Scientific Results

Second sound and the superfluid fraction in a Fermi gas with resonant interactions

Year: 2013

Authors: Sidorenkov L.A., Tey M.K., Grimm R., Hou Y.H., Pitaevskii L., Stringari S.

Autors Affiliation: Institut für Quantenoptik und Quanteninformation, Ŏsterreichische Akademie der Wissenschaften, 6020 Innsbruck, Austria; Institut für Experimentalphysik and Zentrum für Quantenphysik, Universität Innsbruck, 6020 Innsbruck, Austria; Dipartimento di Fisica, Universita di Trento and INO-CNR BEC Center, I-38123 Povo, Italy; Kapitza Institute for Physical Problems, Russian Academy of Sciences, 119334 Moscow, Russia Università degli Studi di Trento, INOA, University of Innsbruck

Abstract: Superfluidity is a macroscopic quantum phenomenon occurring in systems as diverse as liquid helium and neutron stars. It occurs below a critical temperature(1,2) and leads to peculiar behaviour such as frictionless flow, the formation of quantized vortices and quenching of the moment of inertia. Ultracold atomic gases offer control of interactions and external confinement, providing unique opportunities to explore superfluid phenomena. Many such (finite-temperature) phenomena can be explained in terms of a two-fluid mixture(3,4) comprising a normal component, which behaves like an ordinary fluid, and a superfluid component with zero viscosity and zero entropy. The two-component nature of a superfluid is manifest in \’second sound\’, an entropy wave in which the superfluid and the non-superfluid components oscillate with opposite phases (as opposed to ordinary \’first sound\’, where they oscillate in phase). Here we report the observation of second sound in an ultracold Fermi gas with resonant interactions. The speed of second sound depends explicitly on the value of the superfluid fraction(5), a quantity that is sensitive to the spectrum of elementary excitations(6). Our measurements allow us to extract the temperature dependence of the superfluid fraction, a previously inaccessible quantity that will provide a benchmark for theories of strongly interacting quantum gases.

Journal/Review: NATURE

Volume: 498 (7452)      Pages from: 78  to: 81

More Information: We thank E. R. Sanchez Guajardo for his contributions in the early stage of this work, and P. van der Straten for discussions. The Innsbruck team acknowledges support from the Austrian Science Fund (FWF) within SFB FoQuS (project no. F4004-N16). The Trento team acknowledges support from the European Research Council through the project QGBE and from the Provincia Autonoma di Trento. We dedicate the present work to our late friend and colleague A. Griffin, who enthusiastically promoted the idea of measuring second sound in Fermi gases.
DOI: 10.1038/nature12136

Citations: 125
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