89,261 research outputs found
Many-body theory of excitation dynamics in an ultracold Rydberg gas
We develop a theoretical approach for the dynamics of Rydberg excitations in
ultracold gases, with a realistically large number of atoms. We rely on the
reduction of the single-atom Bloch equations to rate equations, which is
possible under various experimentally relevant conditions. Here, we explicitly
refer to a two-step excitation-scheme. We discuss the conditions under which
our approach is valid by comparing the results with the solution of the exact
quantum master equation for two interacting atoms. Concerning the emergence of
an excitation blockade in a Rydberg gas, our results are in qualitative
agreement with experiment. Possible sources of quantitative discrepancy are
carefully examined. Based on the two-step excitation scheme, we predict the
occurrence of an antiblockade effect and propose possible ways to detect this
excitation enhancement experimentally in an optical lattice as well as in the
gas phase.Comment: 12 pages, 8 figure
Collapse and revival of excitations in Bose-Einstein condensates
We study the energies and decay of elementary excitations in weakly
interacting Bose-Einstein condensates within a finite-temperature gapless
second-order theory. The energy shifts for the high-lying collective modes turn
out to be systematically negative compared with the
Hartree-Fock-Bogoliubov-Popov approximation and the decay of the low-lying
modes is found to exhibit collapse and revival effects. In addition,
perturbation theory is used to qualitatively explain the experimentally
observed Beliaev decay process of the scissors mode.Comment: 9 pages, 5 figure
Controlled quantum stirring of Bose-Einstein condensates
By cyclic adiabatic change of two control parameters of an optical trap one
can induce a circulating current of condensed bosons. The amount of particles
that are transported per period depends on the "radius" of the cycle, and this
dependence can be utilized in order to probe the interatomic interactions. For
strong repulsive interaction the current can be regarded as arising from a
sequence of Landau-Zener crossings. For weaker interaction one observes either
gradual or coherent mega crossings, while for attractive interaction the
particles are glued together and behave like a classical ball. For the analysis
we use the Kubo approach to quantum pumping with the associated Dirac monopoles
picture of parameter space.Comment: 12 pages, 8 figure
Lick Slit Spectra of Thirty-Eight Objective Prism QSO Candidates and Low Metallicity Halo Stars
We present Lick Observatory slit spectra of 38 objects which were claimed to
have pronounced ultraviolet excess and emission lines by Zhan \& Chen. Most of
our spectra have FWHM spectral resolutions of about 4~\AA , and relatively high
S/N of about 10 -- 50, although some have FWHM ~\AA ~or lower S/N.
We find eleven QSOs, four galaxies at , twenty-two stars and one
unidentified object with a low S/N spectrum. Six of the QSOs show absorption
systems, including Q0000+027A with a relatively strong associated C~IV
absorption system, and Q0008+008 (V) with a damped Ly
system with an H~I column density of cm. The stars include a
wide variety of spectral types. There is one new DA4 white dwarf at 170~pc, one
sdB at 14~kpc, and three M stars. The rest are of types F, G and K. We have
measured the equivalent widths of the Ca~II~K line, the G-band and the Balmer
lines in ten stars with the best spectra, and we derive metallicities. Seven of
them are in the range ~[Fe/H]~, while the others are less
metal poor. If the stars are dwarfs, then they are at distances of 1 to 7~kpc,
but if they are giants, typical distances will be about 10~kpc.Comment: (Plain Tex, 21 pages, including tables. Send email to
'travell_oir%[email protected]' for 12 pages of figures) To appear in the
%%Astronomical Journal, August, 199
Low Frequency VLA Observations of Abell 754: Evidence for a Cluster Radio Halo and Possible Radio Relics
We present 74 MHz and 330 MHz VLA observations of Abell 754. Diffuse,
halo-like emission is detected from the center of the cluster at both
frequencies. At 330 MHz the resolution of 90'' distinguishes this extended
emission from previously known point sources. In addition to the halo and at a
much lower level, outlying steep-spectrum emission regions straddle the cluster
center and are seen only at 74 MHz. The location, morphology and spectrum of
this emission are all highly suggestive of at least one, and possibly two
