Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Second sound and the superfluid fraction in a resonantly interacting
  Fermi gas   Superfluidity is a macroscopic quantum phenomenon, which shows up below a
critical temperature and leads to a peculiar behavior of matter, with
frictionless flow, the formation of quantized vortices, and the quenching of
the moment of inertia being intriguing examples. A remarkable explanation for
many phenomena exhibited by a superfluid at finite temperature can be given in
terms of a two-fluid mixture comprised of a normal component that behaves like
a usual fluid and a superfluid component with zero viscosity and zero entropy.
Important examples of superfluid systems are liquid helium and neutron stars.
More recently, ultracold atomic gases have emerged as new superfluid systems
with unprecedented possibilities to control interactions and external
confinement. Here we report the first observation of `second sound' in an
ultracold Fermi gas with resonant interactions. Second sound is a striking
manifestation of the two-component nature of a superfluid and corresponds to an
entropy wave, where the superfluid and the non-superfluid components oscillate
in opposite phase, different from ordinary sound (`first sound'), where they
oscillate in phase. The speed of second sound depends explicitly on the value
of the superfluid fraction, a quantity sensitive to the spectrum of elementary
excitations. Our measurements allow us to extract the temperature dependence of
the superfluid fraction, which in strongly interacting quantum gases has been
an inaccessible quantity so far.

13
The categories of this paper are: quant-ph
37
The categories of this paper are: supr-con
25
The categories of this paper are: patt-sol
5
The categories of this paper are: gr-qc
2
The categories of this paper are: math
10
The categories of this paper are: q-bio
9
The categories of this paper are: nlin
34
The categories of this paper are: chem-ph
32
The categories of this paper are: atom-ph
27
The categories of this paper are: comp-gas
22
The categories of this paper are: chao-dyn
35
The categories of this paper are: mtrl-th
11
The categories of this paper are: hep-lat
16
The categories of this paper are: stat
28
The categories of this paper are: alg-geom
21
The categories of this paper are: adap-org