Collective and non-collective excitations in antiferroelectric and ferrielectric liquid crystals studied by dielectric relaxation spectroscopy and electro-optic measurements, Liquid Crystals, 23:5, 723-739,The dynamics of diVerent molecular modes in four antiferroelectric liquid crystal substances have been studied by a combination of spectroscopic methods.The fastest motion is the reorientation around the molecular long axis, here found in the low GHz range by time domain spectroscopy. The reorientation around the short axis has a characteristic frequency of about 10 kHz and is detected by frequency domain spectroscopy in the homeotropic con® guration. As for the collective excitations, the Goldstone and soft modes, characteristic of the ferroelectric phase, have counterparts in the antiferroelectric phase which appear very diVerent. There are two characteristic peaks in the spectrum, one at high frequency, about 100 kHz, the other at low frequency, about 10 kHz. The latter has often been mistaken for short axis reorientation and both have been attributed to soft modes. By combining diVerent experimental techniques and diVerent geometries it can be shown that neither is a soft mode, but both are collective modes of diVerent character: the high frequency mode corresponds tō uctuations where molecules in neighbouring layers are moving in opposite phase, the low frequency mode to phase¯uctuations in the helicoidal superstructure. In materials exhibiting a C* phase in addition to the C * a or C * c phases, an additional strong peak appears in at least one lower-lying phase adjacent to the C* phase. We show that this peak, which we call a hereditary peak, has nothing to do with the antiferroelectric or ferrielectric order, but is
We have studied the three-component liquid crystal mixture reported to exhibit 'thresholdless antiferroelectricity' [Inui et al., J. Mater. Chem., 1996, 6, 671]. We find that the thresholdless or V-shaped switching is obtained in the absence of antiferroelectricity. This analog electro-optic response is due to the field-induced switching of a twisted smectic C* structure stabilized by polar surface interactions and by electrostatic bulk polarization charge interactions. The latter confine the director twist to thin surface regions leaving the bulk of the cell uniform, which gives good extinction at zero field. In sufficiently thin cells, such thresholdless switching can in fact be followed down to much lower temperatures, where the bulk would be antiferroelectric, but is maintained in the cells in the ferroelectric state by hysteresis from surface action.second order and relatively sharp, while the other transitions
The frequency and temperature dependence of the complex dielectric constant was measured near the smectic-C -smectic-A liquid-crystal phase (Sm-C*-Sm-Aj transition for two ferroelectric liquid crystals with a very large spontaneous polarization. The dielectric strengths as well as the corresponding relaxation frequencies of the dielectric modes were determined. Due to the large polarization we were able to resolve the contributions from the soft mode and the Goldstone mode in a comparatively large temperature interval in the Sm-C phase close to the transition to the Sm-A phase. In the Sm-A phase, the relaxation frequency and the inverse dielectric strength of the soft mode decrease linearly when approaching T, . Comparing the experimental data with a recent theoretical calculation of the complex dielectric constant, we conclude that we have a fairly good understanding of the behavior of the complex dielectric constant in the whole Sm-C -Sm-A transition range.
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