Direct evidence for interactions between elementary excitations in superfluid 4He by Raman scattering

In the Raman spectrum of superfluid 4He under saturated vapour pressure at 1.51K, fine structures have been observed at energy shifts closely corresponding to twice the energies of the extremal points of the phonon-roton dispersion curve. From the observed cusps, twice the energies of the maximum and plateau regions of the dispersion curve have been determined to be 2 Delta 1=27.5+or-0.5K and 2 Delta 2=34.8+or-0.5K respectively. The two-roton bound state has been resolved as a pronounced single peak, and twice the roton energy and the attractive binding energy of the two-roton state have been determined to be 2 Delta 0=17.1+or-0.5K and EB=0.5+or-0.3K respectively. The observed fine structures clearly show the existence of the interactions between two elementary excitations having opposite and equal wavevectors.