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Search Results: 1 - 10 of 462224 matches for " A. Lefèvre "
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Edwards measure and the steady state regime of a model with kinetic constraints under tapping
A. Lefèvre
Physics , 2002, DOI: 10.1088/0305-4470/35/43/304
Abstract: We study the tapping dynamics of a one dimensional Ising model with symmetric kinetic constraints. We define and test a variant of the Edwards hypothesis that one may build a thermodynamics for the steady state by using a flat measure over the metastable states with several macroscopic quantities fixed. Various types of tapping are compared and the accuracy of this measure becomes quickly excellent when the number of quantities fixed on average increases, independently of the way the system is excited. We attribute the validity of the naive flat measure at weak tapping to the spatial separation of density defects.
Magnetotactic Bacteria from Extreme Environments
Dennis A. Bazylinski,Christopher T. Lefèvre
Life , 2013, DOI: 10.3390/life3020295
Abstract: Magnetotactic bacteria (MTB) represent a diverse collection of motile prokaryotes that biomineralize intracellular, membrane-bounded, tens-of-nanometer-sized crystals of a magnetic mineral called magnetosomes. Magnetosome minerals consist of either magnetite (Fe 3O 4) or greigite (Fe 3S 4) and cause cells to align along the Earth’s geomagnetic field lines as they swim, a trait called magnetotaxis. MTB are known to mainly inhabit the oxic–anoxic interface (OAI) in water columns or sediments of aquatic habitats and it is currently thought that magnetosomes function as a means of making chemotaxis more efficient in locating and maintaining an optimal position for growth and survival at the OAI. Known cultured and uncultured MTB are phylogenetically associated with the Alpha-, Gamma- and Deltaproteobacteria classes of the phylum Proteobacteria, the Nitrospirae phylum and the candidate division OP3, part of the Planctomycetes- Verrucomicrobia- Chlamydiae (PVC) bacterial superphylum. MTB are generally thought to be ubiquitous in aquatic environments as they are cosmopolitan in distribution and have been found in every continent although for years MTB were thought to be restricted to habitats with pH values near neutral and at ambient temperature. Recently, however, moderate thermophilic and alkaliphilic MTB have been described including: an uncultured, moderately thermophilic magnetotactic bacterium present in hot springs in northern Nevada with a probable upper growth limit of about 63 °C; and several strains of obligately alkaliphilic MTB isolated in pure culture from different aquatic habitats in California, including the hypersaline, extremely alkaline Mono Lake, with an optimal growth pH of >9.0.
A possible experimental test of the thermodynamic approach to granular media
D. S. Dean,A. Lefèvre
Physics , 2002, DOI: 10.1103/PhysRevLett.90.198301
Abstract: We study the steady state distribution of the energy of the Sherrington-Kirkpatrick model driven by a tapping mechanism which mimics the mechanically driven dynamics of granular media. The dynamics consists of two phases: a zero temperature relaxation phase which leads the system to a metastable state, then a tapping which excites the system thus reactivating the relaxational dynamics. Numerically we investigate whether the distribution of the energies of the blocked states obtained agrees with a simple canonical form of the Edwards measure. It is found that this canonical measure is in good agreement with the dynamically measured energy distribution. A possible experimental test of the Edwards measure based on the study here is proposed.
Phase transitions in the steady state behavior of mechanically perturbed spin glasses and ferromagnets
A. Lefèvre,D. S. Dean
Physics , 2001, DOI: 10.1103/PhysRevB.65.220403
Abstract: We analyze the steady state regime of systems interpolating between spin glasses and ferromagnets under a tapping dynamics recently introduced by analogy with the dynamics of mechanically perturbed granular media. A crossover from a second order to first order ferromagnetic transition as a function of the spin coupling distribution is found. The flat measure over blocked states introduced by Edwards for granular media is used to explain this scenario. Annealed calculations of the Edwards entropy are shown to qualitatively explain the nature of the phase transitions. A Monte-Carlo construction of the Edwards measure confirms that this explanation is also quantitatively accurate.
