Observation of vortices and vortex stripes in a dipolar condensate

Abstract

Quantized vortices are a prototypical feature of superfluidity that have been observed in multiple quantum gas experiments. But the occurrence of vortices in dipolar quantum gases—a class of ultracold gases characterized by long-range anisotropic interactions—has not been reported yet. Here we exploit the anisotropic nature of the dipole–dipole interaction of a dysprosium Bose–Einstein condensate to induce angular symmetry breaking in an otherwise cylindrically symmetric pancake-shaped trap. Tilting the magnetic field towards the radial plane deforms the cloud into an ellipsoid, which is then set into rotation. At stirring frequencies approaching the radial trap frequency, we observe the generation of dynamically unstable surface excitations, which cause angular momentum to be pumped into the system through vortices. Under continuous rotation, the vortices arrange into a stripe configuration along the field, in close agreement with numerical simulations.

Vortices in dipolar Bose-Einstein condensates

“…In spite of 20 years of theoretical and experimental endeavour, the first experimental evidence of vortices in a dipolar Bose-Einstein condensate was only observed in an experiment last year [76]. Vortices in a dipolar BEC are predicted to be generated through all the aforementioned methods at the beginning of this article [83].…”

“…2 for a schematic of this process. Recently, this method was employed to produce the first observed vortices in a dipolar gas [76], in which the vortices exhibit remarkable properties due to the dipole-dipole interaction, to be explored in the next section. Rapid magnetostirring is known to produce a different effect, where the atoms remain stationary but the dipole moments rapidly rotate [77].…”

“…We focus our investigations on parameter regimes relevant to the recent experimental observation of vortices in a dipolar BEC [76]. We use an extended Gross-Pitaevskii formalism for the wavefunction Ψ, where the time-dependent equation reads [65,66,105,106]…”

“…In a recent experiment, Klaus et al [76] fixed the tilt angle of the magnetic field at θ = 35 • and investigated the consequences of magnetostirring a dipolar BEC. In what follows, we investigate the significance of this choice of angle.…”

“…3 are the data from Figure 1 of Ref. [76], obtained with the same protocol but with θ = 35 • (orange points). At short times the data agree perfectly, it is only at late times, where the condensate undergoes a much earlier dynamical instability, that the results diverge.…”