content_image/372081.jpg
becomes significantly smaller, however. 

## VII. CONCLUSIONS 

<img src='content_image/372081.jpg'>

In this paper, we have given a detailed analysis of the density response function χ$_{nn}$ ( q, ω ) and the related dynamic structure factor S ( q, ω ) for uniform superfluids in the two-fluid hydrodynamic limit. We have put special emphasis on the second sound contribution to S ( q, ω ) in the case of a strongly interacting Fermi gas superfluid (at unitarity). While many features of the dynamic structure factor we discuss are known in the superfluid $^{4}$He literature [13, 14], several new aspects arise in the case of ultracold superfluid gases. 

First and second sound both appear as resonances in the density response function but their relative weights vary in different experimental probes. Two-photon Bragg scattering, a 

