Demonstration of the antiferroelectric aspect of the helical superstructures in Sm-C*, Sm-Cα*, and Sm-Ca*liquid crystals

Abstract

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 commensurate 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*alpha 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*alpha dielectric response 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 between the antipolar and synpolar states of this phase.