Roton second sound and the roton-roton interaction potential

We report measurements of the roton second sound velocity and absorption at high pressure and low temperature. The velocity measurements show the existence of a dip over a very small range of temperature. Comparing this to the absorption measurements, we show that this dip corresponds to a pure roton second sound regime, which is at a constant number of rotons, in agreement with Maris' predictions. The departure toward the constantchemical-potential regime gives us an estimate of the characteristic frequency of the non-roton-conserving process. The absorption measurements give a roton-roton collision frequency in good agreement with previous measurements at higher temperature. In order to explain its value, we have proposed a new interaction potential between rotons, taking into account the exchange of virtual rotons between them. We then obtain very good agreement with experiment, without any adjustable parameter.Laboratoire associé au Centre National de la Recherche Scientifique.