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Goldstone mode and pair-breaking excitations in atomic Fermi superfluids

Nature Physics volume 13, pages 943–946 (2017)Cite this article

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Abstract

Spontaneous symmetry breaking is a central paradigm of elementary particle physics1, magnetism2, superfluidity3 and superconductivity4. According to Goldstone’s theorem, phase transitions that break continuous symmetries lead to the existence of gapless excitations in the long-wavelength limit5. These Goldstone modes can become the dominant low-energy excitation, showing that symmetry breaking has a profound impact on the physical properties of matter. Here, we present a comprehensive study of the elementary excitations in a homogeneous strongly interacting Fermi gas through the crossover from a Bardeen–Cooper–Schrieffer (BCS) superfluid to a Bose–Einstein condensate (BEC) of molecules using two-photon Bragg spectroscopy. The spectra exhibit a discrete Goldstone mode, associated with the broken-symmetry superfluid phase, as well as pair-breaking single-particle excitations. Our techniques yield a direct determination of the superfluid pairing gap and speed of sound in close agreement with strong-coupling theories.

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Figure 1: Probing homogeneous excitation spectra with focused beam Bragg scattering.
Figure 2: Bragg spectra throughout the BCS–BEC crossover.
Figure 3: Bragg spectra and pairing gap near unitarity.
Figure 4: Speed of sound across the BCS–BEC crossover.

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Acknowledgements

We thank W. Zwerger, G. Strinati, L. Salasnich and Y. Ohashi for sharing their data and comments on the manuscript and P. Hannaford for fruitful discussions. This work was supported by ARC Discovery Project DP130101807. G.M.B. wishes to acknowledge the support of the Villum Foundation via Grant No. VKR023163 and ARC Discovery Project DP160102739.

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Authors and Affiliations

  1. Centre for Quantum and Optical Sciences, Swinburne University of Technology, Melbourne, 3122, Australia

    Sascha Hoinka, Paul Dyke, Marcus G. Lingham & Chris J. Vale

  2. Department of Applied Physics, COMP Centre of Excellence, Aalto University School of Science, FI 00076 Aalto, Finland

    Jami J. Kinnunen

  3. Institut for Fysik og Astronomi, Aarhus Universitet, 8000 Aarhus C, Denmark

    Georg M. Bruun

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  1. Sascha Hoinka
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Contributions

S.H., P.D., M.G.L. and C.J.V. conducted the experimental work and data analysis. J.J.K. and G.M.B. performed the theoretical calculations. All authors contributed to the manuscript preparation.

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Correspondence to Chris J. Vale.

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Hoinka, S., Dyke, P., Lingham, M. et al. Goldstone mode and pair-breaking excitations in atomic Fermi superfluids. Nature Phys 13, 943–946 (2017). https://doi.org/10.1038/nphys4187

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