5,865 research outputs found
Probing States in the Mott Insulator Regime
We propose a method to probe states in the Mott insulator regime produced
from a condensate in an optical lattice. We consider a system in which we
create time-dependent number fluctuations in a given site by turning off the
atomic interactions and lowering the potential barriers on a nearly pure Mott
state to allow the atoms to tunnel between sites. We calculate the expected
interference pattern and number fluctuations from such a system and show that
one can potentially observe a deviation from a pure Mott state. We also discuss
a method in which to detect these number fluctuations using time-of-flight
imaging.Comment: 4 pages, 3 figures. Send correspondence to
[email protected]
Submarine groundwater discharge: an unseen yet potentially important coastal phenomenon
In collaboration with researchers from Florida
State University, Florida Sea Grant introduces an important but poorly known topic:
submarine groundwater discharge. Although nearly invisible, submarine
groundwater discharge influences coastal systems. This brochure helps explain this important phenomenon. (8pp.
Stochastic Gene Expression in a Lentiviral Positive Feedback Loop: HIV-1 Tat Fluctuations Drive Phenotypic Diversity
Stochastic gene expression has been implicated in a variety of cellular
processes, including cell differentiation and disease. In this issue of Cell,
Weinberger et al. (2005) take an integrated computational-experimental approach
to study the Tat transactivation feedback loop in HIV-1 and show that
fluctuations in a key regulator, Tat, can result in a phenotypic bifurcation.
This phenomenon is observed in an isogenic population where individual cells
display two distinct expression states corresponding to latent and productive
infection by HIV-1. These findings demonstrate the importance of stochastic
gene expression in molecular "decision-making."Comment: Supplemental data available as q-bio.MN/060800
Correction: Lipid phosphate phosphatases dimerise, but this interaction is not required for in vivo activity
A Continuum Description of Rarefied Gas Dynamics (I)--- Derivation From Kinetic Theory
We describe an asymptotic procedure for deriving continuum equations from the
kinetic theory of a simple gas. As in the works of Hilbert, of Chapman and of
Enskog, we expand in the mean flight time of the constituent particles of the
gas, but we do not adopt the Chapman-Enskog device of simplifying the formulae
at each order by using results from previous orders. In this way, we are able
to derive a new set of fluid dynamical equations from kinetic theory, as we
illustrate here for the relaxation model for monatomic gases. We obtain a
stress tensor that contains a dynamical pressure term (or bulk viscosity) that
is process-dependent and our heat current depends on the gradients of both
temperature and density. On account of these features, the equations apply to a
greater range of Knudsen number (the ratio of mean free path to macroscopic
scale) than do the Navier-Stokes equations, as we see in the accompanying
paper. In the limit of vanishing Knudsen number, our equations reduce to the
usual Navier-Stokes equations with no bulk viscosity.Comment: 16 page
Temporal coherence, anomalous moments, and pairing correlations in the classical-field description of a degenerate Bose gas
The coherence properties of degenerate Bose gases have usually been expressed
in terms of spatial correlation functions, neglecting the rich information
encoded in their temporal behavior. In this paper we show, using a Hamiltonian
classical-field formalism, that temporal correlations can be used to
characterize familiar properties of a finite-temperature degenerate Bose gas.
