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arXiv:1205.5670 · International Journal of Modern Physics B

GENERATION OF VORTEX DIPOLES IN SUPERFLUID FERMI GAS IN BCS LIMIT

Sandeep Gautam

Published Apr 11, 2013• 1 citation
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Abstract

We theoretically investigate the generation of the vortex dipoles in superfluid Fermi gas in the BCS limit. The vortex dipoles are generated in superfluid either by moving an obstacle above a critical speed or due to the decay of the shock waves obtained on the sudden mixing of two superfluid fragments. We observe that in pancake-shaped traps, the shock waves can lead to the formation of density ripples, which decay into vortex dipoles due to the onset of snake instability.

DOI
10.1142/s0217979213500410

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We theoretically investigate the generation of the vortex dipoles in superfluid Fermi gas in the BCS limit. The vortex dipoles are generated in superfluid either by moving an obstacle above a critical speed or due to the decay of the shock waves obtained on the sudden mixing of two superfluid fragments. We observe that in pancake-shaped traps, the shock waves can lead to the formation of density ripples, which decay into vortex dipoles due to the onset of snake instability.
Vortex dipoles are generated when an obstacle moves through a superfluid above a critical speed. In the case of phase-separated binary condensates, with appropriate interaction parameters in pancake-shaped traps, we show that coreless vortex dipoles are created when a Gaussian obstacle beam traverses across them above a critical speed. As the obstacle passes through the inner component, it carries along a droplet of the outer component. Using the Thomas–Fermi approximation, we show that phase-separated binary condensates can either support vortices with empty or filled cores.
Abstract We study the formation and dynamics of shock waves initiated by a repulsive potential in a superfluid unitary Fermi gas by using the order-parameter equation. In the theoretical framework, the regularization process of shock waves mediated by the quantum pressure term is purely dispersive. Our results show good agreement with the experiment of Joseph et al (2011 Phys. Rev. Lett. 106 150401 ). We reveal that the boxlike-shaped density peak observed in the experiment consists of many vortex rings due to the transverse instability of the dispersive shock wave. In addition, we study the transition from a sound wave to subsonic shock waves as the strength of the repulsive potential increases and show a strong qualitative change in the propagation speed of the wavefronts. For a relatively small strength of the repulsive potential, the propagation speed decreases below the sound speed with the increase of the strength as a scaling behavior. For a large strength where the shock waves are formed by colliding two spatially separated clouds, the speed is still smaller than the sound speed, but remains almost unchanged as the strength increases, which can be interpreted as the same expansion speed of the proliferation of the vortex rings originated from the transverse instability.
Dark solitons in superfluid Bose gases decay through the snake instability mechanism, unless they are strongly confined. Recent experiments in superfluid Fermi gases have also interpreted soliton decay via this mechanism. However, we show using both an effective field numerical simulation and a perturbative analysis that there is a qualitative difference between soliton decay in the BEC and BCS regimes. On the BEC side of the interaction domain, the characteristic snaking deformations are induced by fluctuations of the amplitude of the order parameter, while on the BCS side, fluctuations of the phase destroy the soliton core through the formation of local Josephson currents without the presence of snaking deformations. The latter mechanism is qualitatively different from the snaking instability and this difference should be experimentally detectable.
Vortex dipoles are generated when an obstacle moves through a superfluid. In case of phase-separated binary condensates, with appropriate interaction parameters in pan-cake shaped traps, we show that coreless vortex dipoles are created when a Gaussian obstacle beam traverses across them above a critical speed. As the obstacle passes through the inner component, it carries along a bubble of the outer component. Using Thomas-Fermi approximation, we show that phase-separated binary condensates can either support vortices with empty or filled cores. For time dependent obstacle potentials, ramped down in the present case, relative energy scales of the system influence the dynamical evolution of the binary condensate.
We present numerical calculations of the snake instability in a Fermi superfluid within the Bogoliubov-de Gennes theory of the Bose-Einstein condensate (BEC) to BCS crossover using the random-phase approximation complemented by time-dependent simulations. We examine the snaking behavior across the crossover and quantify the time scale and length scale of the instability. While the dynamics shows extensive snaking before eventually producing vortices and sound on the BEC side of the crossover, the snaking dynamics is preempted by decay into sound due to pair breaking in the deep BCS regime. At the unitarity limit, hydrodynamic arguments allow us to link the rate of snaking to the experimentally observable ratio of inertial to physical mass of the soliton. In this limit we witness an unresolved discrepancy between our numerical estimates for the critical wave number of suppression of the snake instability and recent experimental observations with an ultracold Fermi gas.
The splitting instability of a doubly quantized vortex in the BEC-BCS crossover of a superfluid Fermi gas is investigated by means of a low-energy effective field theory. Our linear stability analysis and nonequilibrium numerical simulations reveal that the character of the instability drastically changes across the crossover. In the BEC limit, the splitting of the vortex into two singly quantized vortices occurs through the emission of phonons, while such an emission is completely absent in the BCS limit. In the crossover regime, the instability and phonon emission are enhanced and the lifetime of a doubly quantized vortex becomes minimal. The emitted phonon can be observed as a spiraling pattern amplified due to the rotational superradiance, known as a mechanism to carry away energy and angular momentum from a spinning black hole. We also investigate the influence of temperature, population imbalance, and three-dimensional effects.