Demonstration of the antiferroelectric aspect of the helical superstructures in SmC*, SmC$_α$* and SmC$_a$* liquid crystals

We show that the helical superstructure in chiral smectic-C-type liquid crystal phases can, if the pitch is short enough, give rise to the tristate switching characteristic of antiferroelectrics even if the local commen- surate order is synpolar as in the ordinary Sm-C* phase. Since the field-induced helix unwinding exhibits a distinct threshold, in contrast to mere helix distortion, two unwinding / rewinding peaks per half cycle of an applied triangular wave voltage can be seen in the current response. By considering this antiferroelectric aspect
*
of the helical modulation we give a simple explanation of why the ultrashort-pitch Sm-C􏰄 phase exhibits
antiferroelectric switching although its dielectric spectroscopy response is qualitatively identical to that of the synpolar Sm-C* phase. Using data from the compound MHPOCBC we show that the Sm-C*􏰄 dielectric re- sponse is well described by continuum theory. We also demonstrate that, if the pitch is very short as in MHPOCBC, helix unwinding / rewinding leave characteristic traces in the electrooptic response even in the commensurately antiferroelectric 􏰐antipolar􏰋 Sm-C*a phase, distinguishable from the switching beween the antipolar and synpolar states of this phase.