5,808 research outputs found
Molecule formation as a diagnostic tool for second order correlations of ultra-cold gases
We calculate the momentum distribution and the second-order correlation
function in momentum space, for molecular dimers
that are coherently formed from an ultracold atomic gas by photoassociation or
a Feshbach resonance. We investigate using perturbation theory how the quantum
statistics of the molecules depend on the initial state of the atoms by
considering three different initial states: a Bose-Einstein condensate (BEC), a
normal Fermi gas of ultra-cold atoms, and a BCS-type superfluid Fermi gas. The
cases of strong and weak coupling to the molecular field are discussed. It is
found that BEC and BCS states give rise to an essentially coherent molecular
field with a momentum distribution determined by the zero-point motion in the
confining potential. On the other hand, a normal Fermi gas and the unpaired
atoms in the BCS state give rise to a molecular field with a broad momentum
distribution and thermal number statistics. It is shown that the first-order
correlations of the molecules can be used to measure second-order correlations
of the initial atomic state.Comment: revtex, 15 pages,8 figure
Taking stock of arctic sea ice and climate
Abstract
The relationship among the cause-and-effect of the Arctic atmosphere, sea ice, and ocean is discussed. The increased vulnerability of the Arctic system to anomalous atmospheric forcing can be argued from the perspective that recent ice loss is the result of a long-term preconditioning to thinner ice. Such consequences demonstrate the difficulties inherent in ascertaining how the atmospheric circulation responds to Arctic, and global, climate change. Later-forming sea ice also leads to less protection from the waves of fall storms, affecting coastal communities such as Kivalina and Shishmaref. The coming decades will provide new insights into the complexities of the Arctic climate system and how changes will affect the biological and human communities within and beyond its boundaries
The Status of the Pion-Nucleon Coupling Constant
A review is given of the various determinations of the different piNN
coupling constants in analyses of the low-energy pp, np, pbarp, and pi-p
scattering data. The most accurate determinations are in the energy-dependent
partial-wave analyses of the NN data. The recommended value is f^2 = 0.075 . A
recent determination of f^2 by the Uppsala group from backward np cross
sections is shown to be model dependent and inaccurate, and therefore
completely uninteresting. We also argue that an accurate determination of f^2
using pp forward dispersion relations is not a realistic option.Comment: 19 pages, latex2e with a4wide.sty, more information is available at
http://NN-OnLine.sci.kun.nl . Invited talk at FBXV, Groningen, The
Netherlands, July 22-26, 1997. Invited talk at MENU97, Vancouver, B.C.,
Canada, July 28 - August 1, 199
Zero sound in a single component fermion - Bose Einstein Condensate mixture
The resonant dynamics of mediated interactions supports zero-sound in a cold
atom degenerate mixture of a single component fermion gas and a Bose-Einstein
condensate (BEC). We characterize the onset of instability in the phase
separation of an unstable mixture and we find a rich collective mode structure
for stable mixtures with one undamped mode that exhibits an avoided crossing
and a Landau-damped mode that terminates.Comment: 4 pages, 2 figure
Partial-Wave Analyses of all Proton-Proton and Neutron-Proton Data Below 500 MeV
In 1993 the Nijmegen group published the results of energy-dependent
partial-wave analyses (PWAs) of the nucleon-nucleon (NN) scattering data for
laboratory kinetic energies below Tlab=350 MeV (PWA93). In this talk some
general aspects, but also the newest developments on the Nijmegen NN PWAs are
reported. We have almost finished a new energy-dependent PWA and will discuss
some typical aspects of this new PWA; where it differs from PWA93, but also
what future developments might be, or should be.Comment: Presentation at the 19th European Conference on Few-Body Problems in
Physics, Groningen, The Netherlands, 23-27 August 2004. 4 pages REVTeX4, no
figure
Retrieval of canopy component temperatures through Bayesian inversion of directional thermal measurements
