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Observation of anisotropic superfluid density in an artificial crystal



Ian Spielman, Mingshu Zhao, Junhent Tao


We experimentally and theoretically investigate the anisotropic speed of sound of an atomic superfluid (SF) Bose-Einstein condensate in a 1D optical lattice. Because the speed of sound derives from the superfluid density, implying that this density is itself anisotropic, and can be expressed as a second rank tensor. We find that the speed of sound is decreased by the optical lattice, and the SF density is concomitantly reduced. This reduction is accompanied by the appearance of a normal fluid in the purely Bose condensed phase. The reduction in SF density---first predicted [A.J.Leggett, Phys. Rev. Lett. \bf 25} 1543--1546 (1970)] in the context of supersolidity---results from the coexistence of superfluidity and density modulations, but is agnostic about the origin of the modulations. We additionally measure the moment of inertia of the system in a scissors mode experiment, demonstrating the existence of rotational flow. As such we shed light on some supersolid properties using imposed, rather than spontaneous, formation of density-order.
Physical Review Letters


superfluid , supersolid , Bose-Einstein condensate


Spielman, I. , Zhao, M. and Tao, J. (2023), Observation of anisotropic superfluid density in an artificial crystal, Physical Review Letters, [online], (Accessed July 21, 2024)


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Created October 18, 2023, Updated February 2, 2024