Regensburg 2010 – scientific programme

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TT: Fachverband Tiefe Temperaturen

TT 15: MLT: Quantum Liquids, Bose-Einstein Condensates, Ultra-cold Atoms, ... 1

TT 15.1: Talk

Tuesday, March 23, 2010, 14:00–14:15, H18

Transport and scale invariance in the unitary Fermi gas — •Tilman Enss and Wilhelm Zwerger — Technische Universität München, Germany

The string theory conjecture of a universal lower bound on the shear viscosity to entropy ratio, characterizing a perfect fluid, has stimulated work from nuclear physics to ultracold atoms. We compute the bulk and shear viscosities of a strongly interacting Fermi gas in three dimensions above the superfluid transition temperature using a diagrammatic technique. It is shown that vertex corrections and the associated Aslamazov-Larkin contributions are crucial to reproduce the correct high-temperature limit and the vanishing of the bulk viscosity due to scale invariance. The resulting shear viscosity to entropy ratio has a minimum value above the superfluid transition that is a factor of five above the string theory bound. The results are compared to recent experimental data on the damping of collective modes of a trapped unitary Fermi gas.