5,487 research outputs found
Spatially resolved photo ionization of ultracold atoms on an atom chip
We report on photo ionization of ultracold magnetically trapped Rb atoms on
an atom chip. The atoms are trapped at 5 K in a strongly anisotropic
trap. Through a hole in the chip with a diameter of 150 m two laser beams
are focussed onto a fraction of the atomic cloud. A first laser beam with a
wavelength of 778 nm excites the atoms via a two photon transition to the 5D
level. With a fiber laser at 1080 nm the excited atoms are photo ionized.
Ionization leads to depletion of the atomic density distribution observed by
absorption imaging. The resonant ionization spectrum is reported. The setup
used in this experiment is not only suitable to investigate BEC ion mixtures
but also single atom detection on an atom chip
Quantum-degenerate mixture of fermionic lithium and bosonic rubidium gases
We report on the observation of sympathetic cooling of a cloud of fermionic
6-Li atoms which are thermally coupled to evaporatively cooled bosonic 87-Rb.
Using this technique we obtain a mixture of quantum-degenerate gases, where the
Rb cloud is colder than the critical temperature for Bose-Einstein condensation
and the Li cloud colder than the Fermi temperature. From measurements of the
thermalization velocity we estimate the interspecies s-wave triplet scattering
length |a_s|=20_{-6}^{+9} a_B. We found that the presence of residual rubidium
atoms in the |2,1> and the |1,-1> Zeeman substates gives rise to important
losses due to inelastic collisions.Comment: 4 pages, 3 figure
Substrate rigidity deforms and polarizes active gels
We present a continuum model of the coupling between cells and substrate that
accounts for some of the observed substrate-stiffness dependence of cell
properties. The cell is modeled as an elastic active gel, adapting recently
developed continuum theories of active viscoelastic fluids. The coupling to the
substrate enters as a boundary condition that relates the cell's deformation
field to local stress gradients. In the presence of activity, the coupling to
the substrate yields spatially inhomogeneous contractile stresses and
deformations in the cell and can enhance polarization, breaking the cell's
front-rear symmetry.Comment: 6 pages, 4 figures, EPL forma
The QCD phase diagram from analytic continuation
We present the crossover line between the quark gluon plasma and the hadron
gas phases for small real chemical potentials. First we determine the effect of
imaginary values of the chemical potential on the transition temperature using
lattice QCD simulations. Then we use various formulas to perform an analytic
continuation to real values of the baryo-chemical potential. Our data set
maintains strangeness neutrality to match the conditions of heavy ion physics.
The systematic errors are under control up to MeV. For the
curvature of the transition line we find that there is an approximate agreement
between values from three different observables: the chiral susceptibility,
chiral condensate and strange quark susceptibility. The continuum extrapolation
is based on 10, 12 and 16 lattices. By combining the analysis for these
three observables we find, for the curvature, the value .Comment: 14 pages, 4 figures, revised versio
Quasi-local contribution to the scalar self-force: Geodesic Motion
We consider a scalar charge travelling in a curved background spacetime. We
calculate the quasi-local contribution to the scalar self-force experienced by
such a particle following a geodesic in a general spacetime. We also show that
if we assume a massless field and a vacuum background spacetime, the expression
for the self-force simplifies significantly. We consider some specific cases
whose gravitational analog are of immediate physical interest for the
calculation of radiation reaction corrected orbits of binary black hole
systems. These systems are expected to be detectable by the LISA space based
gravitational wave observatory. We also investigate how alternate techniques
may be employed in some specific cases and use these as a check on our own
results.Comment: Final Phys. Rev. D version. 24 pages, revtex4. Minor typos correcte
Open ocean carbon monoxide photo-production
Sunlight-initiated photolysis of chromophoric dissolved organic matter (CDOM) is the dominant source of carbon monoxide (CO) in the open-ocean. A modelling study was conducted to constrain this source. Spectral solar irradiance was obtained from two models (GCSOLAR and SMARTS2). Water-column CDOM and total light absorption were modelled using spectra collected along a Meridional transect of the Atlantic ocean using a 200-cm pathlength liquid waveguide UV-visible spectrophotometer. Apparent quantum yields for the production of CO (AQYCO) from CDOM were obtained from a parameterisation describing the relationship between CDOM light absorption coefficient and AQYCO and the CDOM spectra collected. The sensitivity of predicted rates to variations in model parameters (solar irradiance, cloud cover, surface-water reflectance, CDOM and whole water light absorbance, and AQYCO was assessed. The model\u27s best estimate of open-ocean CO photoproduction was 47 +/- 7 Tg CO-C yr-1, with lower and upper limits of 38 and 84 Tg CO-C yr-1, as indicated by sensitivity analysis considering variations in AQYs, CDOM absorbance, and spectral irradiance. These results represent significant constraint of open-ocean CO photoproduction at the lower limit of previous estimates. Based on these results, and their extrapolation to total photochemical organic carbon mineralisation, we recommend a downsizing of the role of photochemistry in the open-ocean carbon cycle.
