Soft mode behaviour of a room temperature ferroelectric liquid crystal

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Abstract
The complex dielectric permittivity was studied in the frequency region 100 Hz ÷ 100 kHz in the vicinity of the SA→ SC* phase transition temperature Tc = 32°C for benzoic acid 4-octyloxy 4–|(2-methyl butyloxy) carbonyl | phenylester. The d.c. electric field of the intensity E = 4 kV/cm was used to unwind the helicoidal structure in S C* phase, therefore the contribution to ϵ from the Goldstone mode was eliminated. It was found that at 100 Hz the temperature dependence of the soft node contribution had the form of the Curie-Weiss law in both SA and SC* phases. At higher frequencies the maximum of ϵ at TC is lowered and at frequencies 10 ÷ 40 kHz a minimum appears. This behaviour was explained by the temperature dependence of the soft mode relaxation frequency fS which was evaluated by fitting the complex permittivity data to the Debye relaxation formula. It was established that fS depended linearly on temperature in SA as well as in SC* phases with a minimum of 1 kHz at TC.