Cascade of Solitonic Excitations in a Superfluid Fermi gas: From Planar Solitons to Vortex Rings and Lines

Abstract

We follow the time evolution of a superfluid Fermi gas of resonantly interacting 6 Li atoms after a phase imprint. Via tomographic imaging, we observe the formation of a planar dark soliton, its subsequent snaking, and its decay into a vortex ring, which, in turn, breaks to finally leave behind a single solitonic vortex. In intermediate stages, we find evidence for an exotic structure resembling the Φ soliton, a combination of a vortex ring and a vortex line. Direct imaging of the nodal surface reveals its undulation dynamics and its decay via the puncture of the initial soliton plane. The observed evolution of the nodal surface represents dynamics beyond superfluid hydrodynamics, calling for a microscopic description of unitary fermionic superfluids out of equilibrium.

Core filling and snaking instability of dark solitons in spin-imbalanced superfluid Fermi gases

“…One can image the soliton in time of flight after a ramp to the BEC limit as done in Refs. [5,12,13].…”

“…They have been studied in a number of physical settings including atomic Bose-Einstein Condensates (BECs) [1] and superfluid Fermi gases of spin-1/2 atoms [2][3][4][5]. They are ubiquitous in quenches [6,7] and can be engineered through phase imprinting protocols [5,[8][9][10][11][12][13]. Previous experimental [7,10,[12][13][14][15] and theoretical work [16][17][18][19][20][21][22][23][24][25][26][27][28][29][30] has discovered that these dark solitons are dynamically unstable to a "snaking" instability transverse to the plane of the soliton in both BECs and Fermi gases.…”

Homogeneous Atomic Fermi Gases

“…This includes the observation of the quasiparticle jump [57] in the momentum distribution of a Fermi liquid, critical fluctuations in the BEC-BCS crossover, and long-lived solitons [58]. Of particular interest are spin imbalanced mixtures that have been studied extensively in harmonic traps [29,30,55,[59][60][61][62], where the trap drives the separation of normal and superfluid phases into a shell structure.…”

Vortex Pinning and Dynamics in the Neutron Star Crust

“…We demonstrated in [28] that the TDSLDA accurately models the formation and decay of solitonic defects -from domain walls into vortex rings and vortex lines. These effects cannot be reproduced without dissipation, and the agreement with experiments [47,48] validates that the so-called one-body dissipation naturally present in the TDSLDA is sufficient to correctly capture vortex dynamics. With the TDSLDA approach, we can thus extract both the magnitude of the vortex-nucleus interaction as well as the dynamical time scales.…”