We have experimentally and theoretically investigated the electric-field effect on the smectic-A–smectic-Cα* phase transition of an antiferroelectric liquid crystal4-(1-methyl-heptyloxycarbonyl)phenyl 4′-octylcarbonyloxybiphenil-4-carboxylate. From the microscopic observations under electric fields and the measurements of the D-E hysteresis loop and apparent tilt angle, the E (electric field)-T (temperature) phase diagram was established, in which a tricritical point was found. From the dielectric measurements under dc electric fields, a soft mode inducing the transition was observed. Furthermore, we have developed a phenomenological theory under dc fields to analyze the experimental results. The origin of the soft mode observed under fields was clarified and the temperature dependence of the relaxation frequency was obtained by the analyses.
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