Recent High-Resolution Neutron Scattering Studies of Liquid ⁴He

Abstract

Neutron scattering has provided much detailed quantitative information on the nature of the elementary excitations of liquid helium and on the interactions between those excitations. It could reasonably be argued that the most significant features of the excitation spectrum of superfluid ⁴He were known by about 1975, following the classic investigations of Cowley and Woods ^1,2 at Chalk River, and of the Brookhaven group ^3,4. The phonon-maxon-roton dispersion curve shown in Fig. 1 had been studied as a function of both temperature and pressure (or equivalently, of density), while the existence of the multiphonon continuum at higher energies had been noted and its relative strength investigated. Nevertheless, neutron scattering studies have continued up to the present day, particularly at Chalk River Nuclear Laboratories and at the Institut Laue-Langevin (I.L.L.). What then was the aim of these second-generation experiments? In their excellent and comprehensive review of neutron scattering studies of solid and liquid helium, published in 1987, Glyde and Svensson⁵ identified several important questions which remained unanswered in the early 1970s. These ranged from uncertainties as to the form of the dispersion relation at small wavevectors to the origin of the oscillations in the peak position and width of the scattering function S(Q, ω) at large wavevector Q and frequency ω. In addition, the relationship, if any, of the unique excitation spectrum to the existence of superfluidity below T_λ remained a question of profound importance.