Investigation of anisotropy in the collective dynamics and thermal diffusivity in the phenyl pyrimidine liquid crystal

Liquid crystals (LCs) are unique heat transfer media that exhibit large anisotropy in thermal properties depending on the direction of the director (n) of the LC state. This phenomenon is the basis of thermal conduction in ordered soft materials, and has been extensively studied to understand thermal conduction in soft materials. In this study, we used inelastic x-ray scattering (IXS) to investigate the anisotropic thermal diffusivity and collective dynamics in the terahertz region of a pyrimidine-type LC (PYP8O8). The dispersion relations at several frequencies corresponding to the LC state were analyzed in the directions parallel and perpendicular to n. The results showed that there was a significant difference between the anisotropy of the sound velocity and that of the attenuation coefficient (P) estimated from the shape parameter of the IXS spectrum. The results for the linewidth of inelastic excitation show Q2 dependence that is far beyond the viscoelastic relaxation of the system, and the anisotropy in the collective dynamics predicted by Forster et al. [Phys. Rev. Lett. 26, 1016 (1971)] in the hydrodynamic region can be extensively applied to the more localized collective motion of LC systems.