19 research outputs found
How to observe dipolar effects in spinor Bose-Einstein condensates
We study a spinor condensate of alkali atoms in F = 1 hyperfine state under
the presence of an oscillating magnetic field. We find resonances which, due to
the dipolar interactions, magnify the transfer of atoms from mF = 1 to mF = 0
Zeeman sublevel. These resonances occur at magnetic fields of the order of
milligaus and are broad enough to enable observation of the famous Einstein-de
Haas effect, which is solely a dipolar effect, in systems of cold alkali gases.Comment: 4 pages, 3 figure
Coherence properties of spinor condensates at finite temperatures
We consider a spinor condensate of 87Rb atoms in its F=1 hyperfine state at
finite temperatures. Putting initially all atoms in m_F=0 component we find
that the system evolves into the state of thermal equilibrium. This state is
approached in a step-like process and when established it manifests itself in
distinguishable ways. The atoms in states m_F=+1 and m_F=-1 start to rotate in
opposite directions breaking the chiral symmetry and showing highly regular
spin textures. Also the coherence properties of the system changes
dramatically. Depending on the strength of spin-changing collisions the system
first enters the stage where the m_F=+1 and m_F=-1 spinor condensate components
periodically loose and recover their mutual coherence whereas their thermal
counterparts get completely dephased. For stronger spin changing collisions the
system enters the regime where also the strong coherence between other
components is built up.Comment: 5 pages, 4 figure
The role of thermal fluctuations in sound propagation in a two-dimensional Fermi gas
We numerically study the transport properties of a two-dimensional Fermi gas
in a weakly and strongly interacting regimes, in the range of temperatures
close to the transition to a superfluid phase. For that we excite sound waves
in a fermionic mixture by using the phase imprinting technique, follow their
evolution, and finally determine both their speed and attenuation. Our
formalism incorporates thermal fluctuations via the ground canonical ensemble
description and with the help of Metropolis algoritm. From numerical
simulations we extract temperature dependence of the sound velocity and
diffusivity as well as the dependence on the interaction strength. We emphasize
the role of virtual vortex-antivortex pairs creation in the process of sound
dissipation
Resonant Einstein-de Haas effect in a rubidium condensate
We numerically investigate a condensate of Rb atoms in an F=1
hyperfine state confined in an optical dipole trap. Assuming the magnetic
moments of all atoms are initially aligned along the magnetic field we observe,
after the field's direction is reversed, a transfer of atoms to other Zeeman
states. Such transfer is allowed by the dipolar interaction which couples the
spin and the orbital degrees of freedom. Therefore, the atoms in
states acquire an orbital angular momentum and start to circulate around the
center of the trap. This is a realization of the Einstein-de Haas effect in
systems of cold gases. We find resonances which amplify this phenomenon making
it observable even in very weak dipolar systems. The resonances occur when the
Zeeman energy on transfer of atoms to state is fully converted to the
rotational kinetic energy.Comment: 4 pages, 5 figure
Formation of soliton trains in Bose-Einstein condensates by temporal Talbot effect
We study the recent observation of formation of matter-wave soliton trains in
Bose-Einstein condensates. We emphasize the role of the box-like confinement of
the Bose-Einstein condensate and find that there exist time intervals for the
opening the box that support the generation of real solitons. When the box-like
potential is switched off outside the existing time windows, the number of
peaks in a train changes resembling missing solitons observed in the
experiment. Our findings indicate that a new way of generating soliton trains
in condensates through the temporal, matter-wave Talbot effect is possible.Comment: 4 pages, 4 figures, new result
Free expansion of a Bose-Einstein condensate at the presence of a thermal cloud
We investigate numerically the free-fall expansion of a Rb atoms
condensate at nonzero temperatures. The classical field approximation is used
to separate the condensate and the thermal cloud during the expansion. We
calculate the radial and axial widths of the expanding condensate and find
clear evidence that the thermal component changes the dynamics of the
condensate. Our results are confronted against the experimental data
Thermally induced instability of a doubly quantized vortex in a Bose-Einstein condensate
We study the instability of a doubly quantized vortex topologically imprinted
on Na condensate, as reported in recent experiment [Phys. Rev. Lett.
\textbf{93}, 160406 (2004)]. We have performed numerical simulations using
three-dimensional Gross-Pitaevskii equation with classical thermal noise.
Splitting of a doubly quantized vortex turns out to be a process that is very
sensitive to the presence of thermal atoms. We observe that even ve ry small
thermal fluctuations, corresponding to 10 to 15% of thermal atoms, ca use the
decay of doubly quantized vortex into two singly quantized vortices in tens of
milliseconds. As in the experiment, the lifetime of doubly quantized vortex i s
a monotonic function of the interaction strength.Comment: 4 pages, 5 figure