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

Nonlinear dielectric effects in liquids: a guided tour

Dielectric relaxation measurements probe how the polarization of a material responds to the application of an external electric field, providing information on structure and dynamics of the sample. In the limit of small fields and thus linear response, such experiments reveal the properties of the material in the same thermodynamic state it would have in the absence of the external field. At sufficiently high fields, reversible changes in enthalpy and entropy of the system occur even at constant temperature, and these will in turn alter the polarization responses. The resulting nonlinear dielectric effects feature field induced suppressions (saturation) and enhancements (chemical effect) of the amplitudes, as well as time constant shifts towards faster (energy absorption) and slower (entropy reduction) dynamics. This review focuses on the effects of high electric fields that are reversible and observed at constant temperature for single component glass-forming liquids. The experimental challenges involved in nonlinear dielectric experiments, the approaches to separating and identifying the different sources of nonlinear behavior, and the current understanding of how high electric fields affect dielectric materials will be discussed. Covering studies from Debye's initial approach to the present state-of-the-art, it will be emphasized what insight can be gained from the nonlinear responses that are not available from dielectric relaxation results obtained in the linear regime.

Strongly nonlinear dynamics of ferroelectric liquid crystals

Molecular reorientation processes induced in thin ferroelectric liquid crystal systems by strong alternating external electric fields are studied both by solving numerically the equation of reorientation motion of molecules and by measuring the electro-optic response of thin samples. It is shown that the occurrence of a wide band in nonlinear response spectra above the Goldstone-mode frequency is a consequence of complex partially uncorrelated molecular reorientations enforced within smectic layers by sufficiently high fields of high enough frequencies. Such nonlinear reorientational motions of molecules are argued to have a character of weakly chaotic long-lasting transients, related to almost periodic modulations of the amplitude of rotational oscillations performed by molecules with the field frequency. These modulations have been numerically proved to proceed with lower frequencies than the field frequency and with space-dependent depths of temporal changes. The occurrence of the modulations has experimentally been confirmed by registering distinct contributions to electro-optic response spectra at frequencies less than the running frequency of the applied electric field.

Linear and non-linear dielectric properties of a short-pitch ferroelectric liquid crystal stabilized by a polymer network

Linear and non-linear dielectric measurements were carried out on a ferroelectric liquid crystal stabilized by an anisotropic polymer network. The polymerization process was achieved at room temperature. It was performed from an achiral monomer in the ferroelectric chiral smectic C phase, exhibiting a very short helical pitch and a large polarization. The linear and non-linear dielectric spectroscopy were also completed by textural morphology as well as structural and ferroelectric characterizations. All these measurements were carried out on a pure ferroelectric liquid crystal material and on composite films containing two polymer concentrations. The increase of the polymer network density leads to a decrease of the dielectric strength determined in the linear and non-linear dielectric spectroscopy. The complementarity between the linear and non-linear dielectric measurements and their confrontation with a theoretical model allowed the simultaneous determination of some physical parameters such as macroscopic polarization, rotational viscosity and twist elastic energy. We also discuss the effect of the polymer network density on the obtained physical parameters.

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.

Nonlinear dielectric response and thermodynamic heterogeneity in liquids

If large amplitude time-dependent fields (e.g., dielectric, magnetic, mechanical) are applied to a sample that displays relaxational modes, some energy of the external field is absorbed by the slow degrees of freedom. The weak coupling of these modes to the phonon bath leads to long persistence times of the resulting higher fictive temperature. Assuming heterogeneities regarding dielectric and thermal relaxation times, extremely strong nonlinear dielectric effects are predicted and experimentally verified. For glycerol at T = 213 K, the dielectric loss measured at 280 kV/cm increases by more than 6% over its low-field value. This nonlinearity shows a characteristic frequency dependence and implies that dielectric and thermal time constants are locally correlated in viscous liquids.