Self diffusion in a system of interacting Langevin particles
D. S. Dean,A. Lefèvre
Physics , 2004, DOI: 10.1103/PhysRevE.69.061111
Abstract: The behavior of the self diffusion constant of Langevin particles interacting via a pairwise interaction is considered. The diffusion constant is calculated approximately within a perturbation theory in the potential strength about the bare diffusion constant. It is shown how this expansion leads to a systematic double expansion in the inverse temperature $\beta$ and the particle density $\rho$. The one-loop diagrams in this expansion can be summed exactly and we show that this result is exact in the limit of small $\beta$ and $\rho\beta$ constant. The one-loop result can also be re-summed using a semi-phenomenological renormalization group method which has proved useful in the study of diffusion in random media. In certain cases the renormalization group calculation predicts the existence of a diverging relaxation time signalled by the vanishing of the diffusion constant -- possible forms of divergence coming from this approximation are discussed. Finally, at a more quantitative level, the results are compared with numerical simulations, in two-dimensions, of particles interacting via a soft potential recently used to model the interaction between coiled polymers.
Lack of functional alpha-lactalbumin prevents involution in Cape fur seals and identifies the protein as an apoptotic milk factor in mammary gland involution
Julie A Sharp, Christophe Lefèvre, Kevin R Nicholas
BMC Biology , 2008, DOI: 10.1186/1741-7007-6-48
Abstract: We show the molecular basis by which alpha-lactalbumin (LALBA), a secreted milk protein, is absent in Cape fur seals and demonstrate an apoptotic function for LALBA when exposed to mammary cells.We propose that apoptosis does not occur in fur seal mammary glands due to lack of LALBA in fur seal milk, allowing evasion of involution during a foraging trip. Our work identifies LALBA as a milk factor that feeds back on the mammary gland to regulate involution.The mammary gland represents one of the most dramatic examples of physiological development. The massive changes of form and function of mammary glands over the life span of a female are characterized by extreme changes in cell proliferation, differentiation, secretion and death, which accompanies pregnancy, lactation and involution upon weaning. While milk is sucked from the mammary gland it provides nutrition and immunity to the young. However, upon milk stasis, due to absence of sucking at weaning, the mammary gland regresses and is remodelled by a process known as involution, which cleanses the gland and returns it to a virgin-like state. Although the mammary gland appears vastly regulated it is also highly susceptible to cancer, with mortality associated with breast cancer rating amongst the highest causes of death for women in the western world.The study of apoptosis in the mammary gland during involution is important for understanding both the normal biology of post-natal regression and the events leading to mammary gland tumorigenesis. Interestingly, some mammals have modified their lactation cycle in order to accommodate and adapt to extreme environmental pressures. Animals such as otariid seals (fur seals and sea lions) exhibit an unusual lactation phenotype [1] which differs from other members of the Pinnipedia family and other mammals. These animals display resistance to mammary gland apoptosis and involution after cessation of sucking, and provide a unique opportunity to investigate aspects of mammary
The effects of nutrient additions on particulate and dissolved primary production in surface waters of three Mediterranean eddies
A. Lagaria,S. Psarra,D. Lefèvre,F. Van Wambeke
Biogeosciences Discussions , 2010, DOI: 10.5194/bgd-7-8919-2010
Abstract: The effects of additions of nitrogen (+N), phosphorus (+P), alone and in combination, were assessed during three microcosm experiments performed with surface waters of three anticyclonic eddies, located in the Western, Central and Eastern Mediterranean. We examined the effects of nutrient additions on rates of dissolved and particulate primary production and on metabolic rates of the osmotrophic community (phytoplankton and heterotrophic prokaryotes). The experiments were performed in June/July 2008 during the BOUM (Biogeochemistry from the Oligotrophic to the Ultra-oligotrophic Mediterranean) cruise. In all three experiments, particulate primary production was significantly stimulated by the additions of nitrogen (+N, +NP) while no effect was observed with the addition of phosphorus alone. Percent extracellular release (PER) showed an inverse relation with total primary production (PPtotal), displaying the lowest values (4–8%) in the +NP treatment. Among the three treatments, the +NP had the strongest effect on the community metabolic rates leading to positive net community production values (NCP>0). These changes of NCP were mainly due to enhanced gross community production (GCP) rather than lower respiration rates (CR). In +NP treatments autotrophic production (whether expressed as GCP or PPtotal) was high enough to fulfil the carbon requirements of the heterotrophic prokaryotes, with phytoplankton and heterotrophic prokaryote production positively correlated. Addition of nitrogen alone (+N) had a smaller effect on community production, resulting in metabolically balanced systems (NCP≈0). Finally, heterotrophic conditions persisted in the +P treatment at the central and eastern stations, and gross production was not sufficient to supply bacterial carbon demand, evidence of a decoupling of phytoplankton production and consumption by heterotrophic prokaryotes.
