Phonons Softening in Tip-Stretched Monatomic Nanowires

It has been shown in recent experiments that electronic transport through a gold monatomic nanowire is dissipative above a threshold voltage due to excitation of phonons via the electron-phonon interaction. We address that data by computing, via density functional theory, the zone boundary longitudinal phonon frequency of a perfect monatomic nanowire during its mechanical elongation. The theoretical frequency that we find for an ideally strained nanowire is not compatible with experiment if a uniformly distributed stretch is assumed. With the help of a semi-empirical Au-Au potential, we model the realistic nanowire stretching as exerted by two tips. In this model we see that strain tends to concentrate in the junctions, so that the mean strain of the nanowire is roughly one half of the ideal value. With this reduced strain, the calculated phonon softening is in much better agreement with experiment.Comment: 9 pages,3 figures, Surface Science, in pres

Volatile inventories in clathrate hydrates formed in the primordial nebula

Examination of ambient thermodynamic conditions suggest that clathrate hydrates could exist in the martian permafrost, on the surface and in the interior of Titan, as well as in other icy satellites. Clathrate hydrates probably formed in a significant fraction of planetesimals in the solar system. Thus, these crystalline solids may have been accreted in comets, in the forming giant planets and in their surrounding satellite systems. In this work, we use a statistical thermodynamic model to investigate the composition of clathrate hydrates that may have formed in the primordial nebula. In our approach, we consider the formation sequence of the different ices occurring during the cooling of the nebula, a reasonable idealization of the process by which volatiles are trapped in planetesimals. We then determine the fractional occupancies of guests in each clathrate hydrate formed at given temperature. The major ingredient of our model is the description of the guest-clathrate hydrate interaction by a spherically averaged Kihara potential with a nominal set of parameters, most of which being fitted on experimental equilibrium data. Our model allows us to find that Kr, Ar and N$_2$ can be efficiently encaged in clathrate hydrates formed at temperatures higher than $\sim$ 48.5 K in the primitive nebula, instead of forming pure condensates below 30 K. However, we find at the same time that the determination of the relative abundances of guest species incorporated in these clathrate hydrates strongly depends on the choice of the parameters of the Kihara potential and also on the adopted size of cages. Indeed, testing different potential parameters, we have noted that even minor dispersions between the different existing sets can lead to non-negligible variations in the determination of the volatiles trapped in clathrate hydrates formed in the primordial nebula.Comment: Accepted for publication in Faraday Discussion

The visibility of the Galactic bulge in optical surveys. Application to the Gaia mission

The bulge is a region of the Galaxy which is of tremendous interest for understanding Galaxy formation. However, measuring photometry and kinematics in it raises several inherent issues, like high extinction in the visible and severe crowding. Here we attempt to estimate the problem of the visibility of the bulge at optical wavelengths, where large CCD mosaics allow to easily cover wide regions from the ground, and where future astrometric missions are planned. Assuming the Besancon Galaxy model and high resolution extinction maps, we estimate the stellar density as a function of longitude, latitude and apparent magnitude and we deduce the possibility of reaching and measuring bulge stars. The method is applied to three Gaia instruments, the BBP and MBP photometers, and the RVS spectrograph. We conclude that, while in the BBP most of the bulge will be accessible, in the MBP there will be a small but significant number of regions where bulge stars will be detected and accurately measured in crowded fields. Assuming that the RVS spectra may be extracted in moderately crowded fields, the bulge will be accessible in most regions apart from the strongly absorbed inner plane regions, because of high extinction, and in low extinction windows like the Baades's window where the crowding is too severe.Comment: 11 pages, 9 figures, accepted for publication in A&A, latex using A&A macro