cluster radio relics. Easily obtained higher resolution, higher sensitivity VLA
observations at both frequencies are required to confirm the extended nature of
the halo-like emission and the 74 MHz relic detections. However, since there is
prior evidence that this cluster is or has recently been in the process of a
major merger event, the possible discovery of relics in this system is of great
interest in light of recent observational and theoretical evidence in favor of
a merger-relic connection. We discuss the possible role the merger shock waves,
which are seen in the X-ray emission, may have played in the formation of the
halo and radio relics in A754.Comment: 15 pages including 4 figures. Accepted for publication by Ap
Model Independent Tests of Skyrmions and Their Holographic Cousins
We describe a new exact relation for large QCD for the long-distance
behavior of baryon form factors in the chiral limit. This model-independent
relation is used to test the consistency of the structure of several baryon
models. All 4D semiclassical chiral soliton models satisfy the relation, as
does the Pomarol-Wulzer holographic model of baryons as 5D Skyrmions. However,
remarkably, we find that the holographic model treating baryons as instantons
in the Sakai-Sugimoto model does not satisfy the relation.Comment: v2. Added references, corrected typo
Spectroscopic measurements of temperature and plasma impurity concentration during magnetic reconnection at the Swarthmore Spheromak Experiment
Electron temperature measurements during counterhelicity spheromak merging studies at the Swarthmore Spheromak Experiment (SSX) [M. R. Brown, Phys. Plasmas 6, 1717 (1999)] are presented. VUV monochromator measurements of impurity emission lines are compared with model spectra produced by the non-LTE excitation kinematics code PRISMSPECT [J. J. MacFarlane et al., in Proceedings of the Third Conference on Inertial Fusion Science and Applications (2004)] to yield the electron temperature in the plasma with 1 ”s time resolution. Average T_e is seen to increase from 12 to 19 eV during spheromak merging. Average C III ion temperature, measured with a new ion Doppler spectrometer (IDS) [C. D. Cothran et al., Rev. Sci. Instrum. 77, 063504 (2006)], likewise rises during spheromak merging, peaking at ~22 eV, but a similar increase in T_i is seen during single spheromak discharges with no merging. The VUV emission line measurements are also used to constrain the concentrations of various impurities in the SSX plasma, which are dominated by carbon, but include some oxygen and nitrogen. A burst of soft x-ray emission is seen during reconnection with a new four-channel detector (SXR). There is evidence for spectral changes in the soft x-ray emission as reconnection progresses, although our single-temperature equilibrium spectral models are not able to provide adequate fits to all the SXR data
Proteoglycan neofunctions: regulation of inflammation and autophagy in cancer biology.
Inflammation and autophagy have emerged as prominent issues in the context of proteoglycan signaling. In particular, two small, leucine-rich proteoglycans, biglycan and decorin, play pivotal roles in the regulation of these vital cellular pathways and, as such, are intrinsically involved in cancer initiation and progression. In this minireview, we will address novel functions of biglycan and decorin in inflammation and autophagy, and analyze new emerging signaling events triggered by these proteoglycans, which directly or indirectly modulate these processes. We will critically discuss the dual role of proteoglycan-driven inflammation and autophagy in tumor biology, and delineate the potential mechanisms through which soluble extracellular matrix constituents affect the microenvironment associated with inflammatory and neoplastic diseases
Sheath ionization model of beam emissions from large spacecraft
An analytical model of the charging of a spacecraft emitting electron and ion beams has been applied to the case of large spacecraft. In this model, ionization occurs in the sheath due to the return current. Charge neutralization of spherical space charge flow is examined by solving analytical equations numerically. Parametric studies of potential large spacecraft are performed. As in the case of small spacecraft, the ions created in the sheath by the returning current play a large role in determining spacecraft potential
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