164 research outputs found
Vocal communication in gibbons
Many non-human primates use vocal communication referentially and also use simple syntax and grammar. However, their comparative vocal repertoires are disappointingly sparse, with many researchers concluding that they have fixed vocal patterns made up of a limited number of discrete units used in a relatively small array of contexts (see McComb & Semple, 2005 for a review). Furthermore, these vocal patterns seem to be innate, under high genetic control with little evidence for vocal learning – something that humans are masters at (Janik & Slater 1997). This leaves us with some questions. Firstly, how did humans become so adept at producing and learning vocal sounds? And, secondly, are there any extant primate species with vocal behaviours that can be directly compared to our own?
Dynamic depletion in a Bose condensate via a sudden increase of the scattering length
We examine the time-dependent quantum depletion of a trapped Bose condensate
arising from a rapid increase of the scattering length. Our solution indicates
that a significant buildup of incoherent atoms can occur within a
characteristic time short compared with the harmonic trap period. We discuss
how the depletion density and the characteristic time depend on the physical
parameters of the condensate
Characterization of elastic scattering near a Feshbach resonance in rubidium 87
The s-wave scattering length for elastic collisions between 87Rb atoms in the
state |f,m_f>=|1,1> is measured in the vicinity of a Feshbach resonance near
1007 G. Experimentally, the scattering length is determined from the mean-field
driven expansion of a Bose-Einstein condensate in a homogeneous magnetic field.
The scattering length is measured as a function of the magnetic field and
agrees with the theoretical expectation. The position and the width of the
resonance are determined to be 1007.40 G and 0.20 G, respectively.Comment: 4 pages, 2 figures minor revisions: added Ref.6, included error bar
Rearranging Edgeworth-Cornish-Fisher Expansions
This paper applies a regularization procedure called increasing rearrangement
to monotonize Edgeworth and Cornish-Fisher expansions and any other related
approximations of distribution and quantile functions of sample statistics.
Besides satisfying the logical monotonicity, required of distribution and
quantile functions, the procedure often delivers strikingly better
approximations to the distribution and quantile functions of the sample mean
than the original Edgeworth-Cornish-Fisher expansions.Comment: 17 pages, 3 figure
Landau-Khalatnikov two-fluid hydrodynamics of a trapped Bose gas
Starting from the quantum kinetic equation for the non-condensate atoms and
the generalized Gross-Pitaevskii equation for the condensate, we derive the
two-fluid hydrodynamic equations of a trapped Bose gas at finite temperatures.
We follow the standard Chapman-Enskog procedure, starting from a solution of
the kinetic equation corresponding to the complete local equilibrium between
the condensate and the non-condensate components. Our hydrodynamic equations
are shown to reduce to a form identical to the well-known Landau-Khalatnikov
two-fluid equations, with hydrodynamic damping due to the deviation from local
equilibrium. The deviation from local equilibrium within the thermal cloud
gives rise to dissipation associated with shear viscosity and thermal
conduction. In addition, we show that effects due to the deviation from the
diffusive local equilibrium between the condensate and the non-condensate
(recently considered by Zaremba, Nikuni and Griffin) can be described by four
frequency-dependent second viscosity transport coefficients. We also derive
explicit formulas for all the transport coefficients. These results are used to
introduce two new characteristic relaxation times associated with hydrodynamic
damping. These relaxation times give the rate at which local equilibrium is
reached and hence determine whether one is in the two-fluid hydrodynamic
region.Comment: 26 pages, 3 postscript figures, submitted to PR
Very high precision bound state spectroscopy near a Rb Feshbach resonance
We precisely measured the binding energy of a molecular state near the
Feshbach resonance in a Rb Bose-Einstein condensate (BEC). Rapid
magnetic field pulses induced coherent atom-molecule oscillations in the BEC.
We measured the oscillation frequency as a function of B-field and fit the data
to a coupled-channels model. Our analysis constrained the Feshbach resonance
position [155.041(18) G], width [10.71(2) G], and background scattering length
[-443(3) a] and yielded new values for , , and . These
results improved our estimate for the stability condition of an attractive BEC.
We also found evidence for a mean-field shift to the binding energy.Comment: 5 pages, 2 figures, submitted to PR
Properties of a Dilute Bose Gas near a Feshbach Resonance
In this paper, properties of a homogeneous Bose gas with a Feshbach resonance
are studied in the dilute region at zero temperature. The stationary state
contains condensations of atoms and molecules. The ratio of the molecule
density to the atom density is . There are two types of excitations,
molecular excitations and atomic excitations. Atomic excitations are gapless,
consistent with the traditional theory of a dilute Bose gas. The molecular
excitation energy is finite in the long wavelength limit as observed in recent
experiments on Rb. In addition, the decay process of the condensate is
studied. The coefficient of the three-body recombination rate is about 140
times larger than that of a Bose gas without a Feshbach resonance, in
reasonably good agreement with the experiment on Na.Comment: 11 pages, 1 figure, comparison between the calculated three-body
recombination rate and the experimental data for Na system has been adde
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
Collapse dynamics of trapped Bose-Einstein condensates
We analyze the implosion and subsequent explosion of a trapped condensate
after the scattering length is switched to a negative value. Our results
compare very well qualitatively and fairly well quantitatively with the results
of recent experiments at JILA.Comment: 4 pages, 3 figure
Quantum corrections to the dynamics of interacting bosons: beyond the truncated Wigner approximation
We develop a consistent perturbation theory in quantum fluctuations around
the classical evolution of a system of interacting bosons. The zero order
approximation gives the classical Gross-Pitaevskii equations. In the next order
we recover the truncated Wigner approximation, where the evolution is still
classical but the initial conditions are distributed according to the Wigner
transform of the initial density matrix. Further corrections can be
characterized as quantum scattering events, which appear in the form of a
nonlinear response of the observable to an infinitesimal displacement of the
field along its classical evolution. At the end of the paper we give a few
numerical examples to test the formalism.Comment: published versio
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