Director-layer fluctuations in the smectic Z_A phase of a ferroelectric nematic liquid crystal

The antiferroelectric smectic-Z (SmZ_A) phase occurs as a function of temperature between para- and ferroelectric nematic phases in certain liquid crystalline compounds [Chen et al., PNAS USA 120 (8), e2217150120 (2023), Nacke et al., Sci. Rep. 14, 4473 (2024)]. The SmZ_A phase is characterized by an alternating polarization field that runs parallel to the smectic layer planes and alternates between them, with a spatial periodicity of ~10 nm. The nematic director coincides with the polarization field so that the SmZ_A phase also realizes the limit of a SmC with the optical axis tilted by 90° from the layer normal. Here we report a dynamic light scattering study of coupled director-layer fluctuations in the SmZ_A phase of two liquid crystals – one a pure compound (DIO), and the other a room temperature ferroelectric nematic mixture. We compare our results to a hydrodynamic model based on the elastic energy of a SmC and an approximation of the viscous dissipation to that for a uniaxial fluid (i.e., weak biaxiality assumption). We obtain estimates of the layer compression and tilt elastic constants that are ~two orders of magnitude smaller than for a typical smectic-A liquid crystal, and discuss the possible effects of the layer polarization on certain viscoelastic parameters.



This research was supported by the NSF grant DMR-2210083 and DMR-2341830.