Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Trapped phonons   We analyze the effect of restricted geometries on the contribution of
Nambu-Goldstone bosons (phonons) to the shear viscosity, $\eta$, of a
superfluid. For illustrative purpose we examine a simplified system consisting
of a circular boundary of radius $R$, confining a two-dimensional rarefied gas
of phonons. Considering the Maxwell-type conditions, we show that phonons that
are not in equilibrium with the boundary and that are not specularly reflected
exert a shear stress on the boundary. In this case it is possible to define an
effective (ballistic) shear viscosity coefficient $\eta \propto \rho_{\rm ph}
\chi R$, where $\rho_{\rm ph}$ is the density of phonons and $\chi$ is a
parameter which characterizes the type of scattering at the boundary. For an
optically trapped superfluid our results corroborate the findings of Refs.
\cite{Mannarelli:2012su, Mannarelli:2012eg}, which imply that at very low
temperature the shear viscosity correlates with the size of the optical trap
and decreases with decreasing temperature.

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The categories of this paper are: physics
37
The categories of this paper are: supr-con
18
The categories of this paper are: econ
15
The categories of this paper are: nucl-ex
6
The categories of this paper are: astro-ph
9
The categories of this paper are: nlin
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The categories of this paper are: cs
29
The categories of this paper are: funct-an
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The categories of this paper are: hep-th
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The categories of this paper are: math
16
The categories of this paper are: stat
11
The categories of this paper are: hep-lat
36
The categories of this paper are: cmp-lg
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The categories of this paper are: comp-gas
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The categories of this paper are: plasm-ph
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The categories of this paper are: atom-ph