各向异性酉费米气体的剪切粘度
The Shear Viscosity in an Anisotropic Unitary Fermi Gas
摘要：
我们考虑了一个由强相互作用的超冷幺正费米气体在谐波约束下组成的系统。我们的分析表明，实验研究的可能性，在这个系统中，各向异性的剪切粘度张量驱动的各向异性的捕获潜力。特别是，我们建议，这个实验装置可以模仿一些功能的各向异性的几何形状，最近已经研究了强耦合场论，其中有一个引力对偶。在这些理论中使用AdS/CFT对应的结果表明，在具有背景线性势的系统中，某些粘度分量可以比熵密度小得多，参数违反Kovtun，Son和Starleman（KSS）提出的界限。这种直觉，以及从玻尔兹曼分析，我们执行的结果表明，违反KSS约束可能会发生在酉费米气体系统时，它受到一个合适的各向异性的捕获势，可以近似为线性的一个合适的参数范围。我们给出了一个具体的建议，实验装置中可能出现的各向异性剪切粘度张量。在这种情况下，也可能观察到剪切粘度的自旋一分量从迄今为止在超冷费米气体中观察到的最低值降低。在极端各向异性的情况下，减少可能足以降低剪切粘度熵比低于建议的KSS的界限，虽然这种制度是难以分析的理论控制的方式。
英文摘要：
We consider a system consisting of a strongly interacting, ultracold unitary Fermi gas under harmonic confinement. Our analysis suggests the possibility of experimentally studying, in this system, an anisotropic shear viscosity tensor driven by the anisotropy in the trapping potential. In particular, we suggest that this experimental setup could mimic some features of anisotropic geometries that have recently been studied for strongly coupled field theories which have a gravitational dual. Results using the AdS/CFT correspondence in these theories show that in systems with a background linear potential, certain viscosity components can be made much smaller than the entropy density, parametrically violating the bound proposed by Kovtun, Son and Starinets (KSS). This intuition, along with results from a Boltzmann analysis that we perform, suggests that a violation of the KSS bound can perhaps occur in the unitary Fermi gas system when it is subjected to a suitable anisotropic trapping potential which may be approximated to be linear in a suitable range of parameters. We give a concrete proposal for an experimental setup where an anisotropic shear viscosity tensor may arise. In such situations, it may also be possible to observe a reduction in the spin one component of the shear viscosity from its lowest value observed so far in ultracold Fermi gases. In extreme anisotropic situations, the reduction may be enough to reduce the shear viscosity to entropy ratio below the proposed KSS bound, although this regime is difficult to analyze in a theoretically controlled manner.
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