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 have 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.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 have 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.

arXiv:2206.05914AAPPS bulletinSep 1, 20229 citations

First and Second Sound in a Compressible 3D Bose Fluid

Timon Hilker, Lena Hannah Dogra, Christoph Eigen +3 more

The two-fluid model is fundamental for the description of superfluidity. In the nearly incompressible liquid regime, it successfully describes first and second sound, corresponding, respectively, to density and entropy waves, in both liquid helium and unitary Fermi gases. Here, we study the two sounds in the opposite regime of a highly compressible fluid, using an ultracold ^{39}K Bose gas in a three-dimensional box trap. We excite the longest-wavelength mode of our homogeneous gas, and observe two distinct resonant oscillations below the critical temperature, of which only one persists above it. In a microscopic mode-structure analysis, we find agreement with the hydrodynamic theory, where first and second sound involve density oscillations dominated by, respectively, thermal and condensed atoms. Varying the interaction strength, we explore the crossover from hydrodynamic to collisionless behavior in a normal gas.The two-fluid model is fundamental for the description of superfluidity. In the nearly incompressible liquid regime, it successfully describes first and second sound, corresponding, respectively, to density and entropy waves, in both liquid helium and unitary Fermi gases. Here, we study the two sounds in the opposite regime of a highly compressible fluid, using an ultracold ^{39}K Bose gas in a three-dimensional box trap. We excite the longest-wavelength mode of our homogeneous gas, and observe two distinct resonant oscillations below the critical temperature, of which only one persists above it. In a microscopic mode-structure analysis, we find agreement with the hydrodynamic theory, where first and second sound involve density oscillations dominated by, respectively, thermal and condensed atoms. Varying the interaction strength, we explore the crossover from hydrodynamic to collisionless behavior in a normal gas.

arXiv:2112.14763Physical Review LettersJun 2, 202216 citations

Sound in a strongly-interacting Fermi gas

Bason E. Clancy, Le Luo, Joseph M. Kinast +3 more

Bulletin of the American Physical SocietyMar 13, 20062 citations

Unitary Fermi superfluid near the critical temperature: Thermodynamics and sound modes from elementary excitations

Giacomo Bighin, Alberto Cappellaro, Luca Salasnich

In superfluid ${}^{4}$ He the first sound refers to the propagation of a density wave while the second sound is related to the propagation of a temperature or entropy wave. The authors show that in superfluid Fermi gases this simplified picture is not always valid. Moreover, they calculate the first and second sound velocities, and the superfluid fraction near the critical point in unitary Fermi gases, and find good agreement with recent experimental results.In superfluid ${}^{4}$He the first sound refers to the propagation of a density wave while the second sound is related to the propagation of a temperature or entropy wave. The authors show that in superfluid Fermi gases this simplified picture is not always valid. Moreover, they calculate the first and second sound velocities, and the superfluid fraction near the critical point in unitary Fermi gases, and find good agreement with recent experimental results.

arXiv:2206.03924Physical Review AJun 30, 20223 citations

In superfluid

{}^{4}

He the first sound refers to the propagation of a density wave while the second sound is related to the propagation of a temperature or entropy wave. The authors show that in superfluid Fermi gases this simplified picture is not always valid. Moreover, they calculate the first and second sound velocities, and the superfluid fraction near the critical point in unitary Fermi gases, and find good agreement with recent experimental results.