Following shortly after my previous journal entry on recent progress in tightening the upper bound on the shear viscosity-to-entropy density ratio (η/s) of the quark-gluon plasma, the competition has struck back. In a letter appearing in Science today
Cao et al. report results from new precision measurements of the η/s ratio in an ultracold gas of trapped fermionic 6Li atoms in the universal regime as a function of temperature.
The shear viscosity is obtained either by observing the time evolution of the aspect ratio of the atom cloud after its release from the trap or by studying the breathing mode of the atom cloud and comparing the observations with hydrodynamic simulations. At high temperature, the viscosity is found to increase like T3/2 as predicted by kinetic theory. At the lowest temperatures reached in the experiment, the ratio reaches values in the range 0.2 < η/s < 0.4. The experiment reached energies per particle E ≈ 0.55 EF, which is about 15% above the ground state energy but below the critical temperature of the Fermi gas, which becomes a superfluid below approximately 0.7 EF.
It is not yet clear how η goes to zero as T → 0. The experimentalists do not see a 1/T2 rise in η at low T as simple two-body physics would predict, but Pauli blocking of 2-body scattering is suppressed by pairing, which can exist above the superfluid transition. The authors also note that the apparent decrease of the η/s ratio as the energy approaches the ground state energy can be influenced by contributions from the cloud edges, which make an increasing contribution to the total entropy as the temperature is reduced.