Hydrodynamics of rotons below 1 °K: Theory and experiments

Using a heat-pulse technique we study the decoupling of phonons and rotons at low temperature in superfluid ⁴He under pressure (0-25 bar). We show that the phonons and the rotons behave as two independent fluids at low temperature. The line shapes, the time of flight, and the linewidths are in good agreement with our theory, using the Khalatnikov theory for the phonon-roton interaction and a dipole-dipole interaction between rotons.