475 research outputs found
Resonant Five-body Recombination in an Ultracold Gas of Bosonic Atoms
We combine theory and experiment to investigate five-body recombination in an
ultracold gas of atomic cesium at negative scattering length. A refined
theoretical model, in combination with extensive laboratory tunability of the
interatomic interactions, enables the five-body resonant recombination rate to
be calculated and measured. The position of the new observed recombination
feature agrees with a recent theoretical prediction and supports the prediction
of a family of universal cluster states at negative that are tied to an
Efimov trimer.Comment: 14 pages, 5 figure
BEC-BCS Crossover of a Trapped Two-Component Fermi Gas with Unequal Masses
We determine the energetically lowest lying states in the BEC-BCS crossover
regime of s-wave interacting two-component Fermi gases under harmonic
confinement by solving the many-body Schrodinger equation using two distinct
approaches. Essentially exact basis set expansion techniques are applied to
determine the energy spectrum of systems with N=4 fermions. Fixed-node
diffusion Monte Carlo methods are applied to systems with up to N=20 fermions,
and a discussion of different guiding functions used in the Monte Carlo
approach to impose the proper symmetry of the fermionic system is presented.
The energies are calculated as a function of the s-wave scattering length a_s
for N=2-20 fermions and different mass ratios \kappa of the two species. On the
BEC and BCS sides, our energies agree with analytically-determined first-order
correction terms. We extract the scattering length and the effective range of
the dimer-dimer system up to \kappa = 20. Our energies for the
strongly-interacting trapped system in the unitarity regime show no shell
structure, and are well described by a simple expression, whose functional form
can be derived using the local density approximation, with one or two
parameters. The universal parameter \xi for the trapped system for various
\kappa is determined, and comparisons with results for the homogeneous system
are presented.Comment: 11 pages, 6 figures, extended versio
Universality in Four-Boson Systems
We report recent advances on the study of universal weakly bound four-boson
states from the solutions of the Faddeev-Yakubovsky equations with zero-range
two-body interactions. In particular, we present the correlation between the
energies of successive tetramers between two neighbor Efimov trimers and
compare it to recent finite range potential model calculations. We provide
further results on the large momentum structure of the tetramer wave function,
where the four-body scale, introduced in the regularization procedure of the
bound state equations in momentum space, is clearly manifested. The results we
are presenting confirm a previous conjecture on a four-body scaling behavior,
which is independent of the three-body one. We show that the correlation
between the positions of two successive resonant four-boson recombination peaks
are consistent with recent data, as well as with recent calculations close to
the unitary limit. Systematic deviations suggest the relevance of range
corrections.Comment: Accepted for publication in special issue of Few-Body Systems devoted
to the Sixth Workshop on the Critical Stability of Quantum Few-Body Systems,
October 2011, Erice, Sicily, Ital
Stability of Inhomogeneous Multi-Component Fermi Gases
Two-component equal-mass Fermi gases, in which unlike atoms interact through
a short-range two-body potential and like atoms do not interact, are stable
even when the interspecies s-wave scattering length becomes infinitely large.
Solving the many-body Schroedinger equation within a hyperspherical framework
and by Monte Carlo techniques, this paper investigates how the properties of
trapped two-component gases change if a third or fourth component are added. If
all interspecies scattering lengths are equal and negative, our calculations
suggest that both three- and four-component Fermi gases become unstable for a
certain critical set of parameters. The relevant length scale associated with
the collapse is set by the interspecies scattering length and we argue that the
collapse is, similar to the collapse of an attractive trapped Bose gas, a
many-body phenomenon. Furthermore, we consider a three-component Fermi gas in
which two interspecies scattering lengths are negative while the other
interspecies scattering length is zero. In this case, the stability of the
Fermi system is predicted to depend appreciably on the range of the underlying
two-body potential. We find parameter combinations for which the system appears
to become unstable for a finite negative scattering length and parameter
combinations for which the system appears to be made up of weakly-bound trimers
that consist of one fermion of each species.Comment: 15 pages, 8 figure
Weakly Bound Cluster States of Efimov Character
We study the behavior of weakly bound clusters and their relation to the
well-known three-body Efimov states. We adopt a model to describe universal
behavior of strongly interacting bosonic systems, and we test its validity by
reproducing predictions of three- and four-body universal states. Then, we
extend our study to larger systems and identify a series of universal cluster
states that can be qualitatively interpreted as adding one particle at a time
to an Efimov trimer. The properties of these cluster states and their
experimental signatures are discussed.Comment: 9 pages, 3 figures, final versio
A quantum many-body spin system in an optical lattice clock
Strongly interacting quantum many-body systems arise in many areas of physics, but their
complexity generally precludes exact solutions to their dynamics. We explored a strongly
interacting two-level system formed by the clock states in ^(87)Sr as a laboratory for the study of
quantum many-body effects. Our collective spin measurements reveal signatures of the
development of many-body correlations during the dynamical evolution. We derived a many-body
Hamiltonian that describes the experimental observation of atomic spin coherence decay,
density-dependent frequency shifts, severely distorted lineshapes, and correlated spin noise. These
investigations open the door to further explorations of quantum many-body effects and
entanglement through use of highly coherent and precisely controlled optical lattice clocks
Energetics and Structural Properties of Trapped Two-Component Fermi Gases
Using two different numerical methods, we study the behavior of two-component
Fermi gases interacting through short-range s-wave interactions in a harmonic
trap. A correlated Gaussian basis-set expansion technique is used to determine
the energies and structural properties, i.e., the radial one-body densities and
pair distribution functions, for small systems with either even or odd , as
functions of the s-wave scattering length and the mass ratio of the
two species. Particular emphasis is put on a discussion of the angular momentum
of the system in the BEC-BCS crossover regime. At unitarity, the excitation
spectrum of the four-particle system with total angular momentum L=0 is
calculated as a function of the mass ratio . The results are analyzed
from a hyperspherical perspective, which offers new insights into the problem.
