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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