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Vortex Ring Dynamics in the Unitary Fermi Gas

Michelle M. Kelley, Michael McNeil Forbes, Aurel Bulgac

Published Jan 1, 2013• 0 citations
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Abstract

A recent article, Yefsah et al., Nature 499, 426 (2013) [1], reported an unusual excitation in a harmonically trapped unitary Fermi gas. The experiment imprints a domain wall in the trap resulting in oscillations that have periods an order of magnitude longer than the trapping period. The authors interpretted the results as ‘heavy solitons,’ but vortex rings provide a more natural explanation of the observed periods. This paper reports the results of simulations of a vortex ring in a harmonic trap to mimic the experiment, using the density functional theory, specically the extended Thomas-Fermi Gross-Pitaevskii model. Vortex rings in harmonic traps experience long oscillatory periods, comparable to the oscillations found in the experiment. It was found that the oscillatory period of the vortex ring increases with the aspect ratio of the trap, and decreases for oscillations with smaller amplitudes of oscillation. Additionally, oscillations experienced anti-damping when the system was not allowed to cool completely, matching well with the experiment’s observed anti-damped oscillations.

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A recent article, Yefsah et al., Nature 499, 426 (2013) [1], reported an unusual excitation in a harmonically trapped unitary Fermi gas. The experiment imprints a domain wall in the trap resulting in oscillations that have periods an order of magnitude longer than the trapping period. The authors interpretted the results as ‘heavy solitons,’ but vortex rings provide a more natural explanation of the observed periods. This paper reports the results of simulations of a vortex ring in a harmonic trap to mimic the experiment, using the density functional theory, specically the extended Thomas-Fermi Gross-Pitaevskii model. Vortex rings in harmonic traps experience long oscillatory periods, comparable to the oscillations found in the experiment. It was found that the oscillatory period of the vortex ring increases with the aspect ratio of the trap, and decreases for oscillations with smaller amplitudes of oscillation. Additionally, oscillations experienced anti-damping when the system was not allowed to cool completely, matching well with the experiment’s observed anti-damped oscillations.
In a recent article, Yefsah et al. [Nature (London) 499, 426 (2013)] report the observation of an unusual excitation in an elongated harmonically trapped unitary Fermi gas. After phase imprinting a domain wall, they observe oscillations almost an order of magnitude slower than predicted by any theory of domain walls which they interpret as a "heavy soliton" of inertial mass some 200 times larger than the free fermion mass or 50 times larger than expected for a domain wall. We present compelling evidence that this "soliton" is instead a quantized vortex ring, by showing that the main aspects of the experiment can be naturally explained within the framework of time-dependent superfluid density functional theories.
We use the time-dependent Gross-Pitaevskii equation to study the motion of a vortex ring produced by phase imprinting on an elongated cloud of cold atoms. Our approach models the experiments of Yefsah et al. [Nature (London) 499, 426 (2013)] on 6{}^{6}6Li in the Bose-Einstein-condensate regime where the fermions are tightly bound into bosonic dimers. We find ring oscillation periods which are much larger than the period of the axial harmonic trap. Our results lend further strength to Bulgac et al.'s arguments (arXiv:1306.4266) that the ``heavy solitons'' seen in those experiments are actually vortex rings. We numerically calculate the periods of oscillation for the vortex rings as a function of interaction strength, trap aspect ratio, and minimum vortex ring radius. In the presence of axial anisotropies the rings undergo complicated internal dynamics where they break into sets of vortex lines, then later combine into rings. These structures oscillate with a similar frequency to simple axially symmetric rings.
We present an investigation of generation, dynamics, and stability of dark solitons in anisotropic Fermi gases for a range of particle numbers and trap aspect ratios within the framework of the order-parameter equation. We calculate the periods of dark solitons oscillating in a trap and find good agreement with the results based on the Bogoliubov--de Gennes equations. By studying the stability of initially off-center dark solitons under various tight transverse confinements in the unitarity limit, we not only give the criterion of dynamical stability, but also find that the soliton and a hybrid of solitons and vortex rings can be characterized by different oscillation periods. The stability criterion is not fulfilled by the parameters of the recent experiment of Yefsah et al. [Nature (London) 499, 426 (2013)]. Therefore, instead of a very slow oscillation as observed experimentally, we find that the created dark soliton undergoes a transverse snake instability with a collapse into vortex rings, which propagate in solitonlike manner with a nearly 2 times larger period.
We present a theoretical study of solitonlike solutions and their dynamics of ultracold superfluid Fermi gases trapped in a time-dependent three-dimensional (3D) harmonic potential with gain or loss. The 3D analytical solitonlike solutions are obtained without introducing any additional integrability constraints used elsewhere. The propagation of both bright- and dark-soliton-like solutions is investigated. We show that the amplitudes of dark-soliton-like solutions exhibit periodic oscillation, whereas those of the bright-soliton-like ones do not show such behavior. Moreover, we highlight that the oscillation periods of dark-soliton-like solutions predicted by our approach are matched very well with those observed in a recent experiment carried out by Yefsah et al. [T. Yefsah, A. T. Sommer, M. J. H. Ku, L. W. Cheuk, W. Ji, W. S. Bakr, and M. W. Zwierlein, Nature (London) 499, 426 (2013)NATUAS0028-083610.1038/nature12338] in both Bose-Einstein condensation and unitarity regimes.
We calculate the frequencies of the Tkachenko oscillations of a vortex lattice in a harmonically trapped superfluid Fermi gas. We use the elastohydrodynamic theory and properly account for the elastic constants, the Thomas-Fermi density profile of the atomic cloud, and the boundary conditions. Thanks to the Fermi pressure, which is responsible for larger cloud radii with respect to the case of dilute Bose-Einstein condensed gases, large vortex lattices are achievable in the unitary limit of infinite scattering length, even at relatively small angular velocities. This opens the possibility of experimentally observing vortex oscillations in the regime where the dispersion relation approaches the Tkachenko law for incompressible fluids and the mode frequency is almost comparable to the trapping frequencies.
We present numerical simulations of phase imprinting experiments in ultracold trapped Fermi gases, which were obtained independently and are in good agreement with recent experimental results. Our focus is on the sequence and evolution of defects using the fermionic time-dependent Ginzburg-Landau equation, which contains dissipation necessary for equilibration. In contrast to other simulations, we introduce small, experimentally unavoidable symmetry breaking, particularly that associated with thermal fluctuations and with the phase-imprinting tilt angle, and we illustrate their dramatic effects. As appears consistent with experiment, the former causes vortex rings in confined geometries to move to the trap surface and rapidly decay into more stable vortex lines. The latter aligns the precessing and relatively long-lived vortex filaments, rendering them difficult to distinguish from solitons.