The slow short-time motions of phospholipid molecules with a focus on the influence of multiple scattering and fitting artefacts

Quasi-elastic neutron scattering is a powerful tool for the study of non-periodic motions in condensed matter, as a detailed line shape analysis can give information about the geometry and rate of the scatterers' displacements. Unfortunately, there are also a number of artefacts which can masquerade as signatures of motions and can therefore lead to erroneous results. Their influence on the evaluation of the motions of the phospholipid dimyristoylphosphatidylcholine (DMPC) is discussed. On a 60 ps timescale, the long-range motion of the molecules has a flow-like character with similar velocities above and below the main phase transition. It is proposed that the concepts of dynamical heterogeneities and 'floppy modes' developed in glass physics provide a framework for explaining the observed behaviour.