  The existence of the phonon-roton minimum has been widely observed for both
the solid and liquid phases but so far there is no sufficient theoretical
explanation of its origin. In this paper we use a range of techniques to study
the dynamics and short-range order for a range of simple materials in their
crystalline, amorphous and liquid phases. We perform inelastic neutron
scattering (INS) experiments of polycrystalline and liquid barium to study the
high-frequency dynamics and understand the mechanisms underlying the atomic
motion. Moreover we perform INS simulations for crystals and supercooled
liquids, compare the collective excitation spectra and identify similarities.
We perform molecular dynamics (MD) simulations for the same materials and
present results of population and bond angle distribution showing a short-range
order dependence of the different phases, expanding the current knowledge in
literature. Finally, we support our findings with a theoretical explanation of
the origin of the phonon-roton minima which is observed in both solids and
liquids. We study this as a classical phenomenon and we base our explanation on
short range interatomic interactions.
