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Birefringence in the Sm A Phase and the Disappearance of Helicoidal Structure in the Sm C* Phase Caused by an Electric Field in DOBAMBC

To understand the exact nature of the Sm A-C* phase transition in the presence of an electric field E, we have studied in (S)-2-methylbutyl p-[(p-decyloxybenzylidene)-amino]-cinnamate (DOBAMBC) the field dependence of (1) critical temperature Tc (E) at which the helicoidal structure in the Sm C* phase disappears, (2) optical rotatory power ρ(φ) along the effective optical axis in the Sm C* phase, and (3) birefringence Δn observed along the layer normal in the Sm A phase. The results obtained are: (1) Tc-Tc (E)∝E2±0.2, where Tc is the critical temperature for the Sm A-C* phase transition at zero field, (2) ρ(φ) appears to stay almost constant when E is small, and (3) the birefringence is mainly due to the hindered rotation, but not to the tilt, of the molecules, and Δn∝|E| when T>(Tc+0.5)°C. These results are discussed in connection with the theoretical considerations by Michelson et al. and by Meyer et al.