Additionally, fixed-node diffusion Monte Carlo calculations are performed for
equal-mass Fermi gases with up to N=30 atoms. We focus on the odd-even
oscillations of the ground state energy of the equal-mass unitary system having
up to N=30 particles, which are related to the excitation gap of the system.
Furthermore, we present a detailed analysis of the structural properties of
these systems.Comment: 22 pages, 21 figure
Spectral Energy Distributions of starburst galaxies in the 900-1200 A range
We present the 970-1175 A spectral energy distributions (SEDs) of 12
starburst galaxies observed with the Far Ultraviolet Spectroscopic Explorer
FUSE. We take benefit of the high spectral resolution of FUSE to estimate a
continuum as much as possible unaffected by the interstellar lines. The
continuum is rather flat with, in few cases, a decrease at lambda <~1050 A, the
amplitude of which being correlated with various indicators of the dust
extinction. The far-UV SEDs are compared with synthetic population models. The
galaxies with almost no extinction have a SED consistent with an on-going star
formation over some Myrs. We derive a mean dust attenuation law in the
wavelength range 965-1140 A by comparing the SED of obscured galaxies to an
empirical dust-free SED. The extinction is nearly constant longward of 1040 A
but rises at shorter wavelengths. We compare our results with other studies of
the extinction for galaxies and stars in this wavelength range.Comment: 11 pages, 6 postscript figures, accepted for publication in Astronomy
& Astrophysic
Salmonella Transiently Reside in Luminal Neutrophils in the Inflamed Gut
Enteric pathogens need to grow efficiently in the gut lumen in order to cause disease and ensure transmission. The interior of the gut forms a complex environment comprising the mucosal surface area and the inner gut lumen with epithelial cell debris and food particles. Recruitment of neutrophils to the intestinal lumen is a hallmark of non-typhoidal Salmonella enterica infections in humans. Here, we analyzed the interaction of gut luminal neutrophils with S. enterica serovar Typhimurium (S. Tm) in a mouse colitis model.Upon S. Tm(wt) infection, neutrophils transmigrate across the mucosa into the intestinal lumen. We detected a majority of pathogens associated with luminal neutrophils 20 hours after infection. Neutrophils are viable and actively engulf S. Tm, as demonstrated by live microscopy. Using S. Tm mutant strains defective in tissue invasion we show that pathogens are mostly taken up in the gut lumen at the epithelial barrier by luminal neutrophils. In these luminal neutrophils, S. Tm induces expression of genes typically required for its intracellular lifestyle such as siderophore production iroBCDE and the Salmonella pathogenicity island 2 encoded type three secretion system (TTSS-2). This shows that S. Tm at least transiently survives and responds to engulfment by gut luminal neutrophils. Gentamicin protection experiments suggest that the life-span of luminal neutrophils is limited and that S. Tm is subsequently released into the gut lumen. This "fast cycling" through the intracellular compartment of gut luminal neutrophils would explain the high fraction of TTSS-2 and iroBCDE expressing intra- and extracellular bacteria in the lumen of the infected gut.
In conclusion, live neutrophils recruited during acute S. Tm colitis engulf pathogens in the gut lumen and may thus actively engage in shaping the environment of pathogens and commensals in the inflamed gut
A general theoretical description of N-body recombination
A formula for the cross section and event rate constant describing
recombination of N particles are derived in terms of general S-matrix elements.
Our result immediately yields the generalized Wigner threshold scaling for the
recombination of N bosons. A semi-analytic formula encapsulates the overall
scaling with energy and scattering length, as well as resonant modifications by
the presence of borromean N-body states near the threshold collision energy in
the entrance channel. We then apply our model to the case of four-boson
recombination into an Efimov trimer and a free atom.Comment: 4 pages, 2 figures RevTe
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