Heat pulse propagation in liquid⁴He revisited

We have studied the propagation of heat pulses in superfluid helium at 0.1 K. Our experiments agree with a simple picture of a propagating cloud of interacting excitations, rather than with the surface rotons invoked by the Exeter group. The "Q-peak" observed in double pulse experiments is interpreted as a shock wave propagating in the cloud. We have observed for the first time that the Q-peak splits up in two distinct modes at intermediate helium pressures. We believe this is evidence fo a decoupling of the phonon and roton gasses as predicted by Castaing.