Superfluidity in inhomogeneous 2D Bose-Einstein condensates

Superfluidity phenomena in the context of thermodynamics, nonlinear, and out-of-equilibrium physics can all be studied with the spectroscopic tool offered by the excitation of sound waves in a Bose-Einstein condensate.

In our experiment, we prepare a quasi-2D ²³Na Bose-Einstein condensate confined in a 2D box potential. We spatially modulate the condensate to create an inhomogeneous density profile, on which the dynamics of low-lying sound excitation is observed. We measure the speed of sound along and orthogonal to the 1D inhomogeneities; the case of a 1D lattice, studied in [1, 2], serves as a reference case. The anisotropic speed of sound reveals a reduction of the component of the superfluid density tensor along the modulation direction. Different inhomogeneities potentially change the superfluid density differently, and we compare each case to the result of Leggett [3].

[1] G. Chauveau, C. Maury, F. Rabec, C. Heintze, G. Brochier, S. Nascimbene, J. Dalibard, J. Beugnon, S. M. Roccuzzo, and S. Stringari, Superfluid Fraction in an Interacting Spatially Modulated Bose-Einstein Condensate, Phys. Rev. Lett. 130, (2023).

[2] J. Tao, M. Zhao, and I. B. Spielman, Observation of Anisotropic Superfluid Density in an Artificial Crystal, Phys. Rev. Lett. 131, (2023).

[3] A. J. Leggett, Can a Solid Be “Superfluid”?, Phys. Rev. Lett. 25, 1543 (1970).