Thermal fluctuations and positional correlations in oriented lipid membranes.

We have determined the positional correlation functions in aligned stacks of fully hydrated phospholipid bilayers from the thermal diffuse scattering measured by nonspecular x-ray reflectivity. While fair agreement can be obtained between experiment and linear smectic theory at length scales above 120 A, significant deviations occur at small r, which are tentatively attributed to collective protrusion modes.