Unusual dielectric and electrical switching behavior in the de Vries smectic A phase of two organosiloxane derivatives.

X-ray, electrical, electro-optical, and dielectric studies in the de Vries smectic A (SmA) phase of organosiloxane derivatives exhibit features surprisingly different from that of a conventional SmA phase. The switching data show a double peak profile, characteristic of an antiferroelectric (AF) structure. A model with the adjacent smectic layers having an AF-like arrangement and no global tilt correlation is proposed. Observed in molecules with differential interactions between the two termini, these findings have wide ramifications in understanding the minimum layer shrinkage of such systems.

Photoinduced effects in the vicinity of the smectic-A-smectic-C*A transition: polarization, tilt angle, and response time studies.

We report detailed measurements of the photoinduced effects on the electric polarization, tilt angle, response time, and rotational viscosity in the vicinity of the smectic-A-antiferroelectric-smectic-C (Sm-C*(A)) transition of a guest-host system consisting of photoactive azobenzene-based guest molecules and nonphotoactive host molecules. In the Sm-C*(A) phase all the parameters, except the tilt angle, exhibit both the primary and secondary photoferroelectric effects. The tilt angle dependence of the polarization in the absence of light and in light-on conditions have been analyzed in terms of the predictions of the generalized mean-field and microscopic models.

Effect of pressure on the dynamics of the photostimulated orientational ordering transition in a liquid crystal.

We have investigated the influence of the applied pressure on the shift in the photostimulated phase transition temperature and the dynamics of the system to undergo such a phase transition. The shift is seen to decrease with increasing pressure, a feature that can be countered, to a certain extent, by increase in the intensity of the light used. Whereas the time scales of the system to undergo such a transition increases with pressure, the thermal back relaxation process to recover the original phase occurs faster at elevated pressures. We propose a simple argument to explain these observations.

Comparative x-ray measurements of a de Vries smectic- material in bulk and confined geometries.

X-ray investigations have been carried out on a material exhibiting the de Vries smectic-A (Sm-A) phase in bulk and confined geometries. In the confined geometry realized by confining the material in an Anopore membrane, while the minimal layer shrinkage feature characteristic of the de Vries materials is retained, the overall change of the layer spacing from the Sm-A to the Sm-C phase is much less compared to the bulk geometry and also the transition gets smeared out. A "crossover" behavior, which has been recently ascribed to different molecular packing arrangements within the Sm-A phase, is seen in both geometries. With the support from data on another non-de Vries type of material we suggest that the associated feature may not be due to a phase transition, but competition between the thermally driven stretching and molecular tilt.

X-ray and dielectric measurements of the smectic- A -hexatic- B transitionin bulk and confined geometries.

We report x-ray and dielectric measurements on a liquid crystal exhibiting a smectic- A -hexatic- B (Sm- A -Hex- B ) transition in bulk and confined geometries. The confined geometry experiments have been performed by filling the compound, n -butyl 4(') - n -hexyloxybiphenyl-4-carboxylate, into Anopore membranes with 20 nm pore size. The surprising result seen in the x-ray studies on both the bulk and the Anopore samples is the presence of unusually strong second harmonic reflection arising from the smectic layers in both the Sm- A and Hex- B phases. In fact, the third harmonic reflection is also observed in the entire temperature range of the Hex- B phase and close to the transition in the Sm- A phase of the bulk sample, with the ratio of I(2qo) / I(qo), and I(3qo) / I(qo) being approximately 1/40 and 1/470, respectively (here I(qo), I(2qo), and I(3qo) correspond to the peak intensities of the first, second, and third harmonic reflections). These features point to the fact that the layering is much better defined in this system, unlike the usual description of pure sinusoidal mass density wave with a typical value of 10(-4) for I(2qo) / I(qo). Another notable feature observed is that whereas the extent of positional correlations of the molecules within the smectic layer, is comparable in the Sm- A phase of the bulk and Anopore samples, the correlations are reduced in the Hex- B phase. In the bulk as well as the Anopore case, the critical behavior of the positional correlations is found to be well described by a power-law expression with the exponent alpha being slightly larger for the Anopore sample. It is also seen that the weak first order nature of the transition seen in the bulk, gets weakened further, perhaps becoming second order. Despite such significant differences between the bulk and Anopore samples, the dynamics associated with the director fluctuations is only slightly influenced by confinement.