1,964 research outputs found
Effects of the trapping potential on a superfluid atomic Fermi Gas
We examine a dilute two-component atomic Fermi gas trapped in a harmonic
potential in the superfluid phase. For experimentally realistic parameters, the
trapping potential is shown to have crucial influence on various properties of
the gas. Using an effective hamiltonian, analytical results for the critical
temperature, the temperature dependence of the superfluid gap, and the energy
of the lowest collective modes are derived. These results are shown to agree
well with numerical calculations. We furthermore discuss in more detail a
previous proposed method to experimentally observe the superfluid transition by
looking at the collective mode spectrum. Our results are aimed at the present
experimental effort to observe a superfluid phase transition in a trapped
atomic Fermi gas.Comment: 2. revised version. Minor mistakes in equation references corrected.
To appear in Phys. Rev.
Hartree-Fock-Bogoliubov theory versus local-density approximation for superfluid trapped fermionic atoms
We investigate a gas of superfluid fermionic atoms trapped in two hyperfine
states by a spherical harmonic potential. We propose a new regularization
method to remove the ultraviolet divergence in the Hartree-Fock-Bogoliubov
equations caused by the use of a zero-range atom-atom interaction. Compared
with a method used in the literature, our method is simpler and has improved
convergence properties. Then we compare Hartree-Fock-Bogoliubov calculations
with the semiclassical local-density approximation. We observe that for systems
containing a small number of atoms shell effects, which cannot be reproduced by
the semiclassical calculation, are very important. For systems with a large
number of atoms at zero temperature the two calculations are in quite good
agreement, which, however, is deteriorated at non-zero temperature, especially
near the critical temperature. In this case the different behavior can be
explained within the Ginzburg-Landau theory.Comment: 12 pages, 8 figures, revtex; v2: references and clarifying remarks
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Multi-channel scattering and Feshbach resonances: Effective theory, phenomenology, and many-body effects
A low energy effective theory based on a microscopic multi-channel
description of the atom-atom interaction is derived for the scattering of
alkali atoms in different hyperfine states. This theory describes all
scattering properties, including medium effects, in terms of the singlet and
triplet scattering lengths and the range of the atom-atom potential and
provides a link between a microscopic description of Feshbach scattering and
more phenomenological approaches. It permits the calculation of medium effects
on the resonance coming from the occupation of closed channel states. The
examination of such effects are demonstrated to be of particular relevance to
an experimentally important Feshbach resonance for K atoms. We analyze a
recent rethermalization rate experiment on K and demonstrate that a
measurement of the temperature dependence of this rate can determine the
magnetic moment of the Feshbach molecule. Finally, the energy dependence of the
Feshbach interaction is shown to introduce a negative effective range inversely
proportional to the width of the resonance. Since our theory is based on a
microscopic multi-channel picture, it allows the explicit calculation of
corrections to commonly used approximations such as the neglect of the
effective range and the treatment of the Feshbach molecule as a point boson.Comment: 10 pages, 5 figures. Typos corrected. Accepted for PR
Laser probing of Cooper-paired trapped atoms
We consider a gas of trapped Cooper-paired fermionic atoms which are
manipulated by laser light. The laser induces a transition from an internal
state with large negative scattering length (superfluid) to one with weaker
interactions (normal gas). We show that the process can be used to detect the
presence of the superconducting order parameter. Also, we propose a direct way
of measuring the size of the gap in the trap. The efficiency and feasibility of
this probing method is investigated in detail in different physical situations.Comment: 9 pages, 8 figure
Life cycle modelling of environmental impacts of application of processed organic municipal solid waste on agricultural land (EASEWASTE)