Evapotranspiration is usually estimated in remote sensing from single temperature value representing both soil and vegetation. This surface temperature is an aggregate over multiple canopy components. The temperature of the individual components can differ significantly, introducing errors in the evapotranspiration estimations. The temperature aggregate has a high level of directionality. An inversion method is presented in this paper to retrieve four canopy component temperatures from directional brightness temperatures. The Bayesian method uses both a priori information and sensor characteristics to solve the ill-posed inversion problem. The method is tested using two case studies: 1) a sensitivity analysis, using a large forward simulated dataset, and 2) in a reality study, using two datasets of two field campaigns. The results of the sensitivity analysis show that the Bayesian approach is able to retrieve the four component temperatures from directional brightness temperatures with good success rates using multi-directional sensors (SrspectraËś0.3, SrgonioËś0.3, and SrAATSRËś0.5), and no improvement using mono-angular sensors (SrËś1). The results of the experimental study show that the approach gives good results for high LAI values (RMSEgrass=0.50 K, RMSEwheat=0.29 K, RMSEsugar beet=0.75 K, RMSEbarley=0.67 K); but for low LAI values the results were unsatisfactory (RMSEyoung maize=2.85 K). This discrepancy was found to originate from the presence of the metallic construction of the setup. As these disturbances, were only present for two crops and were not present in the sensitivity analysis, which had a low LAI, it is concluded that using masked thermal images will eliminate this discrepanc
Self-localized impurities embedded in a one dimensional Bose-Einstein condensate and their quantum fluctuations
We consider the self-localization of neutral impurity atoms in a
Bose-Einstein condensate in a 1D model. Within the strong coupling approach, we
show that the self-localized state exhibits parametric soliton behavior. The
corresponding stationary states are analogous to the solitons of non-linear
optics and to the solitonic solutions of the Schroedinger-Newton equation
(which appears in models that consider the connection between quantum mechanics
and gravitation). In addition, we present a Bogoliubov-de-Gennes formalism to
describe the quantum fluctuations around the product state of the strong
coupling description. Our fluctuation calculations yield the excitation
spectrum and reveal considerable corrections to the strong coupling
description. The knowledge of the spectrum allows a spectroscopic detection of
the impurity self-localization phenomenon.Comment: 7 pages, 5 figure
Degenerate fermion gas heating by hole creation
Loss processes that remove particles from an atom trap leave holes behind in
the single particle distribution if the trapped gas is a degenerate fermion
system. The appearance of holes increases the temperature and we show that the
heating is (i) significant if the initial temperature is well below the Fermi
temperature , and (ii) increases the temperature to
after half of the system's lifetime, regardless of the initial temperature. The
hole heating has important consequences for the prospect of observing
Cooper-pairing in atom traps.Comment: to be published in PR
BRDFs acquired by directional radiative measurements during EAGLE and AGRISAR
Radiation is the driving force for all processes and interactions between earth surface and atmosphere. The amount of
measured radiation reflected by vegetation depends on its structure, the viewing angle and the solar angle. This angular
dependence is usually expressed in the Bi-directional Reflectance Distribution Function (BRDF). This BRDF is not
only different for different types of vegetation, but also different for different stages of the growth. The BRDF therefore
has to be measured at ground level before any satellite imagery can be used the calculate surface-atmosphere
interaction. The objective of this research is to acquire the BRDFs for agricultural crop types.
A goniometric system is used to acquire the BRDFs. This is a mechanical device capable of a complete hemispherical
rotation. The radiative directional measurements are performed with different sensors that can be attached to this
system. The BRDFs are calculated from the measured radiation.
In the periods 10 June - 18 June 2006 and 2 July - 10 July 2006 directional radiative measurements were performed at
three sites: Speulderbos site, in the Netherlands, the Cabauw site, in the Netherlands, and an agricultural test site in
Goermin, Germany. The measurements were performed over eight different crops: forest, grass, pine tree, corn, wheat,
sugar beat and barley. The sensors covered the spectrum from the optical to the thermal domain. The measured radiance
is used to calculate the BRDFs or directional thermal signature.
This contribution describes the measurements and calculation of the BRDFs of forest, grassland, young corn, mature
corn, wheat, sugar beat and barley during the EAGLE2006 and AGRISAR 2006 fieldcampaigns. Optical BRDF have
been acquired for all crops except barley. Thermal angular signatures are acquired for all the crop
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