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

Daniel K Hoffmann,Ludwig Mathey,Vijay Pal Singh,Wolfgang Limmer,Manuel Jäger,Thomas Paintner,Johannes Hecker Denschlag

doi:10.1038/s41467-021-27149-z

Abstract

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 6Li 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.

Highlights

Second sound is a transport phenomenon of quantum liquids that emerges below the critical temperature for superfluidity Tc1–3
In the limit of vanishing temperature T → 0, the two-fluid model predicts that first sound corresponds to a propagating pressure oscillation with constant entropy, while second sound is an entropy oscillation propagating at constant pressure[8]
Second sound has recently been observed by Sidorenkov et al.[20] in a unitary Fermi gas and by Ville et al.[21] and Christodoulou et al.[22] in a twodimensional bosonic SF

Summary

Introduction

Second sound is a transport phenomenon of quantum liquids that emerges below the critical temperature for superfluidity Tc1–3. C-field simulations in the BEC regime match quite well the observed wave dynamics in experiments at interaction strengths of up to ðkFaÞÀ1 1⁄4 1. This applies pressure on the cold cloud on both sides of the laser beam and creates two density wave packets (Fig. 1c) which propagate out in opposite directions along the axial trap axis with the speed u1.

Results

Conclusion