James Franck Institute and Department of Physics, University of Chicago, Chicago, Illinois 60637, USA.
Phys Rev Lett. 2011 Jul 8;107(2):020403. doi: 10.1103/PhysRevLett.107.020403. Epub 2011 Jul 6.
Recent experiments on the shear viscosity η in a unitary Fermi gas fail to see the theoretically predicted upturn in η at the lower T. In this Letter, we compute η in a fashion which is demonstrably consistent with conservation laws and, in the process, provide an understanding of recent experiments. We show that this disagreement with prior theories cannot be readily attributed to the trap, since (via edge effects) trap-averaged viscosities will be larger than their homogeneous counterparts. The small values of η we find can be simply understood; they reflect the fact that the Goldstone bosons (phonons) do not couple to transverse probes such as η, and fermionic excitations, which determine the viscosity, are necessarily absent in the ground state.
最近在幺正费米气体中的剪切黏度 η 的实验未能观察到理论预测的在较低温度下 η 的上升。在这封信中，我们以一种明显符合守恒定律的方式计算了 η，在此过程中，我们对最近的实验有了更深入的了解。我们表明，这种与先前理论的分歧不能轻易归因于陷阱，因为（通过边缘效应），平均黏度会大于其均匀对应的黏度。我们发现 η 值较小可以简单理解；这反映了这样一个事实，即 Goldstone 玻色子（声子）与 η 等横向探针没有耦合，而决定黏度的费米子激发在基态中必然不存在。
