Nonlinear dielectric relaxation spectroscopy of the antiferroelectric liquid crystal 4-(1-trifluoromethyl-pheptyloxycarbonyl) phenyl 4(')-octyloxybiphenyl-4-carboxylate

Second-harmonic dielectric response of an antiferroelectric liquid crystal under dc electric fields

Second-harmonic dielectric response under dc fields in an antiferroelectric liquid-crystal 4-(1-methyl-heptyloxycarbonyl)phenyl 4-octylcarbonyloxybiphenyl- 4-carboxylate (MHPOCBC) has been investigated. The second-harmonic response from the amplitude mode in the smectic- Calpha* phase was observed due to the symmetry breaking by the application of dc electric fields. A Landau theory was developed in order to analyze experimentally obtained frequency dispersions. From the analysis we found a softening of the amplitude mode with increasing the dc field at a temperature close to the smectic- Calpha*-smectic- A phase transition point. It was turned out that the relaxation frequency of the amplitude mode decreases linearly with the applied electric field.

Rotational Brownian motion and nonlinear dielectric relaxation of asymmetric top molecules in strong electric fields

A general theoretical treatment of the nonlinear dielectric response of an assembly of asymmetric top molecules in strong electric fields is presented in the context of the noninertial rotational diffusion model. The calculation proceeds by obtaining an infinite hierarchy of recurrence equations for the expectation values of Wigner's D functions describing nonlinear relaxation of the system. This hierarchy may be used for the evaluation of both transient and ac nonlinear responses in strong electric fields. The solution of this hierarchy is obtained for the particular case of rigid rodlike molecules in superimposed ac and strong dc bias electric fields, allowing one to evaluate the corresponding nonlinear response. The results are in agreement with available experimental data on nonlinear dielectric relaxation of dilute solutions of polar rodlike molecules in nonpolar solvents.

Critical behavior of nonlinear permittivity in the smectic-A phase of chiral liquid crystals

We study the nonlinear permittivity of chiral liquid crystals in the smectic-A (Sm-A) phase near the ferroelectric smectic-C* (Sm-C*) phase and the smectic-C(*)(alpha) (Sm-C(*)(alpha)) phase theoretically and experimentally. The third-order nonlinear permittivity epsilon(3) shows the critical behavior with the exponent of four near the Sm-C* phase and its sign depends on the order of the phase transition. In the case of the Sm-A-Sm-C(*)(alpha) phase transition, the sign inversion of epsilon(3), presumably due to the large fluctuation of order parameter, is observed near the transition temperature.

Nonlinear dielectric relaxation spectroscopy of ferroelectric liquid crystals

The nonlinear dielectric relaxation spectra of ferroelectric liquid crystals (FLCs) have been studied in the chiral smectic-C phase. The linear and third-order nonlinear dielectric spectra show the relaxation corresponding to the fluctuation in the azimuthal angle of directors called the Goldstone mode. We calculated the nonlinear dielectric spectra of the Goldstone mode theoretically by the torque balance equation which describes the dynamics of FLCs under the electric field. The calculated spectra make good agreement with the measured ones. We also evaluated the material constants of FLCs from the best-fitted values of the linear and nonlinear dielectric increment and relaxation time.