A model capable of quantifying the potential environmental impacts of agricultural application of composted or anaerobically digested source-separated organic municipal solid waste (MSW) is presented. In addition to the direct impacts, the model accounts for savings by avoiding the production and use of commercial fertilizers. The model is part of a larger model, Environmental Assessment of Solid Waste Systems and Technology (EASEWASTE), developed as a decisionsupport model, focusing on assessment of alternative waste management options. The environmental impacts of the land application of processed organic waste are quantified by emission coefficients referring to the composition of the processed waste and related to specific crop rotation as well as soil type. The model contains several default parameters based on literature data, field experiments and modelling by the agro-ecosystem model, Daisy. All data can be modified by the user allowing application of the model to other situations. A case study including four scenarios was performed to illustrate the use of the model. One tonne of nitrogen in composted and anaerobically digested MSW was applied as fertilizer to loamy and sandy soil at a plant farm in western Denmark. Application of the processed organic waste mainly affected the environmental impact categories global warming (0.4–0.7 PE), acidification (–0.06 (saving)–1.6 PE), nutrient enrichment (–1.0 (saving)–3.1 PE), and toxicity. The main contributors to these categories were nitrous oxide formation (global warming), ammonia volatilization (acidification and nutrient enrichment), nitrate losses (nutrient enrichment and groundwater contamination), and heavy metal input to soil (toxicity potentials). The local agricultural conditions as well as the composition of the processed MSW showed large influence on the environmental impacts. A range of benefits, mainly related to improved soil quality from long-term application of the processed organic waste, could not be generally quantified with respect to the chosen life cycle assessment impact categories and were therefore not included in the model. These effects should be considered in conjunction with the results of the life cycle assessment
Antiferromagnetic noise correlations in optical lattices
We analyze how noise correlations probed by time-of-flight (TOF) experiments
reveal antiferromagnetic (AF) correlations of fermionic atoms in
two-dimensional (2D) and three-dimensional (3D) optical lattices. Combining
analytical and quantum Monte Carlo (QMC) calculations using experimentally
realistic parameters, we show that AF correlations can be detected for
temperatures above and below the critical temperature for AF ordering. It is
demonstrated that spin-resolved noise correlations yield important information
about the spin ordering. Finally, we show how to extract the spin correlation
length and the related critical exponent of the AF transition from the noise.Comment: 4 pages, 4 figure
Density wave instabilities of tilted fermionic dipoles in a multilayer geometry
We consider the density wave instability of fermionic dipoles aligned by an
external field, and moving in equidistant layers at zero temperature. Using a
conserving Hartree-Fock approximation, we show that correlations between
dipoles in different layers significantly decrease the critical coupling
strength for the formation of density waves when the distance between the
layers is comparable to the inter-particle distance within each layer. This
effect, which is strongest when the dipoles are oriented perpendicular to the
planes, causes the density waves in neighboring layers to be in-phase for all
orientations of the dipoles. We furthermore demonstrate that the effects of the
interlayer interaction can be understood from a classical model. Finally, we
show that the interlayer correlations are important for experimentally relevant
dipolar molecules, including the chemically stable NaK and
KCs, where the density wave regime is within experimental reach.Comment: 18 pages, 11 figures; new version with expanded discussion on
experimental relevance including one new figur
All the colours of the rainbow.
Our perception of colour has always been a source of fascination, so it's little wonder that studies of the phenomenon date back hundreds of years. What, though, can modern scientists learn from medieval literature — and how do we go about it
Enterococcus faecalis bacteremia: please do the echo
Infective endocarditis (IE) caused by Enterococcus faecalis (E. faecalis) is a disease of the elderly with an increasing incidence, often health-care associated and with in-hospital mortality rates around 10-20%. E. faecalis IE is notoriously challenging to diagnose due to unspecific symptoms, often presenting with a complex clinical picture with low-grade fever and only moderately elevated infectious parameters. In a newly published prospective multicenter study using echocardiography to screen E. faecalis bacteremia patients, we found an IE prevalence as high as 26%. The 344 included patients with E. faecalis bacteremia had a mean age of 74 (±12) years confirming that it is indeed a disease of the elderly. The key feature of the study was that echocardiography was performed in all patients including transesophageal echocardiography (TEE) in 74%. Transthoracic echocardiography (TTE) missed vegetations in half of the cases where TEE demonstrated vegetations, underlining the importance of TEE
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