2,645 research outputs found
Optical detection of a BCS phase transition in a trapped gas of fermionic atoms
Light scattering from a spin-polarized degenerate Fermi gas of trapped
ultracold Li-6 atoms is studied. We find that the scattered light contains
information which directly reflects the quantum pair correlation due to the
formation of atomic Cooper pairs resulting from a BCS phase transition to a
superfluid state. Evidence for pairing can be observed in both the space and
time domains.Comment: 8 pages, 4 figures, revte
Growth and Collapse of a Bose Condensate with Attractive Interactions
We consider the dynamics of a quantum degenerate trapped gas of Li-7 atoms.
Because the atoms have a negative s-wave scattering length, a Bose condensate
of Li-7 becomes mechanically unstable when the number of condensate atoms
approaches a maximum value. We calculate the dynamics of the collapse that
occurs when the unstable point is reached. In addition, we use the quantum
Boltzmann equation to investigate the nonequilibrium kinetics of the atomic
distribution during and after evaporative cooling. The condensate is found to
undergo many cycles of growth and collapse before a stationary state is
reached.Comment: Four pages of ReVTeX with four postscript figure
Direct observation of growth and collapse of a Bose-Einstein condensate with attractive interactions
The dynamical behavior of Bose-Einstein condensation (BEC) in a gas with
attractive interactions is striking. Quantum theory predicts that BEC of a
spatially homogeneous gas with attractive interactions is precluded by a
conventional phase transition into either a liquid or solid. When confined to a
trap, however, such a condensate can form provided that its occupation number
does not exceed a limiting value. The stability limit is determined by a
balance between self-attraction and a repulsion arising from position-momentum
uncertainty under conditions of spatial confinement. Near the stability limit,
self-attraction can overwhelm the repulsion, causing the condensate to
collapse. Growth of the condensate, therefore, is punctuated by intermittent
collapses, which are triggered either by macroscopic quantum tunneling or
thermal fluctuation. Previous observation of growth and collapse has been
hampered by the stochastic nature of these mechanisms. Here we reduce the
stochasticity by controlling the initial number of condensate atoms using a
two-photon transition to a diatomic molecular state. This enables us to obtain
the first direct observation of the growth of a condensate with attractive
interactions and its subsequent collapse.Comment: 10 PDF pages, 5 figures (2 color), 19 references, to appear in Nature
Dec. 7 200
The Superfluid State of Atomic Li6 in a Magnetic Trap
We report on a study of the superfluid state of spin-polarized atomic Li6
confined in a magnetic trap. Density profiles of this degenerate Fermi gas, and
the spatial distribution of the BCS order parameter are calculated in the local
density approximation. The critical temperature is determined as a function of
the number of particles in the trap. Furthermore we consider the mechanical
stability of an interacting two-component Fermi gas, both in the case of
attractive and repulsive interatomic interactions. For spin-polarized Li6 we
also calculate the decay rate of the gas, and show that within the mechanically
stable regime of phase space, the lifetime is long enough to perform
experiments on the gas below and above the critical temperature if a bias
magnetic field of about 5 T is applied. Moreover, we propose that a measurement
of the decay rate of the system might signal the presence of the superfluid
state.Comment: 16 pages Revtex including 10 figures, submitted to Phys. Rev.
The Distribution of Dark Matter in a Ringed Galaxy
Outer rings are located at the greatest distance from the galaxy center of
any feature resonant with a bar. Because of their large scale, their morphology
is sensitive to the distribution of the dark matter in the galaxy. We introduce
here how study of these rings can constrain the mass-to-light ratio of the bar,
and so the percentage of dark matter in the center of these galaxies. We
compare periodic orbits integrated in the ringed galaxy NGC 6782 near the outer
Lindblad resonance to the shape of the outer ring. The non-axisymmetric
component of the potential resulting from the bar is derived from a
near-infrared image of the galaxy. The axisymmetric component is derived
assuming a flat rotation curve. We find that the pinched non-self-intersecting
periodic orbits are more elongated for higher bar mass-to-light ratios and
faster bars. The inferred mass-to-light ratio of the bar depends on the assumed
inclination of the galaxy. With an assumed galaxy inclination of i=41 degrees,
for the orbits to be consistent with the observed ring morphology the
mass-to-light ratio of the bar must be high, greater than 70% of a maximal disk
value. For i=45 degrees, the mass-to-light ratio of the bar is of
the maximal disk value. Since the velocity field of these rings can be used to
constrain the galaxy inclination as well as which periodic orbit is represented
in the ring, further study will yield tighter constraints on the mass-to-light
ratio of the bar. If a near maximal disk value for the bar is required, then
either there would be little dark matter within the bar, or the dark matter
contained in the disk of the galaxy would be non-axisymmetric and would rotate
with the bar.Comment: AAS Latex + jpg Figures, Accepted for publication in Ap
Mass Density Profiles of LSB Galaxies
We derive the mass density profiles of dark matter halos that are implied by
high spatial resolution rotation curves of low surface brightness galaxies. We
find that at small radii, the mass density distribution is dominated by a
nearly constant density core with a core radius of a few kpc. For rho(r) ~ r^a,
the distribution of inner slopes a is strongly peaked around a = -0.2. This is
significantly shallower than the cuspy a < -1 halos found in CDM simulations.
