PHONONS AND ROTONS IN LIQUID-HE-4 NEAR THE SUPERFLUID TRANSITION

We determine the behavior of the excitations in liquid He-4, covering the normal and superfluid phases, for a range of temperatures (1 < T < 4.2 K), wave numbers (0.1 < q < 2.2 Angstrom(-1)) and pressures (0 < p < 25 bar). We find that, for all q and p, the T dependence of the ne utron scattering data can be well described by using only one adjustab le parameter, the damping rate of the momentum fluctuations. We establ ish a close similarity between classical liquids, normal-fluid He-4 an d superfluid He-4. For all q-values, there is a marked change with tem perature in the damping rate of the momentum fluctuations, with the ch ange occurring predominantly in a small temperature region just below the superfluid-transition temperature T-lambda. We argue that the rapi d change in this damping rate near T-lambda can be attributed to the r equirements imposed on the He-4-wave function by Bose symmetry. (C) 19 98 Elsevier Science B.V. All rights reserved.