High-resolution measurements of excitations in superfluid ⁴He beyond the roton

High-resolution neutron inelastic scattering studies of the wavevector and temperature dependence of the excitation spectrum of ⁴He in the superfluid and normal-fluid phases have been made in the wavevector region `beyond the roton', 2.0<Q<3.6 Å^-1, at a pressure of 20 bar. The scattering in this region exhibits two separate components: a sharp peak at low energies and a broader continuum at higher energies. The relative intensity of these two peaks is Q-dependent. We find that the two-roton energy, 2Δ, is never exceeded by the sharp peak at this pressure. The data are compared with the density-quasiparticle model of Glyde and Griffin and a tentative analysis of the temperature dependence of the condensate fraction n₀ is made.