Laboratoire de Physique Théorique
Toulouse - UMR 5152

I will discuss sound in one-dimensional Galilean invariant quantum liquids. In contrast to classical fluids, instead of a single sound mode in a broad range of frequencies the system supports two sound modes. The nature of these modes depends on the type of particles forming the quantum liquid. In systems of spin-1/2 fermions, these are the first and second sound modes propagating at two different speeds. Similarly to superfluid helium, the first sound is a wave of density, whereas the second one is a wave of entropy. On the other hand, in spinless quantum liquids the two sound modes have a hybrid nature : they are simultaneous oscillations of both density and entropy. Their speeds are nearly equal, with the difference scaling linearly with the temperature.