Motion of a Solitonic Vortex in the BEC-BCS Crossover

Abstract

We observe a long-lived solitary wave in a superfluid Fermi gas of 6 Li atoms after phase imprinting. Tomographic imaging reveals the excitation to be a solitonic vortex, oriented transverse to the long axis of the cigar-shaped atom cloud. The precessional motion of the vortex is directly observed, and its period is measured as a function of the chemical potential in the BEC-BCS crossover. The long period and the correspondingly large ratio of the inertial to the bare mass of the vortex are in good agreement with estimates based on superfluid hydrodynamics that we derive here using the known equation of state in the BEC-BCS crossover. DOI: 10.1103/PhysRevLett.113.065301 PACS numbers: 67.85.-d, 03.75.Kk, 03.75.Lm, 03.75.Ss Solitary waves that do not spread as they propagate are ubiquitous in nonlinear systems, from classical fluids and fiber optics to superfluids and superconductors. These waves are localized objects with defined energy and mass, and as such they can be described as an effective single particle emerging from a many-body environment. This distinguishes them from larger-scale collective excitations such as shape oscillations of a superfluid, or from perturbative linear excitations such as phonons. Paradigmatic examples of solitary waves in superfluids are planar solitons that separate regions of differing phase, as well as vortex rings or single vortex lines [see Fig. 1(a)]. The direct creation of such localized and highly nonlinear objects "on demand" in ultracold quantum gases allows for an excellent dynamical probe of novel superfluids, such as strongly interacting Fermi gases [1] or spin-orbit coupled Bose-Einstein condensates [2,3].In a recent experiment on fermionic superfluids at MIT [1], long-lived solitary waves were produced that featured a large ratio of inertial to bare (missing) mass of over 200, evidenced by an oscillation period over 15 times longer than the period for a single atom. The observed absorption images suggested the interpretation of the waves as planar solitons, but the longevity as well as the large effective mass ratio were unexpected for this type of defect [4][5][6][7]. Indeed, the nodal plane of a soliton is energetically more costly than the nodal line of a vortex, and planar solitons can decay into lower energy excitations via the snake instability, the undulation of the soliton plane [4]. Several recent works therefore suggested that these solitary waves are vortex rings [8][9][10]. For weakly interacting BoseEinstein condensates, solitons have been created [11,12] and observed to decay into vortex rings [13,14]. The latter further decay into a vortex-antivortex pair that eventually breaks up, leaving behind a single remnant vortex [15][16][17]. The exact process was recently elucidated in a discussion of apparent soliton oscillations observed in weakly interacting BECs [18,19]. In the case of strongly interacting fermionic superfluids, the understanding of such nontrivial dynamics presents a challenging nonequilibrium many-body problem [8,20].In this Letter,...

Multidimensional Josephson vortices in spin-orbit-coupled Bose-Einstein condensates: Snake instability and decay through vortex dipoles

“…The latter, in particular, opens up an interesting way to study the snake instability in twocomponent condensates, in the same manner as it is currently being the case of DS decay in scalar condensates [45,46]. Besides, the decay of JVs in SO coupled condensates provides an excellent playground to investigate the rich dynamics of vortex dipoles as has been previously done in scalar BECs [47,48].…”

Proper phase imprinting method for a dark soliton excitation in a superfluid Fermi mixture

“…A similar procedure was applied in a superfluid Fermi mixture [21] resulting in a local disturbance of the atomic density which oscillated in an harmonic trap much more slowly than predicted for a dark soliton [22][23][24]. In a recent experiment [25] it has been shown that the state created by means of the phase imprinting method evolves very quickly to a so-called vortex soliton -see also theoretical analysis in Refs. [26,27].…”

Traveling Majorana Solitons in a Low-Dimensional Spin-Orbit-Coupled Fermi Superfluid

“…The family of BEC solitons consists of many interesting members, from bright solitons in attractive BECs [6] and gap solitons in optical lattices [7], to dark solitons in repulsively interacting BECs [3][4][5], which are created experimentally by imprinting a sharp and characteristic phase jump into the BEC. Remarkably, dark solitons may also be created in strongly interacting Fermi gases [8][9][10][11][12][13][14][15][16] at the crossover from BECs to Bardeen-Cooper-Schrieffer (BCS) superfluids [17], where phase kinks are encoded in the pairing order parameter. Their recent experimental observation may offer valuable insights into the nature of fermionic superfluidity in the strongly correlated regime [13,18].…”

“…1 is easy to set up [22,28], although the realization of 1D topological superfluids is difficult due to the lack of efficient cooling techniques [22]. Majorana solitons can be created by imprinting a sharp phase jump [13,15]. The observation of their oscillations seems to be an experimental challenge, as the density profile of Majorana solitons remains flat [26,27].…”