![]() This helps for separating the generally weak second sound signals from the first sound ones. As second sound is mainly an entropy wave and first sound is mainly a pressure wave, different excitation schemes give rise to different responses for first and second sound. In addition, we investigate experimentally and theoretically the system response when modifying the excitation scheme. In particular, c-field simulations in the BEC regime match quite well the observed wave dynamics in experiments at interaction strengths of up to ( k F a ) − 1 = 1. Nevertheless, comparing our measurements to existing calculations and interpolations we find reasonable agreement. This is important, because full theoretical calculations are not yet available for the entire strongly interacting regime. ![]() Here, we experimentally investigate how second sound changes across the BEC–BCS crossover. Second sound was also possibly present in an experiment by Meppelink et al. 20 in a unitary Fermi gas and by Ville et al. Second sound has recently been observed by Sidorenkov et al. This includes experiments on first sound (see e.g., ref. In the experiment, this is done by tuning the interaction parameter ( k F a ) − 1, where a is the scattering length, k F = 2 m E F / ℏ the Fermi wavenumber, E F is the Fermi energy, and m the atomic mass.Ī large range of thermodynamical properties of the BEC-BCS crossover has been studied, e.g., in refs. 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– 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– 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– 3.
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