6,470 research outputs found
Effects of temperature on in vitro sediment reworking processes by a gallery biodiffusor, the polychaete Neanthes virens
Temperature-induced variations in bioturbation could affect sediment mixing processes in the marine benthic environment. In this study, sediment reworking by Neanthes virens (Sars), a widely distributed polychaete in muddy sand communities of northern temperate latitudes, was studied under different temperature conditions representing winter (1°C), spring and fall (6°C), summer(13°C), and tide pool (18°C) temperatures in the lower St. Lawrence Estuary, Québec, Canada. Sediment reworking was quantified using inert fluorescent particles (luminophores) deposited at the sediment surface. Based on the 1-D luminophore distributions obtained after 5 and 30 d, the use of the
specific âgallery-biodiffusorâ model allowed us to quantify both biodiffusion (Db) and biotransport (Vb) due to the organisms. Our results showed temperature effects on sediment transport. The lowest biotransport and biodiffusion coefficients were measured at 1 and 6°C and did not change with time. The highest biodiffusion occurred at 13°C for both sampling periods. At 18°C, biodiffusion was intermediate while biotransport was maximal. Differences between the 13°C biodiffusive transport and
the other temperatures increased with time. Low transport values at 1 and 6°C suggest that a quiescent stage exists for this species at these temperatures, with sediment mixing occurring mostly during burrow construction. On the other hand, sediment mixing resulted from both the burrow
construction and maintenance phases at higher temperatures (13 and 18°C)
Tuning Energy Relaxation along Quantum Hall Channels
The chiral edge channels in the quantum Hall regime are considered ideal
ballistic quantum channels, and have quantum information processing
potentialities. Here, we demonstrate experimentally, at filling factor 2, the
efficient tuning of the energy relaxation that limits quantum coherence and
permits the return toward equilibrium. Energy relaxation along an edge channel
is controllably enhanced by increasing its transmission toward a floating ohmic
contact, in quantitative agreement with predictions. Moreover, by forming a
closed inner edge channel loop, we freeze energy exchanges in the outer
channel. This result also elucidates the inelastic mechanisms at work at
filling factor 2, informing us in particular that those within the outer edge
channel are negligible.Comment: 8 pages including supplementary materia
Non-destructive measurement of the transition probability in a Sr optical lattice clock
We present the experimental demonstration of non-destructive probing of the
1S0-3P0 clock transition probability in an optical lattice clock with 87Sr
atoms. It is based on the phase shift induced by the atoms on a weak
off-resonant laser beam. The method we propose is a differential measurement of
this phase shift on two modulation sidebands with opposite detuning with
respect to the 1S0-1P1 transition, allowing a detection limited by the photon
shot noise. We have measured an atomic population of 10^4 atoms with a signal
to noise ratio of 100 per cycle, while keeping more than 95% of the atoms in
the optical lattice with a depth of 0.1 mK. The method proves simple and robust
enough to be operated as part of the whole clock setup. This detection scheme
enables us to reuse atoms for subsequent clock state interrogations,
dramatically reducing the loading time and thereby improving the clock
frequency stability.Comment: 4 pages, 5 figure
Iron in Hot DA White Dwarfs
We present a study of the iron abundance pattern in hot hydrogen-rich (DA)
white dwarfs. The study is based on new and archival far ultraviolet
spectroscopy of a sample of white dwarfs in the temperature range 30,000 K <
T_eff < 64,000 K. The spectra obtained with the Far Ultraviolet Spectroscopic
Explorer along with spectra obtained with the Hubble Space Telescope Imaging
Spectrograph and the International Ultraviolet Explorer sample FeIII to FeVI
absorption lines enabling a detailed iron abundance analysis over a wider range
of effective temperatures than previously afforded. The measurements reveal
abundance variations in excess of two orders of magnitude between the highest
and the lowest temperatures probed, but also show considerable variations (over
one order of magnitude) between objects with similar temperatures and surface
gravities. Such variations in cooler objects may be imputed to accretion from
unseen companions or so-called circumstellar debris although the effect of
residual mass-loss and selective radiation pressure in the hottest objects in
the sample remain dominant.Comment: Accepted for publication in Ap
Direct measurements of anisotropic energy transfers in a rotating turbulence experiment
We investigate experimentally the influence of a background rotation on the
energy transfers in decaying grid turbulence. The anisotropic energy flux
density, , where
is the vector velocity increment over separation , is
determined for the first time using Particle Image Velocimetry. We show that
rotation induces an anisotropy of the energy flux , which
leads to an anisotropy growth of the energy distribution , in agreement with the K\'arm\'an-Howarth-Monin equation.
Surprisingly, our results prove that this anisotropy growth is essentially
driven by a nearly radial, but orientation-dependent, energy flux density .Comment: to appear in Physical Review Letters (July 8, 2011 issue
A simple, efficient, and general treatment of the singularities in Hartree-Fock and exact-exchange Kohn-Sham methods for solids
We present a general scheme for treating the integrable singular terms within
exact exchange (EXX) Kohn-Sham or Hartree-Fock (HF) methods for periodic
solids. We show that the singularity corrections for treating these
divergencies depend only on the total number and the positions of k-points and
on the lattice vectors, in particular the unit cell volume, but not on the
particular positions of atoms within the unit cell. The method proposed here to
treat the singularities constitutes a stable, simple to implement, and general
scheme that can be applied to systems with arbitrary lattice parameters within
either the EXX Kohn-Sham or the HF formalism. We apply the singularity
correction to a typical symmetric structure, diamond, and to a more general
structure, trans-polyacetylene. We consider the effect of the singularity
corrections on volume optimisations and k-point convergence. While the
singularity corrections clearly depends on the total number of k-points, it
exhibits a remarkably small dependence upon the choice of the specific
arrangement of the k-points.Comment: 24 pages, 5 Figures, re-submitted to Phys. Rev. B after revision
Sound generation by impulse excited plates coupled to acoustics cavities.
International audienceThis paper is concerned with vibroacoustics in the time domain. One of the aims is to compare results given by an semi-analytical technique based on the resonance modes with a finite difference technique. An other goal is to describe the response of a fluid-loaded plate (displacement of the structure and sound pressure in the fluid) coupled to a rigid cavity when it is excited by a Ricker wavelet and to see the influence of the excitation on the response of system
Constraints on Light Dark Matter From Core-Collapse Supernovae
We show that light ( 1 -- 30 MeV) dark matter particles can play a
significant role in core-collapse supernovae, if they have relatively large
annihilation and scattering cross sections, as compared to neutrinos. We find
that if such particles are lighter than 10 MeV and reproduce the
observed dark matter relic density, supernovae would cool on a much longer time
scale and would emit neutrinos with significantly smaller energies than in the
standard scenario, in disagreement with observations. This constraint may be
avoided, however, in certain situations for which the neutrino--dark matter
scattering cross sections remain comparatively small.Comment: 4 pages, 1 figur
- âŠ