Life cycle of superfluid vortices and quantum turbulence in the unitary Fermi gas

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Life cycle of superfluid vortices and quantum turbulence in the unitary Fermi gas

Gabriel Wlazłowski, Aurel Bulgac, Michael McNeil Forbes, and Kenneth J. Roche

Phys. Rev. A 91, 031602(R) – Published 12 March 2015

Abstract

The unitary Fermi gas (UFG) offers a unique opportunity to study quantum turbulence both experimentally and theoretically in a strongly interacting fermionic superfluid with the highest vortex line density of any known superfluid. It yields to accurate and controlled experiments and admits the only dynamical microscopic description via time-dependent density-functional theory, apart from dilute bosonic gases, of the crossing and reconnection of superfluid vortex lines conjectured by Feynman [R. P. Feynman, Prog. Low Temp. Phys. 1, 17 (1955)] to be at the origin of quantum turbulence in superfluids at zero temperature. We demonstrate how various vortex configurations can be generated by using well-established experimental techniques: laser stirring and phase imprinting. New imaging techniques demonstrated by Ku et al. [M. J. H. Ku et al., Phys. Rev. Lett. 113, 065301 (2014)] should be able to directly visualize these crossings and reconnections in greater detail than performed so far in liquid helium. We demonstrate the critical role played by the geometry of the trap in the formation and dynamics of a vortex in the UFG and how laser stirring and phase imprint can be used to create vortex tangles with clear signatures of the onset of quantum turbulence.

Received 8 April 2014
Revised 18 July 2014

DOI:https://doi.org/10.1103/PhysRevA.91.031602

Authors & Affiliations

Gabriel Wlazłowski^1,2,*, Aurel Bulgac^2,†, Michael McNeil Forbes^2,3,4,‡, and Kenneth J. Roche^2,5,§

¹Faculty of Physics, Warsaw University of Technology, Ulica Koszykowa 75, 00-662 Warsaw, Poland
²Department of Physics, University of Washington, Seattle, Washington 98195-1560, USA
³Department of Physics & Astronomy, Washington State University, Pullman, Washington 99164-2814, USA
⁴Institute for Nuclear Theory, University of Washington, Seattle, Washington 98195-1550, USA
⁵Pacific Northwest National Laboratory, Richland, Washington 99352, USA

^*gabrielw@if.pw.edu.pl
^†bulgac@uw.edu
^‡mforbes@alum.mit.edu
^§k8r@u.washington.edu

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Issue

Vol. 91, Iss. 3 — March 2015

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Images

Figure 1
(Video available via Ref. [7].) Demonstration of off-axis vortices in the UFG generated by firing a bullet along an axially symmetric trap in a movie from the simulations included with Ref. [6]. Off-axis vortex rings attach to the walls and turn into vortex lines that cross and reconnect. The movie also demonstrates that vortex rings and lines move at similar velocities.
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Figure 2
(Video available via Ref. [7].) Effects of moderate anisotropy $δ = 1 - ω_{y} / ω_{x} \approx 10 %$ (the trap is taller than it is wide). The ring hits the narrow walls on the side, forming two parallel vortices. Without any anharmonicity $C = 0$ (top), these undergo several reconnection, oscillating between a horizontal and a vertical orientation. The presence of an anharmonicity $C / C_{0} \approx 3 %$ breaks the symmetry, eventually expelling one vortex from the system, resulting in a long-lived vortex that oscillates back and forth.
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Figure 3
(Video available via Ref. [7].) Effects of weak anisotropy. The ring now exists long enough to return as a small ring. Without an anharmonicity (top) several oscillations occur before the ring expands and hits the walls forming two parallel vortices. Adding a weak anharmonicity (bottom) causes the small ring to rotate. Once twisted off axis, this ring rapidly converts to a single vortex as shown in [8]. The rapid production of a single vortex from a twisted defect was also described in [17] and provides a convenient way of constructing vortices.
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Figure 4
(Video available via Ref. [7].) The trap on the top is wider than it is tall, the energetically favored configuration would be a vertical vortex, but the initial conditions lead to a slightly more energetic vortex aligned along the longer of the two axes. On the bottom, we imprint an oblique vortex line in a trap with the same asymmetry as the experiment. As it oscillates long the trap, the vortex alignment oscillates about the favored horizontal position, but without significant damping, and continues past this alignment to another oblique position. The presence of significant damping, such as through phonons at finite $T$ , might allow this to relax to a horizontal alignment. However, this relaxation would likely be comparable to the time it takes the vortex to oscillate out of the system; the true minimum energy state has no vortex at all.
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Figure 5
(Video available via Ref. [7].) Generation of quantum turbulence by phase imprint of the vortex lattice. In the left column consecutive frames show (a) a vortex lattice with a knife edge dividing the cloud, (b) just after phase imprint removal of the knife, and (c)–(e) decay of turbulent motion. In the right column we show the corresponding PDFs for longitudinal $v_{| |}$ and transverse $v_{⊥}$ components of collective velocity. Dotted lines show the Gaussian best fit to the data.
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