776 research outputs found
Stability and collapse of a coupled Bose-Einstein condensate
The dynamics of a coupled Bose-Einstein condensate involving trapped atoms in
two quantum states is studied using the time-dependent Gross-Pitaevskii
equation including an interaction which can transform atoms from one state to
the other. We find interesting oscillation of the number of atoms in each of
the states. For all repulsive interactions, stable condensates are formed. When
some of the atomic interactions are attractive, the possibility of collapse is
studied by including an absorptive contact interaction and a quartic three-body
recombination term. One or both components of the condensate may undergo
collapse when one or more of the nonlinear terms are attractive in nature.Comment: 14 Latex pages, 6 postscript figure
Two Epochs of Very Large Array Observations of Water Maser Emission in the active galaxy NGC 6240
Studies of 22GHz H2O maser emission from the merging galaxy NGC 6240 with
double nuclei are presented. Two epochs of Very Large Array (VLA) observations
in the A-configuration in spectral-line mode were carried out at 0.1 arcsec
resolution by covering the redshifted velocity range of ~ 300 km/s with respect
to the systemic velocity of the galaxy. The purpose of these new observations
is twofold: to detect an H2O maser that an earlier VLA observation pinpointed
in the southern nucleus in the northern nucleus as well to clarify the
kinematics of the double nuclei, and to understand the origin of the maser in
the galaxy. In the second epoch, one velocity feature peaking at Vlsr=7491.1
km/s, redshifted by ~200 km/s relative to the systemic velocity, was detected
only toward the southern nucleus. The detection of an H2O maser feature at or
near this velocity had never been reported in earlier observations. However,
including the known velocity features at redshifted velocities, no other
velocity features were observed toward either nuclei throughout these epochs.
The maser remains unresolved at an angular resolution of ~ 0".1, corresponding
to a linear size of less than about 45 pc. The two epochs of VLA observations
show that the maser intensity is variable on timescales of at least three
months, while the correlation between the maser intensity and the radio
continuum intensity is not certain from our data. It is plausible that the
maser in NGC 6240 is associated with the activity of an active galactic nucleus
in the southern nucleus. Alternatively, the maser can be explained by
starforming activity at the site of massive starformation in the galaxy.Comment: 6 Pages, 3 Figures, published in AJ, the journal reference adde
Morphology and kinematics of the ionised gas in early-type galaxies
We present results of our ongoing study of the morphology and kinematics of
the ionised gas in 48 representative nearby elliptical and lenticular galaxies
using the SAURON integral-field spectrograph on the 4.2m William Herschel
Telescope. Making use of a recently developed technique, emission is detected
in 75% of the galaxies. The ionised-gas distributions display varied
morphologies, ranging from regular gas disks to filamentary structures.
Additionally, the emission-line kinematic maps show, in general, regular
motions with smooth variations in kinematic position angle. In most of the
galaxies, the ionised-gas kinematics is decoupled from the stellar counterpart,
but only some of them present signatures of recent accretion of gaseous
material. The presence of dust is very common in our sample and is usually
accompanied by gas emission. Our analysis of the [OIII]/Hbeta emission-line
ratios, both across the whole sample as well as within the individual galaxies,
suggests that there is no unique mechanism triggering the ionisation of the
gas.Comment: 8 pages, 2 figures, submitted to "Adaptive Optics-Assisted
Integral-Field Spectroscopy", Rutten R.G.M., Benn C.R., Mendez J., eds., May
2005, La Palma (Spain), New Astr. Rev. For full resolution PS, see
http://www.strw.leidenuniv.nl/~jfalcon/JFB_AOmeeting_color_hires.ps.g
Minimal Length Uncertainty Relation and the Hydrogen Spectrum
Modifications of Heisenberg's uncertainty relations have been proposed in the
literature which imply a minimum position uncertainty. We study the low energy
effects of the new physics responsible for this by examining the consequent
change in the quantum mechanical commutation relations involving position and
momenta. In particular, the modifications to the spectrum of the hydrogen atom
can be naturally interpreted as a varying (with energy) fine structure
constant. From the data on the energy levels we attempt to constrain the scale
of the new physics and find that it must be close to or larger than the weak
scale. Experiments in the near future are expected to change this bound by at
least an additional order of magnitude.Comment: 8 pages, no figure. Corrected typos, added a reference with comment
Quantum tunneling across spin domains in a Bose-Einstein condensate
Quantum tunneling was observed in the decay of metastable spin domains in
gaseous Bose-Einstein condensates. A mean-field description of the tunneling
was developed and compared with measurement. The tunneling rates are a
sensitive probe of the boundary between spin domains, and indicate a spin
structure in the boundary between spin domains which is prohibited in the bulk
fluid. These experiments were performed with optically trapped F=1 spinor
Bose-Einstein condensates of sodium.Comment: 5 pages, 4 figure
Coupled Bose-Einstein condensate: Collapse for attractive interaction
We study the collapse in a coupled Bose-Einstein condensate of two types of
bosons 1 and 2 under the action of a trap using the time-dependent
Gross-Pitaevskii equation. The system may undergo collapse when one, two or
three of the scattering lengths for scattering of boson with ,
, are negative representing an attractive interaction. Depending
on the parameters of the problem a single or both components of the condensate
may experience collapse.Comment: 5 pages and 9 figures, small changes mad
Observation of metastable states in spinor Bose-Einstein condensates
Bose-Einstein condensates have been prepared in long-lived metastable excited
states. Two complementary types of metastable states were observed. The first
is due to the immiscibility of multiple components in the condensate, and the
second to local suppression of spin-relaxation collisions. Relaxation via
re-condensation of non-condensed atoms, spin relaxation, and quantum tunneling
was observed. These experiments were done with F=1 spinor Bose-Einstein
condensates of sodium confined in an optical dipole trap.Comment: 3 figures included in paper, fourth figure separat
Collective dynamics of internal states in a Bose gas
Theory for the Rabi and internal Josephson effects in an interacting Bose gas
in the cold collision regime is presented. By using microscopic transport
equation for the density matrix the problem is mapped onto a problem of
precession of two coupled classical spins. In the absence of an external
excitation field our results agree with the theory for the density induced
frequency shifts in atomic clocks. In the presence of the external field, the
internal Josephson effect takes place in a condensed Bose gas as well as in a
non-condensed gas. The crossover from Rabi oscillations to the Josephson
oscillations as a function of interaction strength is studied in detail.Comment: 18 pages, 2 figure
Characterization of the nuclear export adaptor protein Nmd3 in association with the 60S ribosomal subunit
3D reconstruction by cryo-EM provides the first structural description of a ribosomal biogenesis factor (Nmd3) in complex with the 60S ribosomal subunit
Numerical study of the coupled time-dependent Gross-Pitaevskii equation: Application to Bose-Einstein condensation
We present a numerical study of the coupled time-dependent Gross-Pitaevskii
equation, which describes the Bose-Einstein condensate of several types of
trapped bosons at ultralow temperature with both attractive and repulsive
interatomic interactions. The same approach is used to study both stationary
and time-evolution problems. We consider up to four types of atoms in the study
of stationary problems. We consider the time-evolution problems where the
frequencies of the traps or the atomic scattering lengths are suddenly changed
in a stable preformed condensate. We also study the effect of periodically
varying these frequencies or scattering lengths on a preformed condensate.
These changes introduce oscillations in the condensate which are studied in
detail. Good convergence is obtained in all cases studied.Comment: 9 pages, 10 figures, accepted in Physical Review
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