Final-state effects and momentum distribution in normal liquid 4He

Abstract

We have performed time-of-flight neutron scattering experiments, measuring the dynamic structure factor S( Q,ω) of liquid 4He with high resolution and very good statistical precision. A continuous range of wavevectors Q from 3 to 17 Å -1 was covered at T=2.5 K, while at T =1.42K the Q-range from 2 to 10Å -1 was covered. As part of a lengthy and careful data reduction procedure, the TOF data were rebinned onto constant- Q representations for use in the subsequent data analysis. Taking instrumental resolution into account, clear oscillations in both the width and position of the recoil peak are observed at both the measured temperatures up to a wavevector in excess of 12Å -1, which are caused by coherence effects. The measured S( Q, ω) is analysed using a recent expression by Glyde, which expresses the intermediate dynamic structure factor in powers of time. Very good agreement is obtained by fitting the normal fluid data to his expression with three free parameters related to the final-state effects and the central moments of the momentum distribution. The momentum distribution in the normal fluid state is found to be significantly non-Gaussian and agrees well with PIMC calculations of the momentum distribution. The extracted value for the mean kinetic energy per atom 〈 K〉 is in good agreement with previous neutron and X-ray scattering measurements.