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arXiv · 1707.00406

Goldstone mode and pair-breaking excitations in atomic Fermi superfluids

Sascha Hoinka·Paul Dyke·Marcus G. Lingham·Jami J. Kinnunen·Georg M. Bruun·Chris J. Vale
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Abstract

Spontaneous symmetry breaking is a central paradigm of elementary particle physics, magnetism, superfluidity and superconductivity. According to Goldstone's theorem, phase transitions that break continuous symmetries lead to the existence of gapless excitations in the long-wavelength limit. These Goldstone modes generally dominate the low-energy excitations, showing that symmetry breaking has a profound impact on the physical properties of matter. Here, we present the first 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 a strong-coupling theory.

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Sascha Hoinka, Paul Dyke, Marcus G. Lingham, Jami J. Kinnunen, Georg M. Bruun, Chris J. Vale. 2017-07-03. Goldstone mode and pair-breaking excitations in atomic Fermi superfluids. https://doi.org/10.1038/nphys4187

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