Seeing the Superfluid Transition of a Gas

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Science  03 Feb 2012:
Vol. 335, Issue 6068, pp. 549-550
DOI: 10.1126/science.1218074

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An important characteristic of a superfluid is the critical temperature Tc, below which it forms from an ordinary fluid, but a more detailed understanding comes from measuring other thermodynamic and quantum-mechanical parameters. For systems in which the superfluid is created by pairing fermions (for example, the electron superfluid in superconductors), Tc is often very low compared to another characteristic temperature of mobile fermions, their Fermi temperature TF. Thus, superconductivity has not been observed at room temperature. On page 563 of this issue, Ku et al. (1) present precise measurements of thermodynamic quantities of an ultracold gas of lithium-6 atoms—fermions—with strong attractive interactions that explicitly show the superfluid transition through changes in the compressibility and the heat capacity. The absolute temperatures here are on the order of only 100 nK, but this Fermi gas provides an example of a high-temperature superfluid: No other system has ever been observed with a transition temperature as high as ∼16% of TF. Scaled to the density of electrons in metals, Tc would occur far above room temperature.