(c) 2006 Elsevier Ltd. All rights reserved
Diffraction of a Bose-Einstein condensate from a Magnetic Lattice on a Micro Chip
We experimentally study the diffraction of a Bose-Einstein condensate from a
magnetic lattice, realized by a set of 372 parallel gold conductors which are
micro fabricated on a silicon substrate. The conductors generate a periodic
potential for the atoms with a lattice constant of 4 microns. After exposing
the condensate to the lattice for several milliseconds we observe diffraction
up to 5th order by standard time of flight imaging techniques. The experimental
data can be quantitatively interpreted with a simple phase imprinting model.
The demonstrated diffraction grating offers promising perspectives for the
construction of an integrated atom interferometer.Comment: 4 pages, 4 figure
Equation of State of Oscillating Brans-Dicke Scalar and Extra Dimensions
We consider a Brans-Dicke scalar field stabilized by a general power law
potential with power index at a finite equilibrium value. Redshifting
matter induces oscillations of the scalar field around its equilibrium due to
the scalar field coupling to the trace of the energy momentum tensor. If the
stabilizing potential is sufficiently steep these high frequency oscillations
are consistent with observational and experimental constraints for arbitrary
value of the Brans-Dicke parameter . We study analytically and
numerically the equation of state of these high frequency oscillations in terms
of the parameters and and find the corresponding evolution of the
universe scale factor. We find that the equation of state parameter can be
negative and less than -1 but it is not related to the evolution of the scale
factor in the usual way. Nevertheless, accelerating expansion is found for a
certain parameter range. Our analysis applies also to oscillations of the size
of extra dimensions (the radion field) around an equilibrium value. This
duality between self-coupled Brans-Dicke and radion dynamics is applicable for
where D is the number of extra dimensions.Comment: 10 two-column pages, RevTex4, 8 figures. Added clarifying
discussions, new references. Accepted in Phys. Rev. D (to appear
Calibration of a single atom detector for atomic micro chips
We experimentally investigate a scheme for detecting single atoms
magnetically trapped on an atom chip. The detector is based on the
photoionization of atoms and the subsequent detection of the generated ions. We
describe the characterization of the ion detector with emphasis on its
calibration via the correlation of ions with simultaneously generated
electrons. A detection efficiency of 47.8% (+-2.6%) is measured, which is
useful for single atom detection, and close to the limit allowing atom counting
with sub-Poissonian uncertainty
AdS and stabilized extra dimensions in multidimensional gravitational models with nonlinear scalar curvature terms 1/R and R^4
We study multidimensional gravitational models with scalar curvature
nonlinearities of the type 1/R and R^4. It is assumed that the corresponding
higher dimensional spacetime manifolds undergo a spontaneous compactification
to manifolds with warped product structure. Special attention is paid to the
stability of the extra-dimensional factor spaces. It is shown that for certain
parameter regions the systems allow for a freezing stabilization of these
spaces. In particular, we find for the 1/R model that configurations with
stabilized extra dimensions do not provide a late-time acceleration (they are
AdS), whereas the solution branch which allows for accelerated expansion (the
dS branch) is incompatible with stabilized factor spaces. In the case of the
R^4 model, we obtain that the stability region in parameter space depends on
the total dimension D=dim(M) of the higher dimensional spacetime M. For D>8 the
stability region consists of a single (absolutely stable) sector which is
shielded from a conformal singularity (and an antigravity sector beyond it) by
a potential barrier of infinite height and width. This sector is smoothly
connected with the stability region of a curvature-linear model. For D<8 an
additional (metastable) sector exists which is separated from the conformal
singularity by a potential barrier of finite height and width so that systems
in this sector are prone to collapse into the conformal singularity. This
second sector is not smoothly connected with the first (absolutely stable) one.
Several limiting cases and the possibility for inflation are discussed for the
R^4 model.Comment: 28 pages, minor cosmetic improvements, Refs. added; to appear in
Class. Quantum Gra
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