COLLECTIVE AND NONCOLLECTIVE EXCITATIONS IN ANTIFERROELECTRIC AND FERRIELECTRIC LIQUID-CRYSTALS STUDIED BY DIELECTRIC-RELAXATION SPECTROSCOPY AND ELECTROOPTIC MEASUREMENTS

The dynamics of different molecular modes in four antiferroelectric li quid crystal substances have been studied by a combination of spectros copic methods. The fastest motion is the reorientation around the mole cular long axis, here found in the low GHz range by time domain spectr oscopy. The reorientation around the short axis has a characteristic f requency of about 10kHz and is detected by frequency domain spectrosco py in the homeotropic configuration. As for the collective excitations , the Goldstone and soft modes, characteristic of the ferroelectric ph ase, have counterparts in the antiferroelectric phase which appear ver y different. There are two characteristic peaks in the spectrum, one a t high frequency, about 100kHz, the other at low frequency, about 10kH z. The latter has often been mistaken for short axis reorientation and both have been attributed to soft modes. By combining different exper imental techniques and different geometries it can be shown that neith er is a soft mode, but both are collective modes of different characte r: the high frequency mode corresponds to fluctuations where molecules in neighbouring layers are moving in opposite phase, the low frequenc y mode to phase fluctuations in the helicoidal superstructure. In mate rials exhibiting a C phase in addition to the C-a* or C-gamma* 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 here ditary peak, has nothing to do with the antiferroelectric or ferrielec tric order, but is just the Goldstone peak from a coexisting C phase. In the same way, a Goldstone mode peak from the C-gamma phase may ap pear in the underlying C-a phase. In a general way, narrow phases lik e C-gamma, being bounded by first order transitions on both sides (C- a-C-gamma*-C*) are likely to show non-characteristic (hereditary) pea ks from both adjacent phases.