Second sound and the superfluid fraction in a resonantly interacting\n Fermi gas

Superfluidity is a macroscopic quantum phenomenon, which shows up below a\ncritical temperature and leads to a peculiar behavior of matter, with\nfrictionless flow, the formation of quantized vortices, and the quenching of\nthe moment of inertia being intriguing examples. A remarkable explanation for\nmany phenomena exhibited by a superfluid at finite temperature can be given in\nterms of a two-fluid mixture comprised of a normal component that behaves like\na usual fluid and a superfluid component with zero viscosity and zero entropy.\nImportant examples of superfluid systems are liquid helium and neutron stars.\nMore recently, ultracold atomic gases have emerged as new superfluid systems\nwith unprecedented possibilities to control interactions and external\nconfinement. Here we report the first observation of `second sound' in an\nultracold Fermi gas with resonant interactions. Second sound is a striking\nmanifestation of the two-component nature of a superfluid and corresponds to an\nentropy wave, where the superfluid and the non-superfluid components oscillate\nin opposite phase, different from ordinary sound (`first sound'), where they\noscillate in phase. The speed of second sound depends explicitly on the value\nof the superfluid fraction, a quantity sensitive to the spectrum of elementary\nexcitations. Our measurements allow us to extract the temperature dependence of\nthe superfluid fraction, which in strongly interacting quantum gases has been\nan inaccessible quantity so far.\n