TEMPERATURE-DEPENDENCE OF THE PHONON AND ROTON EXCITATIONS IN LIQUID-HE-4

We report high-precision neutron scattering measurements of the temperature dependence of the phonon and roton excitations in liquid He4 at saturated vapor pressure in both the superfluid and normal-fluid phases. Two wave vectors were examined in detail: Q=0.4 A-1 in the phonon region and Q=1.925 A-1 at the roton minimum. The goal is to understand excitations in Bose liquids by determining the dynamic structure factor S(Q,) accurately in both phases. At Q=0.4 A-1 the sharp peak in S(Q,) broadens with temperature T, but remains sharp and well defined in the normal phase (T>T) up to T3 K. The rate of increase of the width of S(Q,) shows a break at T, but otherwise S(Q,) is similar in the two phases and a transition could not be identified through S(Q,). In contrast at Q=1.925 A-1, the sharp component of S(Q,) disappears at T and S(Q,) is very different above and below T. This behavior is documented and values of the frequencies and linewidths at several temperatures are reported. © 1990 The American Physical Society.