Zero sound

Zero sound is the name given by Landau to the unique quantum vibrations in quantum Fermi liquids.

This sound can no longer be thought of as a simple wave of compression and rarefaction.

As the shape of Fermi distribution function changes slightly(or largely), zero sound propagates in the direction for the head of Fermi surface with no change of the density of the liquid.

References

Piers Coleman, "introduction to many body physics"