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I am trying to get a handle on the phenomenon of second sound. It most famously occurs in superfluids at finite temperature. Here, according to Landau, superfluid phonons (which exist at zero temperature and constitute 'first sound') are excited by thermal vibrations and thus form a weakly-coupled gas that admits sound waves of its own, corresponding to a second sound wave.

Question 1: Why do we not see second sound in solids? I am aware that in certain very special conditions they are observed, but in the vast majority of solids, they are not. Is this because they are actually not present or are they just very hard to detect in most solids?

Question 2: Are there ever instances of finite-temperature superfluids without second sound modes? If so, under what conditions does this happen?

References would be much appreciated

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  • $\begingroup$ I think one possible ingredient in the answer is that electrons carry entropy in normal solids, so you don't get ballistic transport of phonons, but diffusive transport due to coupling of vibrations to electronic excitations. In the case of the superfluid, the Bose condensate (electron pairs or helium atoms) do not carry entropy, so you diffusive transport is suppressed and you get ballistic second sound. $\endgroup$
    – KF Gauss
    Commented Jan 24, 2020 at 3:48

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