpha-like quartetting) is favored over deuteron condensation (triplet
pairing). This pairing-quartetting competition is expected to be a general
feature of interacting fermion systems, such as the excition-biexciton system
in excited semiconductors. Possible experimental consequences are pointed out.Comment: LaTeX, 11 pages, 2 figures, uses psfig.sty (included), to be
published in Phys. Rev. Lett., tentatively scheduled for 13 April 1998
(Volume 80, Number 15
Cocoa Butter Saturated with Supercritical Carbon Dioxide: Measurements and Modelling of Solubility, Volumetric Expansion, Density and Viscosity
International audienceThe use of supercritical carbon dioxide technology for lipid processing has recently developed at the expense of traditional processes. For designing new processes the knowledge of thermophysical properties is a prerequisite. This work is focused on the characterization of physical and thermodynamic properties of CO2-cocoa butter (CB) saturated mixture. Measurements of density, volumetric expansion, viscosity and CO2 solubility were carried out on CB-rich phase at 313 and 353 K and pressures up to 40 MPa. The experimental techniques have previously been compared and validated. Density measurements of CB and CB saturated with CO2, were performed using the vibrating tube technology at pressures ranging from 0.1 to 25 MPa. Experimental values correlated well with the modified Tait equation. CO2 solubility measurements were coupled to those of density in the same pressures ranges. At 25 MPa, the solubility of CO2 is 31.4 % and 28.7 % at 313 and 353 K. Phase behaviour was investigated using a view cell in order to follow the expansion of the CB-rich phase with the rise in pressure. Volumetric expansion up to 47 % was measured and correlated to the CO2 solubility. Phase inversion was observed at 313 K and 40 MPa. Lastly, we developed an innovative falling ball viscometer for high pressure measurements. Viscosity measurements were carried out up to 25 MPa showing a viscosity reduction up to 90 % upon CO2 dissolution. These results were correlated with two empirical models. A new model here presented, was favourably compared with the Grunberg and Nissan model. All the experimental results are consistent with the available literature data for the CB-CO2 mixture and other fat systems. This work is a new contribution to the characterization of physical and thermodynamic behaviour of fats in contact with CO2 which is necessary to design supercritical fluid processes for fats processing
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Comparing Patterns of Natural Selection Across Species Using Selective Signatures
Comparing gene expression profiles over many different conditions has led to insights that were not obvious from single experiments. In the same way, comparing patterns of natural selection across a set of ecologically distinct species may extend what can be learned from individual genome-wide surveys. Toward this end, we show how variation in protein evolutionary rates, after correcting for genome-wide effects such as mutation rate and demographic factors, can be used to estimate the level and types of natural selection acting on genes across different species. We identify unusually rapidly and slowly evolving genes, relative to empirically derived genome-wide and gene family-specific background rates for 744 core protein families in 30 gamma-proteobacterial species. We describe the pattern of fast or slow evolution across species as the 'selective signature' of a gene. Selective signatures represent a profile of selection across species that is predictive of gene function: pairs of genes with correlated selective signatures are more likely to share the same cellular function, and genes in the same pathway can evolve in concert. For example, glycolysis and phenylalanine metabolism genes evolve rapidly in Idiomarina loihiensis, mirroring an ecological shift in carbon source from sugars to amino acids. In a broader context, our results suggest that the genomic landscape is organized into functional modules even at the level of natural selection, and thus it may be easier than expected to understand the complex evolutionary pressures on a cell
Spontaneous breaking of rotational symmetry in superconductors
We show that homogeneous superconductors with broken spin/isospin symmetry
lower their energy via a transition to a novel superconducting state where the
Fermi-surfaces are deformed to a quasi-ellipsoidal form at zero total momentum
of Cooper pairs. In this state, the gain in the condensation energy of the
pairs dominates over the loss in the kinetic energy caused by the lowest order
(quadrupole) deformation of Fermi-surfaces from the spherically symmetric form.
There are two energy minima in general, corresponding to the deformations of
the Fermi-spheres into either prolate or oblate forms. The phase transition
from spherically symmetric state to the superconducting state with broken
rotational symmetry is of the first order.Comment: 5 pages, including 3 figures, published versio
Molecular regulation of GLUT-4 targeting in 3T3-L1 adipocytes
Insulin stimulates glucose transport in muscle and adipose tissue by triggering the movement of the glucose transporter GLUT-4 from an intracellular compartment to the cell surface. Fundamental to this process is the intracellular sequestration of GLUT-4 in nonstimulated cells. Two distinct targeting motifs in the amino and carboxy termini of GLUT-4 have been previously identified by expressing chimeras comprised of portions of GLUT-4 and GLUT-1, a transporter isoform that is constitutively targeted to the cell surface, in heterologous cells. These motifs-FQQ1 in the NH terminus and LL in the COOH terminus-resemble endocytic signals that have been described in other proteins. In the present study we have investigated the roles of these motifs in GLUT-4 targeting in insulin-sensitive cells. Epitope-tagged GLUT-4 constructs engineered to differentiate between endogenous and transfected GLUT-4 were stably expressed in 3T3-L1 adipocytes. Targeting was assessed in cells incubated in the presence or absence of insulin by subcellular fractionation. The targeting of epitope-tagged GLUT-4 was indistinguishable from endogenous GLUT-4. Mutation of the FQQI motif (F to A) caused GLUT-4 to constitutively accumulate at the cell surface regardless of expression level. Mutation of the dileucine motif (LL to AA) caused an increase in cell surface distribution only at higher levels of expression, but the overall cell surface distribution of this mutant was less than that of the amino-terminal mutants. Both NH- and COOH-terminal mutants retained insulin-dependent movement from an intracellular to a cell surface locale, suggesting that neither of these motifs is involved in the insulin-dependent redistribution of GLUT-4. We conclude that the phenylalanine-based NH-terminal and the dileucine-based COOH-terminal motifs play important and distinct roles in GLUT-4 targeting in 3T3-L1 adipocytes
Two-body correlation functions in nuclear matter with condensate
The density, spin and isospin correlation functions in nuclear matter with a
neutron-proton () condensate are calculated to study the possible
signatures of the BEC-BCS crossover in the low-density region. It is shown that
the criterion of the crossover (Phys. Rev. Lett. {\bf 95}, 090402 (2005)),
consisting in the change of the sign of the density correlation function at low
momentum transfer, fails to describe correctly the density-driven BEC-BCS
transition at finite isospin asymmetry or finite temperature. As an unambiguous
signature of the BEC-BCS transition, there can be used the presence (BCS
regime) or absence (BEC regime) of the singularity in the momentum distribution
of the quasiparticle density of states.Comment: Prepared with RevTeX4, 5p., 4 figure
Thermodynamics of - condensate in asymmetric nuclear matter
We study the neutron-proton pairing in nuclear matter as a function of
isospin asymmetry at finite temperatures and the saturation density using
realistic nuclear forces and Brueckner-renormalized single particle spectra.
Our computation of the thermodynamic quantities shows that while the difference
of the entropies of the superconducting and normal phases anomalously changes
its sign as a function of temperature for arbitrary asymmetry, the grand
canonical potential does not; the superconducting state is found to be stable
in the whole temperature-asymmetry plane. The pairing gap completely disappears
for density-asymmetries exceeding .Comment: 7 pages, including 3 figures, uses revte
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