Crystalline smectic-B films as fluctuating systems: static and dynamic x-ray scattering.

Static and dynamic x-ray scattering has been used to characterize thermal fluctuations in free-standing smectic films in the crystalline-B phase. The results are remarkably similar to those in smectic-A films, in spite of the three-dimensional positional order in the crystalline-B phase. The main difference is the disappearance of the characteristic Landau-Peierls instability of the smectic-A phase. The dynamic nature of the fluctuations is explicitly demonstrated by x-ray photon correlation spectroscopy. This fluctuation behavior of crystalline-B films can be attributed to the small value of the shear elastic constant C44.