The dependence on the helical pitch of the antiferroelectric dielectric modes

In an attempt to determine the physical nature of the low-frequency dielect ric mode in antiferroelectric liquid crystals, we have studied the dielectr ic response in thick planar-oriented samples of Smectic C-A* compounds unde rgoing a helix inversion. If the mode is collective there ought to be a min imum in the dielectric absorption due to the mode, at the temperature at wh ich the helical pitch diverges. Although the effect is not large, we find m inima for both antiferroelectric modes at this point, indicating that both modes are related to the helical director structure. A corresponding experi ment has also been performed on a binary mixture which shows a helix invers ion for a certain mixing ratio. However, because the effect in this case is overshadowed by unavoidable variations in other parameters, no minimum was detected.