Sound propagation is a macroscopic manifestation of the interplay between the equilibrium thermodynamics and the dynamical transport properties of fluids. Here, for a two-dimensional system of ultracold fermions, we calculate the first and second sound velocities across the whole BCS-BEC crossover, and we analyze the system response to an external perturbation. In the low-temperature regime we reproduce the recent measurements [Phys. Rev. Lett. 124, 240403 (2020)] of the first sound velocity, which, due to the decoupling of density and entropy fluctuations, is the sole mode excited by a density probe. Conversely, a heat perturbation excites only the second sound, which, being sensitive to the superfluid depletion, vanishes in the deep BCS regime and jumps discontinuously to zero at the Berezinskii-Kosterlitz-Thouless superfluid transition. A mixing between the modes occurs only in the finite-temperature BEC regime, where our theory converges to the purely bosonic results.
Physical Review A
G.B. acknowledges support from the Austrian Science Fund (FWF), under Project No. M2641-N27. This work was partially supported by the University of Padua, BIRD project “Superfluid properties of Fermi gases in optical potentials.” The authors thank Miki Ota, Tomoki Ozawa, Sandro Stringari, Tilman Enss, Hauke Biss, Henning Moritz, and Nicolò Defenu for fruitful discussions. The authors thank Henning Moritz and Markus Bohlen for providing their experimental data.
Tononi A, Cappellaro A, Bighin G, Salasnich L. Propagation of first and second sound in a two-dimensional Fermi superfluid. Physical Review A. 2021;103(6). doi:10.1103/PhysRevA.103.L061303
Tononi, A., Cappellaro, A., Bighin, G., & Salasnich, L. (2021). Propagation of first and second sound in a two-dimensional Fermi superfluid. Physical Review A. American Physical Society. https://doi.org/10.1103/PhysRevA.103.L061303
Tononi, A., Alberto Cappellaro, Giacomo Bighin, and L. Salasnich. “Propagation of First and Second Sound in a Two-Dimensional Fermi Superfluid.” Physical Review A. American Physical Society, 2021. https://doi.org/10.1103/PhysRevA.103.L061303.
A. Tononi, A. Cappellaro, G. Bighin, and L. Salasnich, “Propagation of first and second sound in a two-dimensional Fermi superfluid,” Physical Review A, vol. 103, no. 6. American Physical Society, 2021.
Tononi A, Cappellaro A, Bighin G, Salasnich L. 2021. Propagation of first and second sound in a two-dimensional Fermi superfluid. Physical Review A. 103(6), L061303.
Tononi, A., et al. “Propagation of First and Second Sound in a Two-Dimensional Fermi Superfluid.” Physical Review A, vol. 103, no. 6, L061303, American Physical Society, 2021, doi:10.1103/PhysRevA.103.L061303.