The temporal coherence of a Bose-Einstein condensate is limited only by the
slow diffusion of its phase, and thus the presence of a condensate is indicated
by a sharp feature in the temporal power spectrum of the field. We show that
the condensate mode can be obtained by averaging the field for a short time in
an appropriate phase-rotating frame, and that for a wide range of temperatures,
the condensate obtained in this approach agrees well with that defined by the
Penrose-Onsager criterion based on one-body (spatial) correlations. For time
periods long compared to the phase diffusion time, the field will average to
zero, as we would expect from the overall U(1) symmetry of the Hamiltonian. We
identify the emergence of the first moment on short time scales with the
concept of U(1) symmetry breaking that is central to traditional mean-field
theories of Bose condensation. We demonstrate that the short-time averaging
procedure constitutes a general analog of the 'anomalous' averaging operation
of symmetry-broken theories by calculating the anomalous thermal density of the
field, which we find to have form and temperature dependence consistent with
the results of mean-field theories.Comment: 11 pages, 6 figures. v3: Final version. Typos fixed, and other minor
change
Seed Treatment for Corn Diseases
In Iowa three destructive corn diseases attack the seed, namely, Dfplc·dfa dry rot, Baslsporlum dry rot and Gibberella dry rot. These dry rots are best known on the ear, but also may attack any part of the plant, lncluding the seed and seedling. The Injury to the seed and to the subsequent yield has been measured during the last six years in 25 counties and found to average 5 bushels per acre. These dry rot organisms llve over on the old stubble In the soil and on the seed and attack the next season\u27s crop
THE EFFECT OF INDIVIDUALISED COACHING INTERVENTIONS ON ELITE YOUNG FAST BOWLERS‘ TECHNIQUE
Fast bowling in cricket is an activity well recognised as having a high injury prevalence. Previous research has associated lower back injury with aspects of fast bowling technique. Coaching interventions that may decrease the likelihood of injury, whilst maintaining or increasing ball speed, remain a priority within the sport. Selected kinematics of the bowling action of 14 elite young fast bowlers were measured using an 18 camera Vicon Motion Analysis System. Subjects were tested before and after a two year coaching intervention period, during which subject-specific coaching interventions were provided. Mann-Whitney tests were used to identify significant differences in the change in the selected kinematics between those bowlers who were coached or un-coached on each specific aspect. Coached athletes demonstrated a significant change in shoulder alignment at back foot contact (more side-on, P = 0.002) and shoulder counter-rotation (decreased, P = 0.001) relative to un-coached athletes. There was no difference in the amount of change in flexion angles of the front or back knee or lower trunk side-flexion between those who received coaching intervention and those that did not. This study shows that specific aspects of fast bowling technique in elite players can change over a two year period and may be attributed to coaching intervention
Greenhouse gas balance over thaw-freeze cycles in discontinuous zone permafrost
Peat in the discontinuous permafrost zone contains a globally significant reservoir of carbon that has undergone multiple permafrost-thaw cycles since the end of the mid-Holocene (~3700 years before present). Periods of thaw increase C decomposition rates which leads to the release of CO2 and CH4 to the atmosphere creating potential climate feedback. To determine the magnitude and direction of such feedback, we measured CO2 and CH4 emissions and modeled C accumulation rates and radiative fluxes from measurements of two radioactive tracers with differing lifetimes to describe the C balance of the peatland over multiple permafrost-thaw cycles since the initiation of permafrost at the site. At thaw features, the balance between increased primary production and higher CH4 emission stimulated by warmer temperatures and wetter conditions favors C sequestration and enhanced peat accumulation. Flux measurements suggest that frozen plateaus may intermittently (order of years to decades) act as CO2 sources depending on temperature and net ecosystem respiration rates, but modeling results suggest that—despite brief periods of net C loss to the atmosphere at the initiation of thaw—integrated over millennia, these sites have acted as net C sinks via peat accumulation. In greenhouse gas terms, the transition from frozen permafrost to thawed wetland is accompanied by increasing CO2 uptake that is partially offset by increasing CH4 emissions. In the short-term (decadal time scale) the net effect of this transition is likely enhanced warming via increased radiative C emissions, while in the long-term (centuries) net C deposition provides a negative feedback to climate warming
Just how long can you live in a black hole and what can be done about it?
We study the problem of how long a journey within a black hole can last.
Based on our observations, we make two conjectures. First, for observers that
have entered a black hole from an asymptotic region, we conjecture that the
length of their journey within is bounded by a multiple of the future
asymptotic ``size'' of the black hole, provided the spacetime is globally
hyperbolic and satisfies the dominant-energy and non-negative-pressures
conditions. Second, for spacetimes with Cauchy surfaces (or an
appropriate generalization thereof) and satisfying the dominant energy and
non-negative-pressures conditions, we conjecture that the length of a journey
anywhere within a black hole is again bounded, although here the bound requires
a knowledge of the initial data for the gravitational field on a Cauchy
surface. We prove these conjectures in the spherically symmetric case. We also
prove that there is an upper bound on the lifetimes of observers lying ``deep
within'' a black hole, provided the spacetime satisfies the
timelike-convergence condition and possesses a maximal Cauchy surface. Further,
we investigate whether one can increase the lifetime of an observer that has
entered a black hole, e.g., by throwing additional matter into the hole.
Lastly, in an appendix, we prove that the surface area of the event horizon
of a black hole in a spherically symmetric spacetime with ADM mass
is always bounded by , provided
that future null infinity is complete and the spacetime is globally hyperbolic
and satisfies the dominant-energy condition.Comment: 20 pages, REVTeX 3.0, 6 figures included, self-unpackin
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