Heavy solitons in a fermionic superfluid

Abstract

Solitons-solitary waves that maintain their shape as they propagate-occur as water waves in narrow canals, as light pulses in optical fibres and as quantum mechanical matter waves in superfluids and superconductors. Their highly nonlinear and localized nature makes them very sensitive probes of the medium in which they propagate. Here we create long-lived solitons in a strongly interacting superfluid of fermionic atoms and directly observe their motion. As the interactions are tuned from the regime of Bose-Einstein condensation of tightly bound molecules towards the Bardeen-Cooper-Schrieffer limit of long-range Cooper pairs, the solitons' effective mass increases markedly, to more than 200 times their bare mass, signalling strong quantum fluctuations. This mass enhancement is more than 50 times larger than the theoretically predicted value. Our work provides a benchmark for theories of non-equilibrium dynamics of strongly interacting fermions.

Generic equilibration dynamics of planar defects in trapped atomic superfluids

“…Indeed, there has been significant controversy and debate over the characterization and decay processes of various defects, notably those associated with a phase imprint in which ∆Φ ∼ π. There the original defect was thought to be a heavy soliton [5], later a vortex ring [7], and still later, as was consistent with predictions from some of our co-authors [8], established to be a solitonic vortex [6]. Similarly, defects formed via the Kibble-Zurek mechanism, using a temperature quench across the BoseEinstein condensation (BEC) transition, were originally mis-identified as solitons [9] and later determined to be solitonic vortices [10].…”

“…In this case, using pairs of 6 Li atoms with a chemical potential µ = k B (120 nK) (approximately equal to that reported in Ref. [5]) is sufficient to set the physical time and length , with data for both density cut and phase imprinting simulations at early times. It shows that in both density cut and phase imprinting perturbations, a sharp phase wall forms (t = 5).…”

“…In addition to suggesting directions for future experimental research, these studies yield further insight into related past experiments and simulations [1][2][3][4][5][6]. In the process we clarify the many different defect sequences which have been reported in the literature.…”

“…While these were designed to measure the speed of sound, these studies very early on elicited somewhat controversial [11] theoretical suggestions that dark solitons had been created in the process [3,[12][13][14][15]. Similar experiments have led to the observation of a variety of defects [1,[16][17][18], while other studies engineered localized excitations through phase imprinting [4][5][6][19][20][21]. The theory associated with these experiments has suggested that solitons, or vortices or vortex rings (in some combination or sequentially) are created in the process [4,[6][7][8][22][23][24][25].…”

An investigation of the Casimir energy for a fermion coupled to the sine-Gordon soliton with parity decomposition

“…More generally, the interaction of fermions with solitonic backgrounds could produce or affect a variety of interesting physical phenomena such as charge and fermion number fractionalization [79][80][81][82][83], hadron physics [68,69,[84][85][86], superfluidity [87,88], superconductivity [89], BoseEinstein condensation [90,91], conducting polymers [83,[92][93][94] and localization of fermions [95][96][97][98]. The spectrum of the fermion can in general be distorted due to the presence of such a background; bound states can appear and continuum states can change as compared with the free fermion.…”