Strong-coupling corrections to ground-state properties of a superfluid Fermi gas

Tajima, Hiroyuki; Wyk, Pieter van; Hanai, Ryo; Kagamihara, Daichi; Inotani, Daisuke; Horikoshi, Munekazu; Ōhashi, Y.

doi:10.1103/physreva.95.043625

Cited by 36 publications

(59 citation statements)

References 51 publications

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“…The corrected data (orange diamonds in Fig. 4) agree well with theoretical calculations of c s /v F throughout the BCS-BEC crossover 12,28,30,32 and also with previous experiments 9-11 . Our measurements are the first to probe sound propagation in a homogeneous Fermi gas, allowing direct comparison with theory, free of complications arising from density inhomogeneities and partial hydrodynamics across the wavefront.…”

supporting

confidence: 88%

“…10). Hence, 25 and theoretical predictions: BCS (black dotted line), T-matrix (green dashed) 26 , Luttinger-Ward (black solid) 12 , Gaussian fluctuation theory (red dash-dotted) 28 , Monte Carlo (brown cross) 27 , operator product expansion (red asterisk) 29 , and extended T-matrix (purple dash-dot-dotted) 30 . Error bars in a-c denote the standard deviation of the measured centre-of-mass displacements.…”

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confidence: 99%

“…A plot of ∆/E F for these three spectra is provided in Fig. 3d along with different calculations 12,[26][27][28][29][30] . The experimentally determined pairing gap can serve as an input parameter for our diagrammatic theory, which uses the chemical potential as a fitting parameter (see Supplementary Information).…”

mentioning

confidence: 99%

“…In the momentum range used here, 0.4 k/k F 0.6 (see Supplementary Information), the dispersion remains close to linear near unitarity, but becomes concave (convex) in the BCS (BEC) regime 21,26,31 . We can estimate this curvature for finite k/k F using the QRPA calculation and correct for it to estimate c s /v F as k → 0 (see Supplementary 18 , density functional (green dash-dash-dotted) 32 , Luttinger-Ward (black solid) 12 , Gaussian fluctuation theory (red dash-dotted) 28 , and extended T-matrix (purple dash-dot-dotted) 30 . Inset shows measured BA mode frequency for clouds at di erent k/k F at unitarity demonstrating linear dispersion.…”

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Goldstone mode and pair-breaking excitations in atomic Fermi superfluids

et al. 2017

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. According to Goldstone's theorem, phase transitions that break continuous symmetries lead to the existence of gapless excitations in the long-wavelength limit 5 . 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-Schrie er (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.When a Hamiltonian is invariant with respect to a continuous symmetry, but the ground state is not, a massless bosonic mode appears in the spectrum of allowed excitations 5 . At temperatures low enough for quantum effects to become prominent, dynamical behaviours, such as superconductivity and superfluidity, are possible only due to the low-energy excitation spectrum. Superfluid and superconducting states break gauge invariance and the resultant Goldstone mode is an oscillation of the phase of the order parameter giving rise to a collective motion of particles that is distinct from single-particle excitations. In superconductors, the Coulomb interaction lifts the collective mode up to the frequency of the classical plasma oscillation 4 , present in the normal phase, such that the Goldstone mode is generally imperceptible 6 . In neutral superfluids, however, the Goldstone mode takes the form of a gapless phonon 7 and provides a dramatic signature of macroscopic order.Ultracold gases of atomic fermions have enabled the creation and study of high-transition-temperature superfluids in the smooth crossover from the BCS to BEC regimes 8 . Both first-sound 9-11 and second-sound 10 propagation have been observed in inhomogeneous Fermi gases, yet the basic elementary excitation spectrum has not been measured. Here, we present a comprehensive study of the low-temperature excitations in a homogeneous Fermi superfluid throughout the whole BCS-BEC crossover. The spectra exhibit both a dominant Goldstone mode, or Bogoliubov-Anderson (BA) phonon, and a single-particle continuum. Our study reveals how the energy and spectral weight of these excitations evolve as a function of the interaction strength. We develop a theory based on the quasiparticle random-phase approximation (QRPA), which provides a good quantitative description of the data.The starting point for our experiments is a harmonically trapped gas of fermionic 6 Li atoms in a balanced mixture of the lowest two hyperfine states with tunable s-wave interactions near a broad Feshbach resonance (see Supplementary Information). Atoms are cooled to temperatures below the superfluid transition te...

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confidence: 88%

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confidence: 99%

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confidence: 99%

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Goldstone mode and pair-breaking excitations in atomic Fermi superfluids

et al. 2017

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“…Pairing fluctuations are taken into account within the framework of an extended T -matrix approximation (ETMA) [39,40,[58][59][60][61]. Using this combined theory, we calculate the local spin susceptibility χ t (r, T ), as well as the spatially averaged one X t (T ), in the whole BCS-BEC crossover region.…”

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confidence: 99%

Spin susceptibility and effects of a harmonic trap in the BCS-BEC crossover regime of an ultracold Fermi gas

2017

Self Cite

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We theoretically investigate magnetic properties of a trapped ultracold Fermi gas. Including pairing fluctuations within the framework of an extended T -matrix approximation (ETMA), as well as effects of a harmonic trap in the local density approximation (LDA), we calculate the local spin susceptibility χ t (r, T ) in the BCS (Bardeen-Cooper-Schrieffer)-BEC (Bose-Einstein condensation) crossover region. We show that pairing fluctuations cause non-monotonic temperature dependence of χ t (r, T ). Although this behavior looks similar to the spin-gap phenomenon associated with pairing fluctuations in a uniform Fermi gas, the trapped case is found to also be influenced by the temperature-dependent density profile, in addition to pairing fluctuations. We demonstrate how to remove this extrinsic effect from χ t (r, T ), to study the interesting spin-gap phenomenon purely originating from pairing fluctuations. Since experiments in cold atom physics are always done in a trap, our results would be useful for the assessment of preformed pair scenario, from the viewpoint of spin-gap phenomenon.

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Multi-body Correlations in SU(3) Fermi Gases

Tajima

Naidon

2018

J Low Temp Phys

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We investigate strong-coupling effects in a three-component atomic Fermi gas. It is a promising candidate for simulating quantum chromodynamics (QCD), and furthermore, the emergence of various phenomena such as color superfluidity and Efimov effect are anticipated in this system. In this paper, we study the effects of two-body and three-body correlations by means of the many-body T -matrix approximation (TMA) as well as the Skorniakov-Ter-Martirosian (STM) equation with medium corrections. We investigate the effects of finite temperature and chemical potential on the trimer binding energy at the superfluid critical point of the unitarity limit.

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Strong-coupling corrections to ground-state properties of a superfluid Fermi gas

Cited by 36 publications

References 51 publications

Goldstone mode and pair-breaking excitations in atomic Fermi superfluids

Goldstone mode and pair-breaking excitations in atomic Fermi superfluids

Spin susceptibility and effects of a harmonic trap in the BCS-BEC crossover regime of an ultracold Fermi gas

Multi-body Correlations in SU(3) Fermi Gases

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