Temperature dependence of the collective excitations of liquid He-4 at high pressure

Systematic high-resolution neutron inelastic scattering measurements have been made of the collective phonon-roton excitations in normal and superfluid He-4 as a function of both pressure (density) and temperature. The IN6 time-of-flight spectrometer at the Institut Laue-Langevin has been used to obtain accurate and continuous data over a wavevector range 0.3 Angstrom(-1) < Q < 2.35 Angstrom(-1) with a resolution of 0.12 meV. We present the experimental data and discuss the energies and widths of the single excitations; changes concomitant with the I transition are discussed. The structure of the multiphonon continuum at pressure is also discussed and is shown to be consistent with theory.