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We performed nonlinear dielectric spectroscopy to study the dynamic properties near the smectic-A-smectic-C(*)(alpha) phase transition point of an antiferroelectric liquid crystal 4-(1-methyl-heptyloxycar- bonyl)phenyl 4-octylcarbonyloxybiphenyl-4-carboxylate (MHPOCBC). A Landau-type theory was developed in order to analyze the experimentally obtained frequency dispersions. From the experimental and theoretical results we successfully obtained the temperature dependence of the relaxation frequency of the soft mode in both the smectic-A and smectic-C(*)(alpha) phases, which gave direct evidence that the transition is brought about by soft mode condensation. Significant features of this transition are also discussed.
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