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Vortex signatures in annular Bose-Einstein condensates

M. Cozzini, B. Jackson, S. Stringari

Published Dec 31, 2006• 0 citations
Publisher

Abstract

We consider a Bose-Einstein condensate confined in a 'Mexican hat' or sombrero potential, with a quartic minus quadratic radial dependence. We find conditions under which the ground state is annular in shape, with a hole in the center of the condensate. Rotation leads to the appearance of stable multiply quantized vortices, giving rise to a superfluid flow around the ring. The collective modes of the system are explored both numerically and analytically using the Gross-Pitaevskii and hydrodynamic equations. Potential experimental schemes to detect vorticity are proposed and evaluated, which include measuring the splitting of collective-mode frequencies, observing expansion following release from the trap, and probing the momentum distribution of the condensate. © 2006 The American Physical Society.

DOI
10.25405/ncl.34627677

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Vortex signatures in annular Bose-Einstein condensates

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We consider a Bose-Einstein condensate confined in a 'Mexican hat' or sombrero potential, with a quartic minus quadratic radial dependence. We find conditions under which the ground state is annular in shape, with a hole in the center of the condensate. Rotation leads to the appearance of stable multiply quantized vortices, giving rise to a superfluid flow around the ring. The collective modes of the system are explored both numerically and analytically using the Gross-Pitaevskii and hydrodynamic equations. Potential experimental schemes to detect vorticity are proposed and evaluated, which include measuring the splitting of collective-mode frequencies, observing expansion following release from the trap, and probing the momentum distribution of the condensate.

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arXiv:cond-mat/0505078•Physical Review A•Aug 29, 2005•38 citations
We consider a Bose-Einstein condensate confined in a 'Mexican hat' or sombrero potential, with a quartic minus quadratic radial dependence. We find conditions under which the ground state is annular in shape, with a hole in the center of the condensate. Rotation leads to the appearance of stable multiply quantized vortices, giving rise to a superfluid flow around the ring. The collective modes of the system are explored both numerically and analytically using the Gross-Pitaevskii and hydrodynamic equations. Potential experimental schemes to detect vorticity are proposed and evaluated, which include measuring the splitting of collective-mode frequencies, observing expansion following release from the trap, and probing the momentum distribution of the condensate. © 2006 The American Physical Society.
We consider a Bose-Einstein condensate confined in a 'Mexican hat' or sombrero potential, with a quartic minus quadratic radial dependence. We find conditions under which the ground state is annular in shape, with a hole in the center of the condensate. Rotation leads to the appearance of stable multiply quantized vortices, giving rise to a superfluid flow around the ring. The collective modes of the system are explored both numerically and analytically using the Gross-Pitaevskii and hydrodynamic equations. Potential experimental schemes to detect vorticity are proposed and evaluated, which include measuring the splitting of collective-mode frequencies, observing expansion following release from the trap, and probing the momentum distribution of the condensate.
We consider a Bose-Einstein condensate confined in a ``Mexican hat'' or sombrero potential, with a quartic minus quadratic radial dependence. We find conditions under which the ground state is annular in shape, with a hole in the center of the condensate. Rotation leads to the appearance of stable multiply quantized vortices, giving rise to a superfluid flow around the ring. The collective modes of the system are explored both numerically and analytically using the Gross-Pitaevskii and hydrodynamic equations. Potential experimental schemes to detect vorticity are proposed and evaluated, which include measuring the splitting of collective-mode frequencies, observing expansion following release from the trap, and probing the momentum distribution of the condensate.
The irrotational nature of superfluids leads to spectacular rotational properties. For the fluid to rotate, its density must locally vanish at a singular point called a quantum vortex. The annular geometry is of great interest for studying superfluidity as the gas can rotate in this geometry around a central hole without requiring any singularity, allowing the existence of persistent currents along the ring with a quantized circulation. In out experiment, we confine cold atoms dressed by a radiofrequency field in an adiabatic potential based on a quadrupolar magnetic trap. The resulting bubble-shaped potential, both very smooth and easily tunable, allows us to produce two types of ring-shaped Bose-Einstein condensates. A first strategy consists in adding a far-detuned light sheet to confine the atoms at the intersection between the bubble and the horizontal plane imposed by the light field – i.e. a ring. We present the implementation and optimization of this trap and demonstrate the possibility to prepare and observe superfluid currents in the ring. A second path exploits the centrifugal force and the anharmonicity of the adiabatic potential to create an effective Mexican hat potential by rotating the trapped atoms at the bottom of the bubble faster than the trap frequency. After having realized such a system, we probe its quadrupolar modes to characterize the rotation. The study of vortex distribution in the rotating gas also shows a thermal melting effect of the finite temperature vortex lattice.
We studied a rotating Bose–Einstein condensate confined in ring trap configurations that can be produced starting with a bubble trap confinement, approximated by a Mexican hat and shift harmonic oscillator potentials. Using a variational technique and perturbation theory, we determined the vortex configurations in this system by varying the interparticle interaction and the angular velocity of the atomic cloud. We found that the phase diagram of the system has macrovortex structures for small positive values of the interaction parameter, and the charge of the central vortex increases with rotation. Strengthening the atomic interaction makes the macrovortex unstable, and it decays into multiple singly charged vortices that arrange themselves in a lattice configuration. We also look for experimentally realizable methods to determine the vortex configuration without relying upon absorption imaging since the structures are not always visible in the latter. More specifically, we study how the vortex distribution affects the collective modes of the condensate by solving the Gross–Pitaevskii equation numerically and by analytical predictions using the sum-rule approach for the frequencies of the modes. These results reveal important signatures to characterize the macrovortices and vortex lattice transitions in the experiments.
We study a rotating Bose-Einstein Condensate (BEC) trapped in a shifted harmonic oscillator and Mexican Hat trap, which approximates the potential of a bubble trap potential in some situations.We determined the vortices configurations that appears in this system by varying the interaction and angular momentum of the atomic cloud.The system phase diagram has the macrovortex configurations for small values of the interaction parameter.The charge of the central vortex increases with increasing rotation.Higher values of the atomic interaction turns the macrovortex unstable.It decays into multiple single-charged vortices that arrange themselves in a lattice.Since the vortices are not always visible in the absorption images, we look for alternative methods to characterize the vortex configuration.More specifically, we search for experimental signatures to determine the transitions in the phase diagram.For that, we study how the BEC's velocity field affects the collective modes of the condensate.The splitting of the modes degenerescence and the increase in the mode frequency with angular velocity, which can be traced to a reduction in the gas compressibility, can be seen as an indirect signature of the presence of the vortices in the trapped BEC.
A two-dimensional rapidly rotating Bose-Einstein condensate in a harmonic plus quartic trap is expected to have unusual vortex states that do not occur in a pure harmonic trap. At a critical rotation speed ${\ensuremath{\Omega}}_{h}$, a central hole appears in the condensate, and at some faster rotation speed ${\ensuremath{\Omega}}_{g}$, the system undergoes a transition to a giant vortex state with pure irrotational flow. Using a time-dependent variational analysis, we study the behavior of an annular condensate with a single concentric ring of vortices. The transition to a giant vortex state is investigated by comparing the energy of the two equilibrium states (the ring of vortices and the giant vortex) and also by studying the dynamical stability of small excitation modes of the ring of vortices.