  We report a high energy-resolution neutron backscattering study, combined
with in-situ diffraction, to investigate slow molecular motions on nanosecond
time scales in the fluid phase of phospholipid bilayers of
1,2-dimyristoyl-sn-glycero-3-phoshatidylcholine (DMPC) and DMPC/40% cholesterol
(wt/wt). A cooperative structural relaxation process was observed. From the
in-plane scattering vector dependence of the relaxation rates in hydrogenated
and deuterated samples, combined with results from a 0.1 microsecond long all
atom molecular dynamics simulation, it is concluded that correlated dynamics in
lipid membranes occurs over several lipid distances, spanning a time interval
from pico- to nanoseconds.
