Low-frequency dielectric relaxations of a nonchiral liquid crystal, 8CB

Low-frequency dielectric measurements are performed on a nonchiral liquid crystal (8CB) for two different cell thicknesses 4.5 mum and 1.6 mum, as functions of temperature at various amplitudes of the probe field. Relaxation peak frequencies are observed at similar to 10 Hz for the 4.5 mum thick sample and similar to 100 Hz for the 1.6 mum cell. When the external-field amplitude was increased, the relaxation peak frequency was shifted to lower frequency for the 4.5 mum cell but hardly changed for the 1.6 mum cell. To explain this odd result from experimental observations, we performed a numerical simulation, where the ionic impurities were assumed to encounter ionization-recombination processes in addition to a translational diffusion. We were able to confirm that there are two separate contributions from ionic impurities to the low-frequency dielectric relaxations in this system-that is, from fast ions in the single-particle diffusion and slow ions in the ionization-recombination-assisted diffusion.