384 research outputs found
Operator-Algebraic Approach to the Yrast Spectrum of Weakly Interacting Trapped Bosons
We present an operator-algebraic approach to deriving the low-lying
quasi-degenerate spectrum of weakly interacting trapped N bosons with total
angular momentum \hbar L for the case of small L/N, demonstrating that the
lowest-lying excitation spectrum is given by 27 g n_3(n_3-1)/34, where g is the
strength of the repulsive contact interaction and n_3 the number of excited
octupole quanta. Our method provides constraints for these quasi-degenerate
many-body states and gives higher excitation energies that depend linearly on
N.Comment: 7 pages, one figur
Effect of anharmonicities in the critical number of trapped condensed atoms with attractive two-body interaction
We determine the quantitative effect, in the maximum number of particles and
other static observables, due to small anharmonic terms added to the confining
potential of an atomic condensed system with negative two-body interaction. As
an example of how a cubic or quartic anharmonic term can affect the maximum
number of particles, we consider the trap parameters and the results given by
Roberts et al. [Phys. Rev. Lett. 86, 4211 (2001)]. However, this study can be
easily transferred to other trap geometries to estimate anharmonic effects.Comment: Total of 5 pages, 3 figures and 1 table. To appear in Phys. Rev.
Bistability and macroscopic quantum coherence in a BEC of ^7Li
We consider a Bose-Einstein condensate (BEC) of in a situation where
the density undergoes a symmetry breaking in real space. This occurs for a
suitable number of condensed atoms in a double well potential, obtained by
adding a standing wave light field to the trap potential. Evidence of
bistability results from the solution of the Gross-Pitaevskii equation. By
second quantization, we show that the classical bistable situation is in fact a
Schr\"odinger cat (SC) and evaluate the tunneling rate between the two SC
states. The oscillation between the two states is called MQC (macroscopic
quantum coherence); we study the effects of losses on MQC.Comment: 8 pages, 11 figures. e-mail: [email protected]
Multi frequency evaporative cooling to BEC in a high magnetic field
We demonstrate a way to circumvent the interruption of evaporative cooling
observed at high bias field for Rb atoms trapped in the (F=2, m=+2)
ground state. Our scheme uses a 3-frequencies-RF-knife achieved by mixing two
RF frequencies. This compensates part of the non linearity of the Zeeman
effect, allowing us to achieve BEC where standard 1-frequency-RF-knife
evaporation method did not work. We are able to get efficient evaporative
cooling, provided that the residual detuning between the transition and the RF
frequencies in our scheme is smaller than the power broadening of the RF
transitions at the end of the evaporation ramp.Comment: 12 pages, 2 figure
Free expansion of Bose-Einstein condensates with quantized vortices
The expansion of Bose-Einstein condensates with quantized vortices is studied
by solving numerically the time-dependent Gross-Pitaevskii equation at zero
temperature. For a condensate initially trapped in a spherical harmonic
potential, we confirm previous results obtained by means of variational methods
showing that, after releasing the trap, the vortex core expands faster than the
radius of the atomic cloud. This could make the detection of vortices feasible,
by observing the depletion of the density along the axis of rotation. We find
that this effect is significantly enhanced in the case of anisotropic
disc-shaped traps. The results obtained as a function of the anisotropy of the
initial configuration are compared with the analytic solution for a
noninteracting gas in 3D as well as with the scaling law predicted for an
interacting gas in 2D.Comment: 5 pages, 6 postscript figure
Bose-Einstein condensation with magnetic dipole-dipole forces
Ground-state solutions in a dilute gas interacting via contact and magnetic
dipole-dipole forces are investigated. To the best of our knowledge, it is the
first example of studies of the Bose-Einstein condensation in a system with
realistic long-range interactions. We find that for the magnetic moment of e.g.
chromium and a typical value of the scattering length all solutions are stable
and only differ in size from condensates without long-range interactions. By
lowering the value of the scattering length we find a region of unstable
solutions. In the neighborhood of this region the ground state wavefunctions
show internal structures not seen before in condensates. Finally, we find an
analytic estimate for the characteristic length appearing in these solutions.Comment: final version, 4 pages, 4 figure
Loading a vapor cell magneto-optic trap using light-induced atom desorption
Low intensity white light was used to increase the loading rate of Rb
atoms into a vapor cell magneto-optic trap by inducing non-thermal desorption
of Rb atoms from the stainless steel walls of the vapor cell. An increased Rb
partial pressure reached a new equilibrium value in less than 10 seconds after
switching on the broadband light source. After the source was turned off, the
partial pressure returned to its previous value in times as short as 10
seconds.Comment: 7 pages, 6 figure
Evaporative cooling of trapped fermionic atoms
We propose an efficient mechanism for the evaporative cooling of trapped
fermions directly into quantum degeneracy. Our idea is based on an electric
field induced elastic interaction between trapped atoms in spin symmetric
states. We discuss some novel general features of fermionic evaporative cooling
and present numerical studies demonstrating the feasibility for the cooling of
alkali metal fermionic species Li, K, and Rb. We also
discuss the sympathetic cooling of fermionic hyperfine spin mixtures, including
the effects of anisotropic interactions.Comment: to be publishe
Instantons and radial excitations in attractive Bose-Einstein condensates
Imaginary- and real-time versions of an equation for the condensate density
are presented which describe dynamics and decay of any spherical Bose-Einstein
condensate (BEC) within the mean field appraoch. We obtain quantized energies
of collective finite amplitude radial oscillations and exact numerical
instanton solutions which describe quantum tunneling from both the metastable
and radially excited states of the BEC of 7Li atoms. The mass parameter for the
radial motion is found different from the gaussian value assumed hitherto, but
the effect of this difference on decay exponents is small. The collective
breathing states form slightly compressed harmonic spectrum, n=4 state lying
lower than the second Bogolyubov (small amplitude) mode. The decay of these
states, if excited, may simulate a shorter than true lifetime of the metastable
state. By scaling arguments, results extend to other attractive BEC-s.Comment: 6 pages, 3 figure
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.
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