Collective excitations in liquid deuterium: Neutron-scattering and correlated-density-matrix results

The collective dynamics of liquid deuterium is studied by means of inelastic neutron scattering (INS) at 20.14 K and under 2 bars of applied pressure using two different incident energies. The low-incident-energy (high-energy-resolution) spectra are studied in detail by means of a time-of-flight (TOF) technique, whereas the finite-frequency response is investigated using a higher incident energy on a triple-axis spectrometer (TAS). The main finding of our work is the observation by the latter experimental technique of well-defined collective excitations up to a momentum transfer of 3.8 A${\mathrm{̊}}^{\mathrm{-}1}$. Comparisons are established with the dispersive behavior observed in other liquid systems as well as with the predictions from recent correlated-density-matrix calculations.