Direct observation of interlayer molecular translational motion in a smectic phase and determination of the layer order parameter

Abstract

The smectic phase shows a layered structure, which is the most fundamental and universal order found in soft materials. However, previous technical limitations made it difficult to fully and microscopically observe the interlayer molecular motions in the smectic phase. We were able to directly measure the interlayer molecular motions by quasielastic scattering spectroscopy using Mössbauer gamma rays for 4^{′}-n-octyl-4-cyanobiphenyl (8CB). From the measured motions, we evaluated the order parameter of the layer structure and explain the macroscopic diffusion coefficient in the microscopic viewpoint. Our methodology can be applied broadly to determine the layer order parameter of various soft materials.