Asymptotic geometry of negatively curved manifolds of finite volume

We study the asymptotic behaviour of simply connected, Riemannian manifolds $X$ of strictly negative curvature admitting a non-uniform lattice $\Gamma$. If the quotient manifold $\bar X= \Gamma \backslash X$ is asymptotically $1/4$-pinched, we prove that $\Gamma$ is divergent and $U\bar X$ has finite Bowen-Margulis measure (which is then ergodic and totally conservative with respect to the geodesic flow); moreover, we show that, in this case, the volume growth of balls $B(x,R)$ in $X$ is asymptotically equivalent to a purely exponential function $c(x)e^{\delta R}$, where $\delta$ is the topological entropy of the geodesic flow of $\bar X$. \linebreak This generalizes Margulis' celebrated theorem to negatively curved spaces of finite volume. In contrast, we exhibit examples of lattices $\Gamma$ in negatively curved spaces $X$ (not asymptotically $1/4$-pinched) where, depending on the critical exponent of the parabolic subgroups and on the finiteness of the Bowen-Margulis measure, the growth function is exponential, lower-exponential or even upper-exponential.Comment: 25 p. This paper replaces arXiv:1503.03971, withdrawn by the authors due to the Theorem 1.1 whose statement is far from the main subject of the paper; for the sake of clearness, this new version concentrates only on the question of volume growth (theorems 1.2, 1.3 and 1.4). Theorem 1.1 of arXiv:1503.03971 is now the subject of another paper (Signed only by 2 authors Sambusetti and Peign\'e) focused on this rigidity problem with a much better presentation of the context and another rigidity resul

Evidence for glutamate-mediated excitotoxic mechanisms during photoreceptor degeneration in the rd1 mouse retina

PURPOSE: Kinetic studies of photoreceptor cell death in the retinal degeneration (rd1) mouse model suggest that photoreceptor degeneration could result from cumulative damage. Since alterations in glutamate metabolism have been described in different models of retinitis pigmentosa, we investigated in the present work whether changes in glutamate turnover occur in the degenerating rd1 retina and whether glutamate-mediated excitotoxic mechanisms may contribute to rod photoreceptor death in this model. METHODS: Free amino acid levels were quantified in rd1 and wild-type retinas using an amino acid analyzer selecting times corresponding to early, intermediate, and terminal phases of rod photoreceptor degeneration. Reverse transcription-polymerase chain reaction (RT-PCR) was used to compare the mRNA expression levels of the glial L-glutamate/L-aspartate transporter GLAST, glutamine synthetase (GS), and vimentin, a marker for retinal glia, between rd1 and wild-type mouse retinas. 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), an antagonist of both AMPA and kainate subtypes of ionotropic glutamate receptors, was then daily administered from postnatal day 3 (PN3) to PN21 to rd1 mice while control rd1 mice received only physiological saline solution (7 per treatment). At PN22, the respective numbers of surviving rods i

Adsorption of H2O2 at the surface of Ih ice, as seen from grand canonical Monte Carlo simulations

Adsorption of H2O2 at the (0 0 0 1) surface of I h ice is investigated by GCMC simulations under tropospheric conditions. The results are in qualitative agreement with experimental data and reveal that the main driving force of the adsorption is the formation of new H2O2-H2O2 rather than H 2O2-water H-bonds. The isotherm belongs to class III and not even its low pressure part can be described by the Langmuir formalism. At low coverages H2O2 prefers perpendicular alignment to the surface, in which they can form three H-bonds with surface waters. At higher coverages parallel alignment, stabilized by H-bonds between neighbouring H 2O2 molecules, becomes increasingly preferred. © 2014 Elsevier B.V. All rights reserved

A biophysical model explains the spontaneous bursting behavior in the developing retina

During early development, waves of activity propagate across the retina and play a key role in the proper wiring of the early visual system. During the stage II these waves are triggered by a transient network of neurons, called Starburst Amacrine Cells (SACs), showing a bursting activity which disappears upon further maturation. While several models have attempted to reproduce retinal waves, none of them is able to mimic the rhythmic autonomous bursting of individual SACs and reveal how these cells change their intrinsic properties during development. Here, we introduce a mathematical model, grounded on biophysics, which enables us to reproduce the bursting activity of SACs and to propose a plausible, generic and robust, mechanism that generates it. The core parameters controlling repetitive firing are fast depolarizing $V$-gated calcium channels and hyperpolarizing $V$-gated potassium channels. The quiescent phase of bursting is controlled by a slow after hyperpolarization (sAHP), mediated by calcium-dependent potassium channels. Based on a bifurcation analysis we show how biophysical parameters, regulating calcium and potassium activity, control the spontaneously occurring fast oscillatory activity followed by long refractory periods in individual SACs. We make a testable experimental prediction on the role of voltage-dependent potassium channels on the excitability properties of SACs and on the evolution of this excitability along development. We also propose an explanation on how SACs can exhibit a large variability in their bursting periods, as observed experimentally within a SACs network as well as across different species, yet based on a simple, unique, mechanism. As we discuss, these observations at the cellular level have a deep impact on the retinal waves description.Comment: 25 pages, 13 figures, submitte