Nonlinear dielectric response of antiferroelectric liquid crystals in the smectic c_α* phase

Nonlinear dielectric response of antiferroelectric liquid crystals has been studied in the smectic C_α* (SmC_α*) phase. The linear dielectric spectrum shows a single relaxation of Debye type and its relaxation frequency is as high as one hundred kHz. The profile of the third-order nonlinear dielectric spectrum varies in complex fashion with temperature. In the vicinity of the SmA-SmC_α* phase transition, experimentally obtained nonlinear spectra are well described by those calculated with phenomenological theory of Landau type. The soft mode of the SmC_α* phase shows critical slowing-down near the transition temperature. In the lower-temperature region of the SmC_α* phase, the contribution from the soft mode of the SmC_α* phase reduces and the other relaxation mode with Debye-type spectrum appears at several hundred Hz in the nonlinear spectrum. The appearance of this low-frequency mode suggests that the cooperative fluctuation of directors over long range exists in the SmC_α* phase.