Superlattice structures observed in the extraordinary phase sequence and analyzed by the phenomenological Landau model and the partially molecular model.

We draw several electric-field-temperature (E-T) phase diagrams with electric-field-induced birefringence contours in the nOHFBBB1M7 (n=10) and nOTBBB1M7 (n=11) (C11) mixture system by changing the C11 concentration carefully; some of the mixtures show the unusual extraordinary phase sequence where subphases with the four-, five-, and six-layer superlattice structures emerge above the smectic-C(*) main phase. We try to understand the results in terms of two complementary models that have so far been proposed: the phenomenological Landau model of phase transitions by Dolganov et al. [P. V. Dolganov et al., Phys. Rev. E 86, 020701(R) (2012)] and the partially molecular Emelyanenko-Osipov model [A. V. Emelyanenko and M. A. Osipov, Phys. Rev. E 68, 051703 (2003)]. The observed E-T phase diagram can be well reproduced by the phenomenological model. An emergence of the subphase with the four-layer superlattice structure above smectic-C(*) is also understandable in terms of the partially molecular model. We discuss the pros and cons of the two models as well.

Degeneracy lifting due to thermal fluctuations around the frustration point between anticlinic antiferroelectric SmC(A)* and synclinic ferroelectric SmC*.

In the binary mixture phase diagram of MC881 and MC452, the borderline between anticlinic antiferroelectric SmC(A)(*) and synclinic ferroelectric SmC(*) becomes apparently parallel to the temperature ordinate axis at the critical concentration r(c). The free energy difference between SmC(A)(*) and SmC^{*} is extremely small in a wide temperature range near r(c). In such circumstances, by observing Bragg reflection spectra due to the director helical structure and electric-field-induced birefringence, we have observed the continuous change from SmC(A)(*) to SmC(*) for r/~r(c). These intriguing phenomena have been explained, successfully at least in the high-temperature region, by a thermal equilibrium between the synclinic and anticlinic orderings and the resulting Boltzmann distribution for the ratio between them; the thermal equilibrium is considered to be attained in a nonuniform defect-assisted way through solitary waves moving around dynamically. We have also discussed qualitatively an important role played by the effective long-range interlayer interactions in the low-temperature region.

Experimental study of de Vries properties in antiferroelectric smectic liquid crystals.

Results of the experimental study on different antiferroelectric liquid crystal (AFLC) materials are presented using a number of techniques such as the optical birefringence, electro-optics and the measurements of optical thickness of free-standing films. Despite differences in the molecular structures of the various AFLC materials studied, these are found to exhibit a de Vries type of smecticA (SmA) properties in a temperature range higher than SmC. This correlation leads to the conclusion that these two classes of liquid crystals are related to each other. Furthermore, we suggest that these arise from the same physical mechanism, namely the existence of the weak synclinic (or reduced anticlinic) correlations between the neighbouring molecular tilt directions.

Experimental study of high-temperature smectic- C_{FI2}{ *} phase in chiral smectic liquid crystals that exhibit phase-sequence reversal.

We report the results of an experimental study of a recently observed phase sequence reversal of smectic-C_{FI2}{ *} [ SmC;{ *}(q_{T}=1/2); a four layer antiferroelectric] phase appearing in the temperature range above the smectic-C{ *} (SmC;{ *}) phase from the results of optical birefringence, spontaneous polarization, selective reflection, conoscopy, and dielectric spectroscopy. The SmC_{FI2}{ *} phase is observed in an antiferroelectric liquid crystalline compound, 10OHF, in a temperature range above that of SmC{ *} phase and is found to be thermodynamically monotropic, i.e., it appears only upon cooling from SmC_{alpha}{ *} phase. This is also unstable as if it is once transformed to SmC{ *} by the application of the bias, it does not return to its original state unless the sample is heated and cooled again in the absence of the bias. Nevertheless this phase is stabilized by the addition of a chiral smectic compound 9OTBBB1M7 (abbreviated as C9), having a wide temperature range of the SmC_{FI2}{ *} phase. The temperature range of the low temperature SmC{ *} decreases with increase in the concentration of C9 and for a concentration of 55 wt. %, SmC{ *} disappears and the transition takes place directly from SmC_{FI2}{ *} to the crystalline phase on cooling. The existence of such a high-temperature SmC_{FI2}{ *} phase is also supported by a phenomenological model.

Time-resolved measurements of the dynamics of the photoinduced smectic-C* alpha-smectic-A transition.

We present the study of the dynamics of the recently reported photoinduced smectic-C(*)(alpha)-smectic-A transition. High resolution time-resolved dielectric dispersion measurements carried out during the photoisomerization process demonstrate that the magnitude of the uv intensity mimics the role played by temperature in determining the behavior of the soft mode relaxation. We also show that the uv intensity dependence of the soft mode relaxation frequency f(R) in the photoinduced smectic-A phase can be described with a functional form similar to that derived for the temperature dependence of f(R) and compare the experimentally determined critical exponent with the theoretically predicted for the Ising and three-dimensional XY universality classes. Our study illustrates an interesting feature, namely, the magnitude of light intensity can be treated like a thermodynamic variable such as temperature and pressure to study phase transitions in general.

Comparative x-ray and dielectric measurements of smectic A-smectic-C* transition in bulk and confined geometries.

Comparative x-ray and dielectric measurements have been made on a liquid crystal exhibiting a smectic-A-chiral-smectic-C (smectic-C*) transition in bulk and confined geometries. It is observed that confining the material in Anopore membranes having 200-nm pore size leads to the following features: (1) the temperature dependence of the x-ray layer spacing shows a qualitatively different behavior, (2) in the smectic-A phase the soft mode relaxation frequency increases by a factor of 2.5, and (3) in the smectic-C* phase the relaxation frequency of the Goldstone mode increases dramatically by as much as 400 times, perhaps owing to a partial unwinding of the helix by the surface induced field.

Photoinduced effects in the vicinity of the smectic-C(*)(alpha)-smectic-A transition.

We report the first results of photoinduced effects near the smectic-C(*)(alpha)-smectic-A transition in a binary liquid crystalline material. Dielectric dispersion measurements show that the softening of the tilt fluctuation mode in the vicinity of the transition gets enhanced in the presence of the UV light.

Photoinduced nematic-isotropic phase transition: a case for the random-field Ising model.

Experiments have been carried out on a photoactive guest-nonphotoactive host system exhibiting a photoinduced nematic-isotropic phase transition. It is shown that the phenomenon of photoinduced shift in the transition temperature is similar to the induction of cooperative chiral order in "sergeant-soldier" copolymer systems. An expression resembling the one given by the random-field model proposed for the latter system is seen to quantitatively describe the shift in the transition temperature as a function of the magnitude of the radiation.