Study of the SA-SC* phase transition by a dielectric method

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Abstract
A previously described experimental procedure has been used to perform the large frequency range dielectric study of a ferroelectric liquid crystal oriented in planar geometry (i.e. helix axis perpendicular to the measurement electric field). In addition to the usual relaxation mechanisms observed in liquid crystals, the experimental data show the existence of two relaxation modes (Goldstone mode and soft mode) characteristic of the SA-S C* phase transition. The dielectric strength and the critical frequency of the relaxation mechanisms connected with these two modes were obtained throughout the measurement temperature domain. In the case of the soft mode, this was possible in the SC* phase thanks to the high polarization of the sample and the superimposition on the measurement electric field of a DC electric field high enough to unwind the helix. The experimental data are discussed in terms of a theoretical model based on a Landau-type expansion of the free energy density variation at the SA-SC* phase transition. The main disagreements between the theoretical and the experimental data are connected with the fact that the theoretical model fails in predicting the temperature dependence of the helix pitch. Quantitative evaluations of the friction coefficients were deduced from the data

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