Thinning transitions and fluctuations of freely suspended smectic-A films as studied by specular and diffuse X-ray scattering

The fluctuations in freely suspended smectic-A films have been studied using specular and diffuse X-ray scattering. Such films thin layer-by-layer above the bulk transition temperature of the smectic-A nematic phase transition. The electron density profiles obtained from specular reflectivity demonstrate that thinning originates from nematic layers being expelled from the interior of the film. In addition, using diffuse scattering the spectral dependence of the displacement-displacement correlation function could be measured down to molecular dimensions, with a (2 + 2) scattering geometry at a synchrotron source. At long in-plane length scales the fluctuations of the smectic layers were found to be conformal; i.e. top and bottom of the film fluctuated in unison. However, at increasing wave vector transfers, conformality was progressively lost.