972 research outputs found
TWO NEW SHORT-PERIOD CEPHEIDS
The General Catalogue of Variable Stars gives periods of slightly less than three-quarters of a day for the stars NO Cas and CN Tau. However, new photometry demonstrates that their periods are actually 2.6 and 1.8 days, respectively, and they are thus classical Cepheids. Fourier decompositions of their light curves are performed, and they are found to be members of a class of Cepheids with periods less than three days which may be related to the s-Cepheids. These two stars represent the shortest and longest known members of this class and thus are very useful in defining its properties in the Fourier diagrams
Ab initio Quantum and ab initio Molecular Dynamics of the Dissociative Adsorption of Hydrogen on Pd(100)
The dissociative adsorption of hydrogen on Pd(100) has been studied by ab
initio quantum dynamics and ab initio molecular dynamics calculations. Treating
all hydrogen degrees of freedom as dynamical coordinates implies a high
dimensionality and requires statistical averages over thousands of
trajectories. An efficient and accurate treatment of such extensive statistics
is achieved in two steps: In a first step we evaluate the ab initio potential
energy surface (PES) and determine an analytical representation. Then, in an
independent second step dynamical calculations are performed on the analytical
representation of the PES. Thus the dissociation dynamics is investigated
without any crucial assumption except for the Born-Oppenheimer approximation
which is anyhow employed when density-functional theory calculations are
performed. The ab initio molecular dynamics is compared to detailed quantum
dynamical calculations on exactly the same ab initio PES. The occurence of
quantum oscillations in the sticking probability as a function of kinetic
energy is addressed. They turn out to be very sensitive to the symmetry of the
initial conditions. At low kinetic energies sticking is dominated by the
steering effect which is illustrated using classical trajectories. The steering
effects depends on the kinetic energy, but not on the mass of the molecules.
Zero-point effects lead to strong differences between quantum and classical
calculations of the sticking probability. The dependence of the sticking
probability on the angle of incidence is analysed; it is found to be in good
agreement with experimental data. The results show that the determination of
the potential energy surface combined with high-dimensional dynamical
calculations, in which all relevant degrees of freedon are taken into account,
leads to a detailed understanding of the dissociation dynamics of hydrogen at a
transition metal surface.Comment: 15 pages, 9 figures, subm. to Phys. Rev.
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.
Macroscopic Quantum Tunneling of a Bose-Einstein Condensate with Attractive Interaction
A Bose-Einstein condensate with attractive interaction can be metastable if
it is spatially confined and if the number of condensate bosons is below
a certain critical value . By applying a variational method and the
instanton techinique to the Gross-Pitaevskii energy functional, we find
analytically the frequency of the collective excitation and the rate of
macroscopic quantum tunneling (MQT). We show that near the critical point the
tunneling exponent vanishes according to and that MQT
can be a dominant decay mechanism of the condensate for very close to
.Comment: RevTex 4 pages with 1 postscript figure. Accepted for publication in
Physical Review Letter
A Variational Sum-Rule Approach to Collective Excitations of a Trapped Bose-Einstein Condensate
It is found that combining an excitation-energy sum rule with Fetter's trial
wave function gives almost exact low-lying collective-mode frequencies of a
trapped Bose-Einstein condensate at zero temperature.Comment: 11 pages, 2 figures, Revte
Stability analysis of the D-dimensional nonlinear Schroedinger equation with trap and two- and three-body interactions
Considering the static solutions of the D-dimensional nonlinear Schroedinger
equation with trap and attractive two-body interactions, the existence of
stable solutions is limited to a maximum critical number of particles, when D
is greater or equal 2. In case D=2, we compare the variational approach with
the exact numerical calculations. We show that, the addition of a positive
three-body interaction allows stable solutions beyond the critical number. In
this case, we also introduce a dynamical analysis of the conditions for the
collapse.Comment: 6 pages, 7 figure
Semiclassical Solution of the Quantum Hydrodynamic Equation for Trapped Bose-condensed Gas in the l=0 Case
In this paper the quantum hydrodynamic equation describing the collective,
low energy excitations of a dilute atomic Bose gas in a given trapping
potential is investigated with the JWKB semiclassical method. In the case of
spherically symmetric harmonic confining potential a good agreement is shown
between the semiclassical and the exact energy eigenvalues as well as wave
functions. It is also demonstrated that for larger quantum numbers the
calculation of the semiclassical wave function is numerically more stable than
the exact polynomial with large alternating coefficients.Comment: 12 pages, 7 figure
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Intra-arterial Onyx Embolization of Vertebral Body Lesions
While Onyx embolization of cerebrospinal arteriovenous shunts is well-established, clinical researchers continue to broaden applications to other vascular lesions of the neuraxis. This report illustrates the application of Onyx (eV3, Plymouth, MN) embolization to vertebral body lesions, specifically, a vertebral hemangioma and renal cell carcinoma vertebral body metastatic lesion
Stability of a vortex in a small trapped Bose-Einstein condensate
A second-order expansion of the Gross-Pitaevskii equation in the interaction
parameter determines the thermodynamic critical angular velocity Omega_c for
the creation of a vortex in a small axisymmetric condensate. Similarly, a
second-order expansion of the Bogoliubov equations determines the (negative)
frequency omega_a of the anomalous mode. Although Omega_c = -omega_a through
first order, the second-order contributions ensure that the absolute value
|omega_a| is always smaller than the critical angular velocity Omega_c. With
increasing external rotation Omega, the dynamical instability of the condensate
with a vortex disappears at Omega*=|omega_a|, whereas the vortex state becomes
energetically stable at the larger value Omega_c. Both second-order
contributions depend explicitly on the axial anisotropy of the trap. The
appearance of a local minimum of the free energy for a vortex at the center
determines the metastable angular velocity Omega_m. A variational calculation
yields Omega_m=|\omega_a| to first order (hence Omega_m also coincides with the
critical angular velocity Omega_c to this order). Qualitatively, the scenario
for the onset of stability in the weak-coupling limit is the same as that found
in the strong-coupling (Thomas-Fermi) limit.Comment: 8 pages, RevTe
Mean-field analysis of collapsing and exploding Bose-Einstein condensates
The dynamics of collapsing and exploding trapped Bose-Einstein condensat es
caused by a sudden switch of interactions from repulsive to attractive a re
studied by numerically integrating the Gross-Pitaevskii equation with atomic
loss for an axially symmetric trap. We investigate the decay rate of
condensates and the phenomena of bursts and jets of atoms, and compare our
results with those of the experiments performed by E. A. Donley {\it et al.}
[Nature {\bf 412}, 295 (2001)]. Our study suggests that the condensate decay
and the burst production is due to local intermittent implosions in the
condensate, and that atomic clouds of bursts and jets are coherent. We also
predict nonlinear pattern formation caused by the density instability of
attractive condensates.Comment: 7 pages, 8 figures, axi-symmetric results are adde
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