Second sound with ultracold atoms: A brief historical account

Abstract

We briefly review the research on second sound in ultracold atomic physics, with emphasis on strongly interacting unitary Fermi gases with infinitely large $s$-wave scattering length. Second sound is a smoking-gun feature of superfluidity in any quantum superfluids. The observation and characterization of second sound in ultracold quantum gases has been a long-standing challenge, and in recent years there are rapid developments due to the experimental realization of a uniform box-trap potential. The purpose of this review is to present a brief historical account of the key research activities on second sound over the past two decades. We summarize the initial theoretical works that reveal the characteristics of second sound in a unitary Fermi gas, and introduce its first observation in a highly elongated harmonic trap. We then discuss the most recent measurement on second sound attenuation in a uniform setup, which may open a new era to understand quantum transport near quantum criticality in the strongly interacting regime. The observation of second sound in homogeneous weakly interacting Bose condensates in both two and three dimensions are also briefly introduced.

21 pages, 22 figures

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• Many-Body Physics with Ultracold Gases
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• Vortices and Superfluidity in a Strongly Interacting Fermi Gas
• Weakly bound dimers of fermionic atoms
• Collective excitations of a degenerate gas at the BEC-BCS crossover
• Universal Quantum Viscosity in a Unitary Fermi Gas
• Homogeneous Atomic Fermi Gases
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• Equation of state of a superfluid Fermi gas in the BCS-BEC crossover
• Viscosity and scale invariance in the unitary Fermi gas
• Holographic stress tensor for non-relativistic theories
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• Observation of first and second sound in a BKT superfluid
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• Shear viscosity of a universal Fermi gas near the superfluid phase transition
• Excitation Spectrum and Superfluid Gap of an Ultracold Fermi Gas
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• First and second sound in a strongly interacting Fermi gas
• Quantitative comparison between theoretical predictions and experimental results for Bragg spectroscopy of a strongly interacting Fermi superfluid
• Variational theory of two-fluid hydrodynamic modes at unitarity
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• Low-temperature thermodynamics of the unitary Fermi gas: superfluid fraction, first sound and second sound
• Quantum critical thermal transport in the unitary Fermi gas
• Hydrodynamic Relaxation in a Strongly Interacting Fermi Gas
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• First and second sound of a unitary Fermi gas in highly elongated harmonic traps

Cited by in corpus (6)

• Thermally stable p-wave repulsive Fermi polaron without a two-body bound state
• Unconventional superconductivity of an altermagnetic metal: Polarized BCS and inhomogeneous Fulde-Ferrell-Larkin-Ovchinnikov states
• Dynamic structure factor of one-dimensional Fermi superfluid with spin-orbit coupling
• Fermi spin polaron and dissipative Fermi-polaron Rabi dynamics
• Dynamic structure factor of two-dimensional Fermi superfluid with Rashba spin-orbit coupling
• First and second sound in two-dimensional bosonic and fermionic superfluids