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arXiv:0910.4326 · Lectures given at

Dissipative superfluids, from cold atoms to quark matter

Massimo Mannarelli, Cristina Manuel

Published Oct 22, 2009• 2 citations
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Abstract

Some results about dissipative processes in superfluids are presented. We focus on fermionic superfluidity and restrict our analysis to the contribution of phonons to bulk viscosity, shear viscosity and thermal conductivity. At sufficiently low temperatures phonons give the dominant contribution to the transport coefficients if all the other low energy excitation of the system are gapped. We first consider a system of cold fermionic atoms close to the unitarity limit. Then we turn to the superfluid phase of quark matter that may be realized at high baryonic density.

DOI
10.48550/arxiv.0910.4326

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Some results about dissipative processes in superfluids are presented. We focus on fermionic superfluidity and restrict our analysis to the contribution of phonons to bulk viscosity, shear viscosity and thermal conductivity. At sufficiently low temperatures phonons give the dominant contribution to the transport coefficients if all the other low energy excitation of the system are gapped. We first consider a system of cold fermionic atoms close to the unitarity limit. Then we turn to the superfluid phase of quark matter that may be realized at high baryonic density.
We compute the coefficients of bulk viscosity for a nonrelativistic superfluid corresponding to a fermionic system close to the unitarity limit. We consider the low temperature regime assuming that the transport properties of the system are dominated by phonons. To compute the coefficients of bulk viscosity we use kinetic theory in the relaxation time approximation and the low energy effective field theory of the corresponding system. We show that the three independent bulk viscosity coefficients, ${\ensuremath{\zeta}}_{1},{\ensuremath{\zeta}}_{2},{\ensuremath{\zeta}}_{3}$, associated with irreversible flows vanish for phonons with a linear dispersion law. Considering a phonon dispersion law with a cubic term in momentum we find that in the conformal limit ${\ensuremath{\zeta}}_{1}={\ensuremath{\zeta}}_{2}=0$, while ${\ensuremath{\zeta}}_{3}$ is nonzero. Including a conformal breaking term that arises for a large but finite sss-wave scattering length, aaa, at the leading order in 1/a1/a1/a we obtain that ${\ensuremath{\zeta}}_{1}\ensuremath{\propto}1/a$ and ${\ensuremath{\zeta}}_{2}\ensuremath{\propto}1/{a}^{2}$.
In this paper, we examine in a unified fashion dissipative transport in strongly correlated systems. We thereby demonstrate the connection between "bad metals" (such as the high temperature superconductors) and "perfect fluids" (such as the ultracold Fermi gases, near unitarity). One motivation of this work is to communicate to the high energy physics community some of the central unsolved problems in high TcT_cTc​ superconductors. Because of interest in the nearly perfect fluidity of the cold gases and because of new tools such as the AdS/CFT correspondence, this better communication may lead to important progress in a variety of different fields. A second motivation is to draw attention to the great power of transport measurements which more directly reflect the excitation spectrum than, say, thermodynamics and thus strongly constrain microscopic theories of correlated fermionic superfluids. Our calculations show that bad metal and perfect fluid behavior is associated with the presence of a normal state excitation gap which suppresses the effective number of carriers leading to anomalously low conductivity and viscosity above the transition temperature TcT_cTc​. Below TcT_cTc​ we demonstrate that the condensate collective modes ("phonons") do not couple to transverse probes such as the shear viscosity. As a result, our calculated shear viscosity at low TTT becomes arbitrarily small as observed in experiments. In both homogeneous and trap calculations we do not find the upturn in ηηη or η/sη/sη/s (where sss is the entropy density) found in most theories. In the process of these studies we demonstrate compatibility with the transverse sum rule and find reasonable agreement with both viscosity and cuprate conductivity experiments.
We compute the shear viscosity of a superfluid atomic Fermi gas in the unitarity limit. The unitarity limit is characterized by a divergent scattering length between the atoms, and it has been argued that this will result in a very small viscosity. We show that in the low temperature TTT limit the shear viscosity scales as ${\ensuremath{\xi}}^{5}∕{T}^{5}$, where the universal parameter $\ensuremath{\xi}$ relates the chemical potential and the Fermi energy, $\ensuremath{\mu}=\ensuremath{\xi}{\ensuremath{\epsilon}}_{F}$. Combined with the high temperature expansions of the viscosity our results suggest that the viscosity has a minimum near the critical temperature Tc{T}_{c}Tc​. A na\"{\i}ve extrapolation indicates that the minimum value of the ratio of viscosity over entropy density is within a factor of $\ensuremath{\sim}5$ of the proposed bound $\ensuremath{\eta}∕s\ensuremath{\ge}\ensuremath{\hbar}∕(4\ensuremath{\pi}{k}_{B})$.
Transport coefficients of superfluid dipolar Fermi gas are calculated by using the Boltzmann equation approach. The interaction between Bogoliubov-Bogoliubov quasiparticles in the collision integral is considered in binary process. The shear viscosities nxz↑ = nyy↑,nxy↑; nxz↑ ≈ nyz↑; and nu↑ are proportional to T-6; T-8; and T-10 , respectively. Also, we have found the elements of the diffusive thermal conductivities Kxx↑= Kyy↑ with temperature dependency T-5 and Ku↑ which is proportional to T-7 and other components which are zero.
We derive the Boltzmann equation describing long-wavelength low-frequency transport and relaxation processes in a superfluid Fermi liquid close to the transition temperature Tc{T}_{c}Tc​. In the superfluid, the quasiparticle number is not conserved and therefore one has to take into account the decay of one quasiparticle into three and the inverse process, as well as the two-quasiparticle scattering process. We calculate the collision term in the Boltzmann equation to first order in the superfluid gap $\ensuremath{\Delta}$ and show that it is related rather simply to the corresponding normal-state collision term. As applications of the Boltzmann equation, we solve it exactly to calculate the shear viscosity, and the intrinsic spin relaxation rate. The shear viscosity drops as $\ensuremath{\Delta}\ensuremath{\propto}{({T}_{c}\ensuremath{-}T)}^{\frac{1}{2}}$ for temperatures just below Tc{T}_{c}Tc​, and we determine the coefficient of $\ensuremath{\Delta}$ as a function of the normal-state collision probability. Leggett and Takagi's characteristic spin-relaxation time is shown to be equal to the relaxation time of a normal-state quasiparticle at the Fermi energy at Tc{T}_{c}Tc​. The results provide one with a useful consistency check on experimental measurements which is independent of any assumption about the normal-state collision probability.