Scale-Invariant Hydrodynamics and Quantum Viscosity in Fermi Gases*

An optically-trapped gas of spin 1/2-up and spin 1/2-down $^{\mathrm{6}}$Li atoms, tuned near a collisional (Feshbach) resonance, provides a unique paradigm for testing predictions that cross interdisciplinary boundaries, from high temperature superconductors to nuclear matter. At resonance, the dilute atomic cloud becomes the most strongly interacting, non-relativistic fluid known: Shock waves are produced when two clouds collide. We observe scale-invariant hydrodynamic expansion of a resonantly interacting gas and determine the quantum shear viscosity $\eta =\alpha {\kern 1pt}\hbar n$, with n the density, as a function of interaction strength and temperature, from nearly the ground state through the superfluid phase transition. We extract the local shear viscosity coefficient $\alpha {\kern 1pt}$ from cloud-averaged data, using iterative methods borrowed from image processing, and observe previously hidden features, which are compared to recent predictions.\\[4pt] In collaboration with Ethan Elliott and James Joseph, Physics Department, North Carolina State University.