Observing the Rosensweig instability of a quantum ferrofluid

Abstract

Ferrofluids exhibit unusual hydrodynamic effects owing to the magnetic nature of their constituents. As magnetization increases, a classical ferrofluid undergoes a Rosensweig instability and creates self-organized, ordered surface structures or droplet crystals. Quantum ferrofluids such as Bose-Einstein condensates with strong dipolar interactions also display superfluidity. The field of dipolar quantum gases is motivated by the search for new phases of matter that break continuous symmetries. The simultaneous breaking of continuous symmetries such as the phase invariance in a superfluid state and the translational symmetry in a crystal provides the basis for these new states of matter. However, interaction-induced crystallization in a superfluid has not yet been observed. Here we use in situ imaging to directly observe the spontaneous transition from an unstructured superfluid to an ordered arrangement of droplets in an atomic dysprosium Bose-Einstein condensate. By using a Feshbach resonance to control the interparticle interactions, we induce a finite-wavelength instability and observe discrete droplets in a triangular structure, the number of which grows as the number of atoms increases. We find that these structured states are surprisingly long-lived and observe hysteretic behaviour, which is typical for a crystallization process and in close analogy to the Rosensweig instability. Our system exhibits both superfluidity and, as we show here, spontaneous translational symmetry breaking. Although our observations do not probe superfluidity in the structured states, if the droplets establish a common phase via weak links, then our system is a very good candidate for a supersolid ground state.

An Arrested Implosion

“…Finally, we observe lifetimes of several hundreds of milliseconds, similar to what we reported in Ref. [12], which confirm a strong stabilization mechanism.…”

“…Thus, away from Feshbach resonances at the mean-field level the dipolar interaction dominates with ε dd;bg ¼ a dd =a bg ≃ 1.45. In a previous work [12], we have reported the observation of an instability of a dipolar Bose-Einstein condensate (BEC); the resulting state of this instablity is characterized by the existence of apparent droplets. These droplets cannot be explained by a stabilization by one-body quantum pressure [13], and as such are not solitons in the strict sense.…”

Droplets of Trapped Quantum Dipolar Bosons

“…Notice that the first choice is close to the experimental setting of Ref. [14] where a x ∼ a z √ 3, while in the second model the aspect ratio is different. With these settings, different values of σ lead to different ground-state configurations.…”

“…In the following we use dimensionless quantities, introducing a characteristic length scale r 0 = mC dd /(4πh 2 ) and a characteristic energy scale E 0 =h 2 /(mr 2 0 ). Inspired by recent experimental measurements [14], we have considered different pancake-shaped harmonic traps with oscillator lengths a x = a y > a z (a α = h/(mω α )). Two different choices, leading to more than one stable droplet, have been used: Trap1 with a x = 1.20, a z = 0.6, and Trap2 with a x = 1.38, a z = 0.45.…”

Excitations and stability of weakly interacting Bose gases with multibody interactions

“…On the other hand, the presence of an asymmetric harmonic trapping in combination with short-range repulsive interactions can eliminate such instabilities, opening the way to the study of interesting many-body physics with long-range interactions [59,60]. Recent experiments with dipolar BECs showed that under certain conditions where instability is expected from a standard Bogoliubov approach, dense clusters with many atoms can occur [61][62][63][64][65], which are expected to be superfluid [66]. Two interpretations have been proposed to explain the stabilization of this phase, namely the presence of weak 3-body interactions [11,12] and beyond mean-field effects (Lee-Huang-Yang type corrections) [67][68][69].…”