Overdamped Phonons in Fluid Helium at 4 K

Abstract
From inelastic neutron scattering we determine the dynamic structure factor Sq,ω of fluid helium at 4 K and 1 bar as a function of frequency ω for wave numbers 2.5<q<30 nm1. Sq,ω is completely given by only two phonon contributions located at the sound propagation frequencies ±ωsq. For 18<q<21 nm1 they are overdamped, i.e., ωsq=0.