118 research outputs found
Generation of linear waves in the flow of Bose-Einstein condensate past an obstacle
The theory of linear wave structures generated in Bose-Einstein condensate
flow past an obstacle is developed. The shape of wave crests and dependence of
amplitude on coordinates far enough from the obstacle are calculated. The
results are in good agreement with the results of numerical simulations
obtained earlier. The theory gives a qualitative description of experiments
with Bose-Einstein condensate flow past an obstacle after condensate's release
from a trap.Comment: 11 pages, 3 figures, to be published in Zh. Eksp. Teor. Fi
Three-dimensional vortex dynamics in Bose-Einstein condensates
We simulate in the mean-field limit the effects of rotationally stirring a
three-dimensional trapped Bose-Einstein condensate with a Gaussian laser beam.
A single vortex cycling regime is found for a range of trap geometries, and is
well described as coherent cycling between the ground and the first excited
vortex states. The critical angular speed of stirring for vortex formation is
quantitatively predicted by a simple model. We report preliminary results for
the collisions of vortex lines, in which sections may be exchanged.Comment: 4 pages, 4 figures, REVTeX 3.1; Submitted to Physical Review A (6
March 2000
Critical velocity in cylindrical Bose-Einstein condensates
We describe a dramatic decrease of the critical velocity in elongated
cylindrical Bose-Einstein condensates which originates from the non-uniform
character of the radial density profile. We discuss this mechanism with respect
to recent measurements at MIT.Comment: 3 pages, 2 eps figures, revised according to referee's comment
Breakdown of superfluidity of an atom laser past an obstacle
The 1D flow of a continuous beam of Bose-Einstein condensed atoms in the
presence of an obstacle is studied as a function of the beam velocity and of
the type of perturbing potential (representing the interaction of the obstacle
with the atoms of the beam). We identify the relevant regimes:
stationary/time-dependent and superfluid/dissipative; the absence of drag is
used as a criterion for superfluidity. There exists a critical velocity below
which the flow is superfluid. For attractive obstacles, we show that this
critical velocity can reach the value predicted by Landau's approach. For
penetrable obstacles, it is shown that superfluidity is recovered at large beam
velocity. Finally, enormous differences in drag occur when switching from
repulsive to attractive potential.Comment: 15 pages, 6 figure
Formation, dynamics and stability of coreless vortex dipoles in phase-separated binary condensates
We study the motion of the Gaussian obstacle potential created by blue
detuned laser beam through a phase-separated binary condensate in
pancake-shaped traps. For the velocity of the obstacle above a critical
velocity, we observe the generation of vortex dipoles in the outer component
which can penetrate the inner component. This is equivalent to finite, although
small, transport of outer component across the inner component. In the inner
component, the same method can lead to the formation of coreless vortex
dipoles.Comment: 12 pages, 9 figure
Observation of Superfluid Flow in a Bose-Einstein Condensed Gas
We have studied the hydrodynamic flow in a Bose-Einstein condensate stirred
by a macroscopic object, a blue detuned laser beam, using nondestructive {\em
in situ} phase contrast imaging. A critical velocity for the onset of a
pressure gradient has been observed, and shown to be density dependent. The
technique has been compared to a calorimetric method used previously to measure
the heating induced by the motion of the laser beam.Comment: 4 pages, 5 figure
Critical velocity for a toroidal Bose-Einstein condensate flowing through a barrier
We consider the setup employed in a recent experiment (Ramanathan et al 2011
Phys. Rev. Lett. 106 130401) devoted to the study of the instability of the
superfluid flow of a toroidal Bose-Einstein condensate in presence of a
repulsive optical barrier. Using the Gross-Pitaevskii mean-field equation, we
observe, consistently with what we found in Piazza et al (2009 Phys. Rev. A 80
021601), that the superflow with one unit of angular momentum becomes unstable
at a critical strength of the barrier, and decays through the mechanism of
phase slippage performed by pairs of vortex-antivortex lines annihilating.
While this picture qualitatively agrees with the experimental findings, the
measured critical barrier height is not very well reproduced by the
Gross-Pitaevskii equation, indicating that thermal fluctuations can play an
important role (Mathey et al 2012 arXiv:1207.0501). As an alternative
explanation of the discrepancy, we consider the effect of the finite resolution
of the imaging system. At the critical point, the superfluid velocity in the
vicinity of the obstacle is always of the order of the sound speed in that
region, . In particular, in the hydrodynamic regime
(not reached in the above experiment), the critical point is determined by
applying the Landau criterion inside the barrier region. On the other hand, the
Feynman critical velocity is much lower than the observed critical
velocity. We argue that this is a general feature of the Gross-Pitaevskii
equation, where we have with being a
small parameter of the model. Given these observations, the question still
remains open about the nature of the superfluid instability.Comment: Extended versio
Collective excitations of trapped Bose condensates in the energy and time domains
A time-dependent method for calculating the collective excitation frequencies
and densities of a trapped, inhomogeneous Bose-Einstein condensate with
circulation is presented. The results are compared with time-independent
solutions of the Bogoliubov-deGennes equations. The method is based on
time-dependent linear-response theory combined with spectral analysis of
moments of the excitation modes of interest. The technique is straightforward
to apply, is extremely efficient in our implementation with parallel FFT
methods, and produces highly accurate results. The method is suitable for
general trap geometries, condensate flows and condensates permeated with vortex
structures.Comment: 6 pages, 3 figures small typos fixe
Superfluid behaviour of a two-dimensional Bose gas
Two-dimensional (2D) systems play a special role in many-body physics.
Because of thermal fluctuations, they cannot undergo a conventional phase
transition associated to the breaking of a continuous symmetry. Nevertheless
they may exhibit a phase transition to a state with quasi-long range order via
the Berezinskii-Kosterlitz-Thouless (BKT) mechanism. A paradigm example is the
2D Bose fluid, such as a liquid helium film, which cannot Bose-condense at
non-zero temperature although it becomes superfluid above a critical phase
space density. Ultracold atomic gases constitute versatile systems in which the
2D quasi-long range coherence and the microscopic nature of the BKT transition
were recently explored. However, a direct observation of superfluidity in terms
of frictionless flow is still missing for these systems. Here we probe the
superfluidity of a 2D trapped Bose gas with a moving obstacle formed by a
micron-sized laser beam. We find a dramatic variation of the response of the
fluid, depending on its degree of degeneracy at the obstacle location. In
particular we do not observe any significant heating in the central, highly
degenerate region if the velocity of the obstacle is below a critical value.Comment: 5 pages, 3 figure
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