While the observed distribution of alpha does have a tail towards such extreme
values, the derived value of alpha is found to depend on the spatial resolution
of the rotation curves: a ~ -1 is found only for the least well resolved
galaxies. Even for these galaxies, our data are also consistent with constant
density cores (a = 0) of modest (~ 1 kpc) core radius, which can give the
illusion of steep cusps when insufficiently resolved. Consequently, there is no
clear evidence for a cuspy halo in any of the low surface brightness galaxies
observed.Comment: To be published in ApJ Letters. 6 pages. Uses aastex and
emulateapj5.sty Typo in Eq 1 fixe
Stabilizing an Attractive Bose-Einstein Condensate by Driving a Surface Collective Mode
Bose-Einstein condensates of Li have been limited in number due to
attractive interatomic interactions. Beyond this number, the condensate
undergoes collective collapse. We study theoretically the effect of driving
low-lying collective modes of the condensate by a weak asymmetric sinusoidally
time-dependent field. We find that driving the radial breathing mode further
destabilizes the condensate, while excitation of the quadrupolar surface mode
causes the condensate to become more stable by imparting quasi-angular momentum
to it. We show that a significantly larger number of atoms may occupy the
condensate, which can then be sustained almost indefinitely. All effects are
predicted to be clearly visible in experiments and efforts are under way for
their experimental realization.Comment: 4 ReVTeX pages + 2 postscript figure
A Neutral Hydrogen Survey of Polar-Ring Galaxies: I. Green Bank Observations of the Northern Sample
We present the results of a neutral hydrogen survey conducted with the Green
Bank 140-foot radio telescope of 47 northern objects in the polar-ring galaxy
atlas of Whitmore \etal\ (1990). We detected 39 of these above our detection
limit of 1.7 \hbox{Jy\CDOT\KMS}; the average measured flux of 21 Jy\CDOT\KMS\
corresponds to an average neutral hydrogen mass of \MSUN
for a Hubble constant of \KMS \ Mpc. For the polar-ring
galaxies in our sample that have also been observed with radio arrays, we find
that the 21\AMIN\ (FWHM) Green Bank beam often includes much more flux than
found by the synthesis instruments for the polar rings alone; some of these
galaxies are known to have gas-rich companions. We compare the neutral hydrogen
content of the sample to the blue luminosity and IRAS fluxes. The \HI-to-blue-
light ratios of the confirmed and probable polar rings are around unity in
solar units, indicating that polar ring galaxies (or their environments) are as
gas-rich as typical irregular galaxies. For their blue luminosity, the
confirmed polar rings are underluminous in the far-infrared, as compared with
the rest of the sample. They are also FIR-underluminous for their \HI\ masses,
which suggests that most of the gas in the ring may be in stable orbits, rather
than flowing inward to trigger star formation in the central galaxy. The more
disordered class of `related objects,' which includes a number of obvious
mergers, is highly luminous in the far-infrared.Comment: 34 pages, LaTeX file, Institute for Advanced Study number AST 93/4
Self-Trapping, Quantum Tunneling and Decay Rates for a Bose Gas with Attractive Nonlocal Interaction
We study the Bose-Einstein condensation for a cloud of Li atoms with
attractive nonlocal (finite-range) interaction in a harmonic trap. In addition
to the low-density metastable branch, that is present also in the case of local
interaction, a new stable branch appears at higher densities. For a large
number of atoms, the size of the cloud in the stable high-density branch is
independent of the trap size and the atoms are in a macroscopic quantum
self-trapped configuration. We analyze the macroscopic quantum tunneling
between the low-density metastable branch and the high-density one by using the
istanton technique. Moreover we consider the decay rate of the Bose condensate
due to inelastic two- and three-body collisions.Comment: 5 pages, 4 figures, submitted to Phys. Rev.
Lupus-TR-3b: A Low-Mass Transiting Hot Jupiter in the Galactic Plane?
We present a strong case for a transiting Hot Jupiter planet identified
during a single-field transit survey towards the Lupus Galactic plane. The
object, Lupus-TR-3b, transits a V=17.4 K1V host star every 3.91405d.
Spectroscopy and stellar colors indicate a host star with effective temperature
5000 +/- 150K, with a stellar mass and radius of 0.87 +/- 0.04M_sun and 0.82
+/- 0.05R_sun, respectively. Limb-darkened transit fitting yields a companion
radius of 0.89 +/- 0.07R_J and an orbital inclination of 88.3 +1.3/-0.8 deg.
Magellan 6.5m MIKE radial velocity measurements reveal a 2.4 sigma K=114 +/-
25m/s sinusoidal variation in phase with the transit ephemeris. The resulting
mass is 0.81 +/- 0.18M_J and density 1.4 +/- 0.4g/cm^3. Y-band PANIC image
deconvolution reveal a V>=21 red neighbor 0.4'' away which, although highly
unlikely, we cannot conclusively rule out as a blended binary with current
data. However, blend simulations show that only the most unusual binary system
can reproduce our observations. This object is very likely a planet, detected
from a highly efficient observational strategy. Lupus-TR-3b constitutes the
faintest ground-based detection to date, and one of the lowest mass Hot
Jupiters known.Comment: 4 pages, 4 figures, accepted for publication in ApJ
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