82 research outputs found
Quantum Vortex in a Vectorial Bose-Einstein Condensate
Quantum vortices in the multi-component Bose-Einstein condensation (BEC) are
investigated theoretically. It is found that three kinds of the vortex
configurations are possible and their physical properties are discussed in
details, including the density distribution and the spin texture. By using the
Bogoliubov theory extended to the three component BEC, the collective modes for
these vortices are evaluated. The local vortex stability for these vortices are
examined in light of the existence of the negative eigenvalue, yielding a
narrow magnetization window for the local intrinsic stable region where the
multi-components work together to stabilize a vortex in a self-organized way.Comment: 8 pages, 14 eps figure
Spin textures in slowly rotating Bose-Einstein Condensates
Slowly rotating spin-1 Bose-Einstein condensates are studied through a
variational approach based upon lowest Landau level calculus. The author finds
that in a gas with ferromagnetic interactions, such as Rb, angular
momentum is predominantly carried by clusters of two different types of
skyrmion textures in the spin-vector order parameter. Conversely, in a gas with
antiferromagnetic interactions, such as Na, angular momentum is carried
by -disclinations in the nematic order parameter which arises from spin
fluctuations. For experimentally relevant parameters, the cores of these
-disclinations are ferromagnetic, and can be imaged with polarized light.Comment: 14 pages, 12 low resolution bitmapped figures, RevTeX4. High
resolution figures available from author. Suplementary movies available from
autho
Excitation spectrum in a cylindrical Bose-Einstein gas
Whole excitation spectrum is calculated within the Popov approximation of the
Bogoliubov theory for a cylindrical symmetric Bose-Einstein gas trapped
radially by a harmonic potential. The full dispersion relation and its
temperature dependence of the zero sound mode propagating along the axial
direction are evaluated in a self-consistent manner. The sound velocity is
shown to depend not only on the peak density, but also on the axial area
density. Recent sound velocity experiment on Na atom gas is discussed in light
of the present theory.Comment: 4 pages, 5 eps figure
Collective Oscillations of Vortex Lattices in Rotating Bose-Einstein Condensates
The complete low-energy collective-excitation spectrum of vortex lattices is
discussed for rotating Bose-Einstein condensates (BEC) by solving the
Bogoliubov-de Gennes (BdG) equation, yielding, e.g., the Tkachenko mode
recently observed at JILA. The totally symmetric subset of these modes includes
the transverse shear, common longitudinal, and differential longitudinal modes.
We also solve the time-dependent Gross-Pitaevskii (TDGP) equation to simulate
the actual JILA experiment, obtaining the Tkachenko mode and identifying a pair
of breathing modes. Combining both the BdG and TDGP approaches allows one to
unambiguously identify every observed mode.Comment: 5 pages, 4 figure
Structure and Stability of Vortices in Dilute Bose-Einstein Condensates at Ultralow Temperatures
We compute the structure of a quantized vortex line in a harmonically trapped
dilute atomic Bose-Einstein condensate using the Popov version of the
Hartree-Fock-Bogoliubov mean-field theory. The vortex is shown to be
(meta)stable in a nonrotating trap even in the zero-temperature limit, thus
confirming that weak particle interactions induce the condensed gas a
fundamental property characterizing ``classical'' superfluids. We present the
structure of the vortex at ultralow temperatures and discuss the crucial effect
of the thermal gas component to its energetic stability.Comment: 4 pages, 4 figures, accepted for publication in Phys. Rev. Let
Mermin-Ho vortex in ferromagnetic spinor Bose-Einstein condensates
The Mermin-Ho and Anderson-Toulouse coreless non-singular vortices are
demonstrated to be thermodynamically stable in ferromagnetic spinor
Bose-Einstein condensates with the hyperfine state F=1. The phase diagram is
established in a plane of the rotation drive vs the total magnetization by
comparing the energies for other competing non-axis-symmetric or singular
vortices. Their stability is also checked by evaluating collective modes.Comment: 4 pages, 4 figure
Observation of spinor dynamics in optically trapped 87Rb Bose-Einstein Condensates
We measure spin mixing of F=1 and F=2 spinor condensates of 87Rb atoms
confined in an optical trap. We determine the spin mixing time to be typically
less than 600 ms and observe spin population oscillations. The equilibrium spin
configuration in the F=1 manifold is measured for different magnetic fields and
found to show ferromagnetic behavior for low field gradients. An F=2 condensate
is created by microwave excitation from F=1 manifold, and this spin-2
condensate is observed to decay exponentially with time constant 250 ms.
Despite the short lifetime in the F=2 manifold, spin mixing of the condensate
is observed within 50 ms.Comment: 4 pages, 6 figure
Stability of vortices in rotating taps: a 3d analysis
We study the stability of vortex-lines in trapped dilute gases subject to
rotation. We solve numerically both the Gross-Pitaevskii and the Bogoliubov
equations for a 3d condensate in spherically and cilyndrically symmetric
stationary traps, from small to very large nonlinearities. In the stationary
case it is found that the vortex states with unit and charge are
energetically unstable. In the rotating trap it is found that this energetic
instability may only be suppressed for the vortex-line, and that the
multicharged vortices are never a local minimum of the energy functional, which
implies that the absolute minimum of the energy is not an eigenstate of the
operator, when the angular speed is above a certain value, .Comment: 10 pages, 7 figures in EPS forma
Double Phase Transitions in Magnetized Spinor Bose-Einstein Condensation
It is investigated theoretically that magnetized Bose-Einstein condensation
(BEC) with the internal (spin) degrees of freedom exhibits a rich variety of
phase transitions, depending on the sign of the interaction in the spin
channel. In the antiferromagnetic interaction case there exist always double
BEC transitions from single component BEC to multiple component BEC. In the
ferromagnetic case BEC becomes always unstable at a lower temperature, leading
to a phase separation. The detailed phase diagram for the temperature vs the
polarization, the spatial spin structure, the distribution of non-condensates
and the excitation spectrum are examined for the harmonically trapped systems.Comment: 6 pages, 7 figures. Submitted to J. Phys. Soc. Jp
Vortex stabilization in Bose-Einstein condensate of alkali atom gas
A quantized vortex in the Bose-Einstein condensation (BEC), which is known to
be unstable intrinsically, is demonstrated theoretically to be stabilized by
the finite temperature effect. The mean-field calculation of Popov
approximation within the Bogoliubov theory is employed, giving rise to a
self-consistent solution for BEC confined by a harmonic potential. Physical
origin of this vortex stabilization is investigated. An equivalent effect is
also proved to be induced by an additional pinning potential at the vortex
center produced by a focused laser beam even at the lowest temperature. The
self-consistent solutions give detailed properties of a stable vortex, such as
the spatial profiles of the condensate and non-condensate, the particle current
density around the core, the whole excitation spectrum, and their temperature
dependences.Comment: 11 pages, 17 eps figure
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