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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 instability1 and creates self-organized, ordered surface structures2 or droplet crystals3. Quantum ferrofluids such as Bose–Einstein condensates with strong dipolar interactions also display superfluidity4. The field of dipolar quantum gases is motivated by the search for new phases of matter that break continuous symmetries5,6. 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 condensate7. By using a Feshbach resonance to control the interparticle interactions, we induce a finite-wavelength instability8 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 state9,10,11.

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Figure 1: Growth of a microscopic droplet crystal.
Figure 2: Evaluation of the structures and lifetime analysis.
Figure 3: Hysteresis of pattern formation.

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Acknowledgements

We thank A. Griesmaier for support at the early stage of the experiment and D. Zajec, D. Peter, H. P. Büchler and L. Santos for discussions. This work is supported by the German Research Foundation (DFG) within SFB/TRR21. H.K. acknowledges support by the ‘Studienstiftung des deutschen Volkes’.

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All authors discussed the results, made critical contributions to the work and contributed to the writing of the manuscript.

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Correspondence to Holger Kadau or Tilman Pfau.

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Extended data figures and tables

Extended Data Figure 1 Atom trap-loss spectroscopy for two Feshbach resonances.

Atom-loss spectroscopy mapping Feshbach resonances of 164Dy. The number of atoms (blue circles) and temperature (red diamonds) are normalized. a, The atom-number minimum shows the centre of the Feshbach resonance at B0 = 1.326(3) G, while the temperature is maximized at B0 + ΔB, with ΔB = 8(5) mG. We prepared stable BECs at B = 1.285(3) G (dashed black line) before we induced an instability for lower magnetic-field values. b, For the data presented in the main text, we used a resonance at B0 = 7.117(3) G (the atom-number minimum) with ΔB = 51(15) mG. Stable BECs were created at B = 6.962(3) G (dashed black line).

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Kadau, H., Schmitt, M., Wenzel, M. et al. Observing the Rosensweig instability of a quantum ferrofluid. Nature 530, 194–197 (2016). https://doi.org/10.1038/nature16485

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