In the following, we focus on the sound speed close to the trap center. Our analysis shows that the sound propagation slows down as the pulse approaches the edge of the cloud where the particle density decreases. As a consequence, the measured sound speed should be considered a mean value. Please note that the central sound speed is determined over a local area of the cloud where the density varies by about 30%. The center positions are determined via a Gaussian fit. To obtain the speed of sound, we examine how the center position of each wave packet changes with time. The propagating waves produce a density modulation of only a few percent of the peak density and can be considered as a weak perturbation of the system. We observe two density wave packets which propagate with first sound velocity from the trap center towards the edge of the cloud (two bright traces, marked with red arrows). The time-ordered stack shows the propagation of the sound waves along the axial direction x as a function of time. Each profile was obtained by integrating the measured column density of the atomic cloud along the transverse (i.e., y-) direction (see “Methods” for details). For the given interaction strength, we used T c = 0.37 T F (see Supplementary Note 1).įigure 1d is a time-ordered stack of one-dimensional line density profiles of the atom cloud 31. In the experiment, this is done by tuning the interaction parameter \(\approx (1.61\pm 0.05)\), B = 735 G and a temperature of T = (220 ± 30) nK = (0.30 ± 0.06) T F, which corresponds to T = (0.80 ± 0.15) T c, where T c is the critical temperature. In particular, an ultracold fermionic quantum gas with a tunable Feshbach resonance offers a unique opportunity to access various sorts of superfluidity in one system, ranging continuously between a Bose-Einstein condensate (BEC) of bosonic molecules, a resonant SF, and a SF gas of Cooper pairs (BCS superfluid) 9, 10, 11. ![]() With the advent of ultracold quantum gases, with tunable interactions, these dependencies can now be studied. The properties of a SF naturally depend on parameters such as its temperature and the interaction strength between its particles. In the limit of vanishing temperature T → 0, the two-fluid model predicts that first sound (i.e., standard sound waves) corresponds to a propagating pressure oscillation with constant entropy, while second sound is an entropy oscillation propagating at constant pressure 8. ![]() The NF component carries all the entropy and has non-zero viscosity. The SF component has no entropy and flows without dissipation. It was experimentally discovered 4 in 1944 in He II 5 and was described with a hydrodynamic two-fluid model 2, 6, 7, 8 which treats He II as a mixture of a superfluid (SF) and a normal fluid (NF). Second sound is a transport phenomenon of quantum liquids that emerges below the critical temperature for superfluidity T c 1, 2, 3.
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