McClear: a new model estimating downwelling solar radiation at ground level in clear-sky conditions
M. Lefèvre,A. Oumbe,P. Blanc,B. Espinar
Atmospheric Measurement Techniques Discussions , 2013, DOI: 10.5194/amtd-6-3367-2013
Abstract: A new fast clear-sky model called McClear was developed to estimate the downwelling shortwave direct and global irradiances received at ground level under clear skies. McClear implements a fully physical modelling replacing empirical relations or simpler models used before. It exploits the recent results on aerosol properties, and total column content in water vapor and ozone produced by the MACC project (Monitoring Atmosphere Composition and Climate). It accurately reproduces the irradiance computed by the libRadtran reference radiative transfer model with a computational speed approximately 105 times greater by adopting the abaci, or look-up tables, approach combined with interpolation functions. It is therefore suited for geostationary satellite retrievals or numerical weather prediction schemes with many pixels or grid points, respectively. McClear irradiances were compared to 1 min measurements made in clear-sky conditions in several stations within the Baseline Surface Radiation Network in various climates. For global, respectively direct, irradiance, the correlation coefficient ranges between 0.95 and 0.99, resp. 0.86 and 0.99. The bias is comprised between 14 and 25 W m 2, resp. 49 and +33 W m 2. The RMSE ranges between 20 W m 2 (3% of the mean observed irradiance) and 36 W m 2 (5%), resp. 33 W m 2 (5%) and 64 W m 2 (10%). These results are much better than those from state-of-the-art models. This work demonstrates the quality of the McClear model combined with MACC products, and indirectly the quality of the aerosol properties modeled by the MACC reanalysis.
The number of metastable states in the generalized random orthogonal model
R. Cherrier,D. S. Dean,A. Lefèvre
Physics , 2002, DOI: 10.1088/0305-4470/36/14/301
Abstract: We calculate the number of metastable states in the generalized random orthogonal model. The results obtained are verified by exact numerical enumeration for small systems sizes but taking into account finite size effects. These results are compared with those for Hopfield model in order to examine the effect of strict orthonormality of neural network patterns on the number of metastable states.
The role of the interaction matrix in mean-field spin glasses
R. Cherrier,D. S. Dean,A. Lefèvre
Physics , 2002, DOI: 10.1103/PhysRevE.67.046112
Abstract: Mean-field models of 2-spin Ising spin glasses with interaction matrices taken from ensembles which are invariant under O(N) transformations are studied. A general study shows that the nature of the spin glass transition can be deduced from the eigenvalue spectrum of the interaction matrix. A simple replica approach is derived to carry out the average over the O(N) disorder. The analytic results are confirmed by extensive Monte Carlo simulations for large system sizes and by exact enumeration for small system sizes.
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