X-ray study of the correlations in the thermal fluctuations of free-standing smectic-A films

The first quantitative experimental study is reported of the displacement-displacement correlations in the thermal fluctuations of freely suspended smectic-A films, performed by combining specular and diffuse x-ray scattering. For the first time we are able to separate the long wavelength thermal fluctuations from the local smectic disorder, and obtain a direct measure of the smectic bend and compression elastic constants as well as the surface tension. The local contribution to the total fluctuation profile is found to be considerable. The results are well described by the theory of Holyst.