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Observation of roton mode population in a dipolar quantum gas

Nature Physics volume 14, pages 442–446 (2018)Cite this article

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Abstract

The concept of a roton, a special kind of elementary excitation forming a minimum of energy at finite momentum, has been essential for the understanding of the properties of superfluid 4He (ref. 1). In quantum liquids, rotons arise from the strong interparticle interactions, whose microscopic description remains debated2. In the realm of highly controllable quantum gases, a roton mode has been predicted to emerge due to magnetic dipole–dipole interactions despite their weakly interacting character3. This prospect has raised considerable interest4,5,6,7,8,9,10,11,12; yet roton modes in dipolar quantum gases have remained elusive to observations. Here we report experimental and theoretical studies of the momentum distribution in Bose–Einstein condensates of highly magnetic erbium atoms, revealing the existence of the long-sought roton mode. Following an interaction quench, the roton mode manifests itself with the appearance of symmetric peaks at well-defined finite momentum. The roton momentum follows the predicted geometrical scaling with the inverse of the confinement length along the magnetization axis. From the growth of the roton population, we probe the roton softening of the excitation spectrum in time and extract the corresponding imaginary roton gap. Our results provide a further step in the quest towards supersolidity in dipolar quantum gases13.

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Fig. 1: Roton mode in a dBEC.
Fig. 2: Observed roton peaks and characteristic scalings.
Fig. 3: Dynamics of the roton mode.
Fig. 4: Population growth and roton gap.

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Acknowledgements

We are particularly grateful to B. Blakie for many inspiring exchanges. We thank D. O’Dell, M. Baranov, E. Demler, A. Sykes, T. Pfau, I. Ferrier-Barbut and H. P. Büchler for fruitful discussions, and G. Natale for his support in the final stage of the experiment. This work is dedicated to the memory of D. Jin and her inspiring example. The Innsbruck group is supported through an ERC Consolidator Grant (RARE, no. 681432) and a FET Proactive project (RySQ, no. 640378) of the EU H2020. L.C. is supported within a Marie Curie Project (DipPhase, no. 706809) of the EU H2020. F.W. and L.S. thank the DFG (SFB 1227 DQ-mat). All authors thank the DFG/FWF (FOR 2247). Part of the computational results presented have been achieved using the HPC infrastructure LEO of the University of Innsbruck.

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Authors and Affiliations

  1. Institut für Experimentalphysik, Universität Innsbruck, Innsbruck, Austria

    L. Chomaz, D. Petter, G. Faraoni, S. Baier, J. H. Becher, M. J. Mark & F. Ferlaino

  2. Institut für Quantenoptik und Quanteninformation, Österreichische Akademie der Wissenschaften, Innsbruck, Austria

    R. M. W. van Bijnen, M. J. Mark & F. Ferlaino

  3. Dipartimento di Fisica e Astronomia, Università di Firenze, Sesto Fiorentino, Italy

    G. Faraoni

  4. Institut für Theoretische Physik, Leibniz Universität Hannover, Hannover, Germany

    F. Wächtler & L. Santos

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  1. L. Chomaz
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Contributions

F.F., L.C., D.P., G.F., M.J.M., J.H.B. and S.B. conceived and supervised the experiment and collected the experimental data. L.C. analysed the data. R.M.W.v.B. developed the Bogoliubov–de Gennes calculations. F.W., R.M.W.v.B. and L.S. performed the real-time simulations. L.S. derived the analytical model and the SSM. L.C., F.F., R.M.W.v.B. and L.S. wrote the paper with contributions from all of the authors.

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Correspondence to F. Ferlaino.

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Supplementary Figure 1–3, Supplementary Table 1–2, Supplementary References

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Chomaz, L., van Bijnen, R.M.W., Petter, D. et al. Observation of roton mode population in a dipolar quantum gas. Nature Phys 14, 442–446 (2018). https://doi.org/10.1038/s41567-018-0054-7

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