Second sound in the crossover from the Bose-Einstein condensate to the Bardeen-Cooper-Schrieffer superfluid

• Published in: Nature communications Vol. 12; no. 1; pp. 7074 - 6
• Main Authors: Hoffmann, Daniel K., Singh, Vijay Pal, Paintner, Thomas, Jäger, Manuel, Limmer, Wolfgang, Mathey, Ludwig, Hecker Denschlag, Johannes
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 06-12-2021; Nature Publishing Group; Nature Portfolio
• Subjects: 639/766/119/2791; 639/766/119/2795; 639/766/119/999; Bose-Einstein condensates; Entropy; Fluids; Helium; Humanities and Social Sciences; Interpolation; Matter & antimatter; multidisciplinary; Propagation; Science; Science (multidisciplinary); Sound; Sound propagation; Sound waves; Superfluidity; Thermodynamic properties
• ISSN: 2041-1723; 2041-1723
• DOI: 10.1038/s41467-021-27149-z

Second sound is an entropy wave which propagates in the superfluid component of a quantum liquid. Because it is an entropy wave, it probes the thermodynamic properties of the quantum liquid. Here, we study second sound propagation for a large range of interaction strengths within the crossover between a Bose-Einstein condensate (BEC) and the Bardeen-Cooper-Schrieffer (BCS) superfluid, extending previous work at unitarity. In particular, we investigate the strongly-interacting regime where currently theoretical predictions only exist in terms of an interpolation in the crossover. Working with a quantum gas of ultracold fermionic
6
Li atoms with tunable interactions, we show that the second sound speed varies only slightly in the crossover regime. By varying the excitation procedure, we gain deeper insight on sound propagation. We compare our measurement results with classical-field simulations, which help with the interpretation of our experiments.
The second sound is an entropy oscillation propagating with constant pressure. Here the authors demonstrate the variation of the second sound wave in the crossover from the Bose-Einstein condensate to the Bardeen-Cooper-Schrieffer superfluid using 6Li.