 |
Center for Nonlinear Studies |
Kinetic theory has shown that in normal scale-invariant systems, the shear viscosity is proportional to the pressure and their ratio is the relaxation time. This is an example of a relation connecting thermodynamic quantities (the pressure) and transport coefficients (the shear viscosity). The unitary Fermi gas consists of two-component fermions on the verge of forming two-body bound states and is an example of a scale-invariant system. At low temperatures, the presence of superfluid can change the relation. By implementing a gauge-invariant linear response theory, we found the shear viscosity is related not only to the pressure and relaxation time, but also to the superfluid density and an additional response function from the shear momentum transfer via the Cooper pairs. We have tested the relation with and without pairing fluctuations which are crucial in describing the BCS-BEC crossover and reached qualitatively the same conclusion. Moreover, the relation works in the presence of population imbalance between the two components. A direct measurement of the relaxation time in quantum gases can be challenging, and the relation may offer a constraint for determining it.
Host: Shizeng Lin