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

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.

First and second sound in a dilute Bose gas across the BKT transition

“…As second method, we employ a step-pulse perturbation of the density, which is motivated by references [5,13]. To perturb the density we use the Gaussian potential…”

“…We find a driving-strength dependent density response and the excitation of two well-resolved sound peaks is observed for strong driving strengths only. As a secondary comparison, we excite the two sound modes using a step-pulse perturbation of the density [5,13]. Finally, we compare the sound velocity results of the driven response, the step-pulse perturbation and the DSF with the measurements [6].…”

Unitary Fermi superfluid near the critical temperature: thermodynamics and sound modes from elementary excitations

“…Moving to Fermi gases, the situation across the BCS-BEC crossover is significantly more involved [32]: while the experimental setups is certainly not comparable to Helium, there have been cases where a density-perturbing protocol excited just a single mode [50,53].…”

“…Therefore, besides the values of u 1 and u 2 in Eq. ( 27), in order to provide a more complete characterization of the experimental picture, we also have to consider the amplitudes modes W 1 and W 2 of the response to a density perturbation [32,50,54], i.e.…”

“…On a conceptual standpoint it can be derived on a hydrodynamic basis by connecting thermodynamic and transport quantities within the framework of Landau twofluid theory [23,31], with no need -in principle -to refer to the particular features of the microscopic constituents. From an experimental perspective, it has been recently shown that both modes predicted by the above mentioned Landau theory can be excited by a density-perturbing protocol driven by external laser fields [32].…”